surface stresses associated with arrested dykes in rift zones many theoretical models predict that arrested dykes may generate major grabens at rift zone surfaces arrested dyke tips in eroded rift zones however are normally not associated with major grabens or normal faults that could be generated by dyke induced stresses ahead of the tips and normal faults and grabens tend to be less common in those parts of eroded rift zones where dykes are comparatively abundant similarly there are feeder dykes as well as dykes arrested a few metres below the surface that do not generate faults or grabens at the surface here i propose that this discrepancy between theoretical models and field observations may be explained by the mechanical layering of the crust numerical models presented here show that abrupt changes in young s moduli layers with high dyke normal compressive stresses stress barriers and weak horizontal contacts have large effects on the dyke induced stress fields for the models considered the surface tensile stresses induced by arrested dykes are normally too small to lead to significant fault or graben formation at the rift zone surface the only significant dyke induced surface tensile stresses 2 mpa in these models are for a dyke tip arrested at 1 km depth below the surface of a rift zone with a weak contact at 400 m depth and subject to extension that tensile stress however peaks above the ends of the weak horizontal contact which in the model considered occur at distances of 4 km to either side of the dyke and shows no simple relation to the depth to the dyke tip thus for a layered crust with weak contacts straightforward inversion of surface geodetic data to infer dyke geometries may result in unreliable results

effective tension shear relationships in extensional fissure swarms axial rift zone of northeastern iceland abstract the geometry of fracture systems in selected areas of the active krafla fissure swarm mid atlantic ridge northeastern iceland is analysed based on geodetic analysis of the present day topography at the top of holocene basaltic lava flows which fill the axial rift zone the deformation of this initially horizontal surface can be reconstructed extensional deformation is localised at all scales and block tilting though present remains minor using simple models of the surface expression of normal faults the geometrical characteristics of the topographic features related to active deformation during tectonic volcanic events are quantitatively analysed at crustal depths of about 1 km normal faults are present and have an average 70 dip comparison with the dip data of older normal faults observed in the uplifted and eroded shoulders of the rift zone at palaeodepths of 1 2 km indicates that this dip determination is valid comparisons between the local case study and structural analyses of active fissure swarms on a larger scale suggest that normal faulting plays a major role in the middle section of the thin newly formed brittle crust of the rift zone in the axial oceanic rift zone of ne iceland the extensional deformation in the upper crust is dominated by horizontal tension and shear of normal sense their relative importance depending on depth absolute tension dominates in the uppermost several hundred metres of the crust resulting in the development of fissure swarms effective tension plays an important role at a deeper level 2 5 km because of the presence of magmatic fluid pressure from magma chambers which feed dyke injections at crustal depths of about 1 km normal shear prevails along fault planes which dip 60 75 this importance of normal shear at moderate depth between upper and lower crustal levels where tension prevails is pointed out within the extensional context of rifting these variations of tectonic behaviour with depth are controlled by both the lithostatic pressure and the effective tension induced by the presence of magmatic fluid pressure

localisation of normal faults in multilayer sequences abstract existing conceptual growth models for faults in layered sequences suggest that faults first localise in strong and brittle layers and are later linked in weak and ductile layers we use the discrete element method for modelling the growth of a normal fault in a brittle ductile multilayer sequence the modelling reveals that faults in brittle ductile sequences at low confining pressure and high strength contrast localise first as mode i fractures in the brittle layers low amplitude monoclinal folding prior to failure is accommodated by ductile flow in the weak layers the initially vertically segmented fault arrays are later linked via shallow dipping faults in the weak layers faults localise therefore as geometrically and kinematically coherent arrays of fault segments in which abandoned fault tips or splays are a product of the strain localisation process and do not necessarily indicate linkage of initially isolated faults the modelling suggests that fault tip lines in layered sequences are more advanced in the strong layers compared with weak layers where the difference in propagation distance is most likely related to strength and or ductility contrast layer dependent variations in fault localisation rates generate fringed rather than smooth fault tip lines in multilayers

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

high resolution fault displacement mapping from three dimensional seismic data evidence for dip linkage during fault growth abstract detailed mapping using high resolution three dimensional seismic data has revealed a number of sub horizontal anomalies in the distribution of vertical displacement throw on the planes of growth faults in the gulf of mexico recognition of these anomalies is highly sensitive to the interval at which the fault displacements are sampled because they represent local decreases in throw which are confined to small discrete parts of the fault planes the distribution of the anomalies is inconsistent with model displacement fields of quasi elliptical concentric contours and is therefore incompatible with models of fault growth by uniform slip distribution and radial tip line propagation an alternative model is proposed whereby the evolution of a fault plane is established by the propagation and linkage of precursor fault segments in the dip direction dip linkage overlap and linkage of fault tips in the dip direction results in relay structures that are sub parallel to fault strike and therefore displacement minima that are sub horizontal on normal and thrust faults however since they are orthogonal to the main slip direction on the fault these structures have a low preservation potential and are therefore unlikely to be well resolved on cross sectional seismic profiles

archaean oil migration in the witwatersrand basin of south africa abstract the late archaean witwatersrand supergroup of south africa hosts the largest known gold uranium pyrite ore deposits oil preserved in fluid inclusions in quartz grains in siliciclastic sedimentary rocks of that supergroup implies that hydrocarbon generation and migration occurred during the archaean and may have been involved in mineralization processes through reference to phanerozoic analogues oil bearing fluid inclusions entrapped in healed microfractures in detrital quartz grains and in early syntaxial quartz overgrowths imply that the onset of oil migration coincided with early to intermediate stages of burial while intra granular porosity was still preserved multiple generations of oil migration are indicated by i oil inclusions within early diagenetic cements at different levels in the stratigraphic succession ii more than one type of oil in entrapment sites iii oil entrapment in multiple stages of the quartz paragenetic sequence oil generation and migration are considered to have occurred throughout and for some considerable time after development of the witwatersrand basin consistent with progressive burial and kerogen maturation in more than one tectonic regime oil bearing fluid inclusions within detrital sandstone fragments suggest that oil migration also occurred in a sedimentary succession on the kaapvaal craton prior to 2 9 ga oil in the witwatersrand supergroup was most likely derived from multiple source areas with the principal source probably being shales within the lower witwatersrand supergroup the hydrocarbon migration history of the basin has important implications for understanding the textural relationship between gold bituminized oil and uraninite in the giant gold uranium pyrite ore deposits

results of a magnetotelluric traverse across western oregon crustal resistivity structure and the subduction of the juan de fuca plate abstract as part of project emslab we have collected and analysed wideband magnetotelluric data along an east west transect in western oregon preliminary modelling of the data using one dimensional inversions based upon rotationally invariant earth response functions was followed by finite element two dimensional modelling the models produced indicate the presence of an electrical conductor beneath the oregon coast range dipping eastward at 12 18 from a depth of 23 32 km we believe that this conductor includes the thrust surface of the subducting juan de fuca plate and or adjacent water saturated rocks its high conductance about 200 s is thought to be due to one or more of the following mechanisms 1 sediments subducted atop and with the juan de fuca plate 2 saline fluids produced by dehydration of the former or 3 seawater contained within subducted oceanic basalts there is a distinct possibility that the high conductivity is due primarily to the presence of subducted sediments in contrast with the notion that the subduction of young buoyant lithosphere retards sediment subduction at this convergent margin the conductive layer is overlain by relatively resistive rocks presumed to be accreted oceanic lithosphere model determined resistivities for the upper part of the coast range section are in good agreement with deep well log data a strong electrical contrast appears in the determinant phase pseudosection between the coast range and the willamette valley suggesting a structural boundary between the two provinces a surficial conductor is present in the valley to depths of 1 2 km and is due to alluvial fill induction arrow data show the geomagnetic coast effect and a smaller effect by the willamette valley alluvial fill

overview of electromagnetic methods applied in active volcanic areas of western united states abstract a better understanding of active volcanic areas in the united states through electromagnetic geophysical studies received foundation from the many surveys done for geothermal exploration in the 1970 s investigations by governmental industrial and academic agencies include but are not limited to mapping of the cascades long valley mono area the jemez volcanic field yellowstone park and an area in colorado for one example mt konocti in the mayacamas mountains california gravity magnetic and seismic as well as electromagnetic methods have all been used in an attempt to gain a better understanding of the subsurface structure in each of these volcanic regions anomalous zones were mapped when conductive these anomalies were interpreted to be correlated with hydrothermal activity and not to represent a magma chamber electrical and electromagnetic geophysical methods can offer valuable information in the understanding of volcanoes by being the method which is most sensitive to change in temperature and therefore can best map heat budget and hydrological character to aid in prediction of eruptions

hydrothermal alteration in the witwatersrand goldfields abstract alteration is recorded throughout much of the 3 km thick upper witwatersrand succession in south africa and extends for 300 km around the basin and 50 km into the basin the evidence for this alteration comes from petrography mineralogy geochemistry rock fabrics and stratigraphy the alteration conceived as a giant halo enveloping each major goldfield requires a major hydrothermal event focused on the auriferous upper wits central rand group the scale and nature of the alteration and its relationship to gold distribution on various scales strongly support a hydrothermal model for gold deposition such a model can account for the 50 000 t of gold mined from the witwatersrand goldfields the model works within the context of the major advances in understanding of the geological framework of the witwatersrand basin including the recognition of the importance of post burial processes it is highly significant that the age of witwatersrand sedimentation is much older than 2700 my and thus the host rocks for gold were deposited at least 100 my before the major global archean greenstone gold event the shale units from 40 major mines covering the seven goldfields are examined to establish the nature and distribution of the alteration and to make the connection between the fluid infiltration that caused the alteration and the introduction of gold into the witwatersrand basin in the hydrothermal model described here an auriferous low salinity fluid is produced at about 500 c during the metamorphism of mafic material that was below and adjacent to the witwatersrand basin large shear zones in the footwall of goldfields are the inferred fluid pathways into the upper wits succession and then unconformities smaller shear zones veins and the reef packages guide local fluid flow in this model gold precipitation occurs in response to the reaction of the fluid with carbon and iron bearing rocks that are abundant in the reef packages immediately overlying unconformity surfaces gold precipitation occurred at about 350 c and 10 km depth with the implication of a high geothermal gradient approaching 35 c per km the alteration of the shales but lack of abundant gold in them is predicted from the paucity of appropriate rocks or minerals in the shale units to precipitate gold but fluid infiltration still changed their composition fundamentally the source of the witwatersrand gold its present distribution and the alteration are explained within this hydrothermal model with similarities to the models invoked for other major gold provinces it is worth noting that quartz pebble conglomerate is a poor term for this gold type and has almost certainly limited exploration opportunities globally

deep downward fluid percolation driven by localized crust dilatation in iceland from a detailed seismic survey on a transform like plate setting in iceland we propose a spatial relationship between highly deformed crustal columns and areas of low vp vs ratio which are best explained by high fluid pressures at the base of the seismogenic crust combined with geological observations our seismic data suggest transient and local increases in crust permeability at specific highly dilating areas triggered by dynamical stresses from high magnitude regional earthquakes fluids are then trapped at supercritical conditions in the deep brittle crust which in turn increases the capacity of the rocks to deform through cracking and faulting

evolution of vertical faults at an extensional plate boundary southwest iceland vertical faults having both opening and vertical displacements are common in southwest iceland and hypotheses vary regarding whether they propagated to the surface from below or from the surface downward we address this issue through a study of vertical faults and associated surface fracture zones in regions of both oblique and normal spreading in southwest iceland individual fracture segments are commonly rotated out of the general trend of the fracture zone suggesting oblique motion along subsurface normal faults these faults commonly breach the upper hinge lines of narrow monoclinal folds that flank many fault traces on the hanging wall side based on these field observations and the results of numerical models we propose that 60 758 dipping normal faults in the subsurface propagated to the surface from below vertical fractures formed at the upper tips of the faults at depths of between 250 and 500 m 25 50 of the fault length in response to stress concentrations along the tip line model results indicate that narrow monoclinal folds develop at the surface above these vertical fractures which subsequently breach the monoclines along the upper hinge line forming vertical fault scarps and open fissures at the surface if vertical fractures utilize pre existing cooling joints in basalt to connect directly to the surface the hanging wall is able to pull apart from the footwall without the development of a surface monocline along the fault trace q 2003 elsevier ltd all rights reserved

tectonics and magma dynamics coupling in a dyke swarm of iceland abstract to study the dynamics of dyke and fissure swarms in iceland we analyse the distribution of the dykes across the tertiary alftafjordur dyke swarm and that of open fissures and faults across the active krafla fissure swarm within these narrow 5 to 10 km wide and long length width ratio 10 swarms deformation is accommodated both by tectonics and magmatism the dyke swarm built during the westward regional tilting of the host rock lava pile the populations of dykes suggest that a characteristic thickness at about 3 m is involved during dyking dykes show a quasi periodic distribution across the swarm and form clusters spaced by about 2 5 km the analysis of the fissure distribution in the krafla volcanic system reveals a cluster spacing of about half of that of the dyke clusters we discuss the potential origins of these features as a result of magma dynamics host rock properties and tectonics clustering possibly reflects the intrinsic mechanical response to stretching of the upper levels of the brittle crust we propose a three dimensional model of dyke swarms in which magma likely migrates laterally within the upper crust from igneous centres where high magmatic supply rates locally perturb the regional stress field

normal fault growth in layered basaltic rocks the role of strain rate in fault evolution abstract conceptual models for the evolution of dilatant faults in volcanic rift settings involve a step wise growth pattern involving upward propagation of subsurface faults and surface monocline formation which are breached by subvertical open faults immature discontinuous normal faults are considered representative of the early stages of mature linked faults that accommodate extensional strains we consider the evolution of surface breaking normal faults using a comparison of the distribution and geometry of normal faults from two volcanic rift zones the koa e fault system hawai i and the krafla fissure swarm ne iceland field mapping highlights similarities to current predicted geometries but also prominent differences that are not reconciled by current models variable deformation styles record magma supply changes within the rift zones which drive local strain rate gradients building on existing studies we present a conceptual model of fault growth that accounts for spatial and temporal changes in strain rate within the deforming regions we propose that faults in separate rift systems may not advance through the same stages of evolution and that faults within individual rift systems can show differing growth patterns variations in surface strains may be indicative of subsurface magmatic system changes with important implications for our understanding of volcano tectonic coupling

a detrital model for the origin of gold and sulfides in the witwatersrand basin based on re os isotopes abstract the re os systematics of gold and sulfides from the witwatersrand basin were utilized to determine whether the gold is detrital or was introduced by hydrothermal solutions from outside the basin gold from a gravity concentrate from the western areas gold plant and gold from the vaal reef have very high os concentrations of approximately 73 to 10000 ppb and 3 to 32 ppb re resulting in 187re 188os ratios of 0 010 to 0 185 the gold has subchondritic 187os 188os ratios between 0 1056 to 0 1099 and an average value of 0 1067 rhenium depletion ages trd range from 3 5 ga to 2 9 ga with a median age of 3 3 ga pyrite from the vaal reef have os concentrations ranging from 0 26 to 0 68 ppb re concentrations of 1 7 to 2 8 ppb and187re 188os ratios of approximately 14 to 87 the pyrite samples have measured 187os 188os ratios of 0 84 to 4 7 and define an isochron with an age of 2 99 0 11 ga mswd 0 77 the os isotopic data from the direct measurement of gold preclude introduction of gold to the witwatersrand basin from crustally derived metamorphic or hydrothermal fluids between 2 7 to 2 0 ga the unradiogenic 187os 188os ratios old trd ages of the western areas and vaal reef gold samples as well as the contemporaneously old age of the vaal reef pyrite are consistent with detrital deposition of gold during the formation of the witswatersrand basin the os data will allow for minor hydrothermal remobilization and or overprinting of hydrothermal gold on preexisting detrital gold grains but does not support the introduction of gold solely by hydrothermal fluids

geometry kinematics and deformation rates along the active normal fault system in the southern apennines implications for fault growth throw rates throws and kinematic data have been collected from a localised system of active normal faults in the southern apennines italy to assess how its growth history differs from that for a distributed fault system in the central apennines both show evidence for fault interaction in that 1 faults located centrally along strike have higher throws and throw rates than distal faults and 2 cumulative throw and throw rate profiles summed across strike show central maxima with values decreasing to zero at fault system tips however although throw rates increased through time in the central apennines we are unable to resolve such changes if they exist for the southern apennines specifically throw rates derived from offsets of 18 ka geomorphic surfaces are consistent with total throws in the southern apennines when extrapolated back through time but overestimate total throws in the central apennines by a factor of 2 4 suggesting an increase in throw rate through time for the latter we discuss why some fault systems appear to have throw rates that are constant through time whilst others accelerate during growth interaction

structural architecture of a highly oblique divergent plate boundary segment abstract the reykjanes peninsula in southwest iceland is a highly oblique spreading segment of the mid atlantic ridge oriented about 30 from the direction of absolute plate motion we present a complete and spatially accurate map of fractures for the reykjanes peninsula with a level of detail previously unattained our map reveals a variability in the pattern of normal oblique and strike slip faults and open fractures which reflects both temporal and spatial strain partitioning within the plate boundary zone fracture density varies across the length and width of the peninsula with density maxima at the ends and at the northern margin of the zone of volcanic activity fractures with similar strike cluster into distinct structural domains which can be related to patterns of faulting predicted for oblique extension and to their spatial distribution with respect to volcanic fissure swarms additional structural complexity on the reykjanes peninsula can be reconciled with magmatic periodicity and associated temporal strain partitioning implied by gps data as well as locally perturbed stress fields individual faults show variable slip histories indicating that they may be active during both magmatic and amagmatic periods associated with different strain fields

is a cation ordering transition of the mg fe olivine phase in the mantle responsible for the shallow mantle seismic discontinuity beneath the indian craton 1 we use first principles molecular dynamics simulations to study the behavior of cation ordering in the non equivalent octahedral sites of mg fe olivine solid solutions our theoretical calculations confirm the previous experimental finding that mg2 and fe2 can invert their octahedral site occupancy at a critical temperature assuming that the site preference of fe changes discontinuously between two states in which it is completely restricted to either m1 or m2 sites we have calculated the transition temperature tt between the two extreme states under ambient pressure tt is calculated to be 520 c that agrees fairly with the experimental finding in which however the ordering state changed discontinuously over a much smaller range of the site occupancy of fe tt is found to be pressure sensitive showing an increase by 30 to 100 c per unit gpa depending upon the iron content using the indian continental geotherm we estimate a depth of around 75 km corresponding to the calculated transition pressure and temperature of cation ordering which matches well with the depth for the hales discontinuity marked by a jump of shear wave velocity by 4 for olivine solid solutions with 12 5 iron the ordering transition increases vs from 4 5 to 4 7 km s both the inferences viz depth of discontinuity and magnitude of velocity increase find support from the modeling of teleseismic earthquake waveforms recorded over broadband seismographs on the dharwar craton this leads us to infer that the cation ordering transition in ferromagnesian olivine might be a potential factor for the hales discontinuity

cenozoic active margin and shallow cascades structure cocorp results from western oregon two deep seismic reflection profiles totaling 98 km were recorded by the consortium for continental reflection profiling cocorp in western oregon in 1984 providing some of the first onshore deep seismic reflection data from the forearc and arc of an active convergent margin these data are of lower quality than most cocorp data but when combined with other geological and geophysical data they provide some useful insights into the subduction zone beneath oregon and into extensional structures in the cascades line 1 crossed the eocene sediments and underlying pillow basalts of the coast range and western willamette valley reflections from the willamette valley clearly define the lower and western boundary of flat lying rhythmically bedded eocene sediments these sediments appear to be underlain by as much as 8 km of seismically transparent eocene pillow basalts layered reflections at depths of 8 16 km may indicate a remnant crust upon which the eocene pillow basalts were erupted east dipping reflections at depths of 35 40 km may represent the decollement above the subducting juan de fuca plate oregon line 2 crosses the cenozoic volcanic arc terrains of the western and high cascades interpretation of cocorp seismic data suggests modification of existing models for normal faulting that postulate a symmetric graben or grabens in the cascades of oregon in the high cascades along cocorp oregon line 2 reflections suggest a large gently west dipping half graben with major offset only on the west side of the range the seismic reflection data along with other geological and geophysical data suggest that the high cascades were built on a series of blocks rather than a single graben the blocks were differentially faulted during the pliocene seismic data from the western cascades indicate several normal faults with down to the east offset including a major previously unidentified fault of probable miocene age

field characteristics of laterally emplaced dikes anatomy of an exhumed miocene dike swarm in reydarfjordur eastern iceland abstract many dikes of the tertiary breiddalur dike swarm in reydarfjordur have a great lateral extent but are vertically upward and or downward discontinuous field relationships indicate that they are not the feeders of the lava pile they intrude their characteristics can be best interpreted as them constituting the fossil expression of lateral magma emplacement analogous to that occurring in the actively spreading krafla volcanic center of the neovolcanic zone average dike thickness increases clearly with depth and the thickness versus depth relationship is different from that described for other tertiary dike swarms in eastern iceland it is suggested that dikes represent the extent of finite spreading which at higher levels in the crust is expressed by fissures and other extensional structures

from surface fault traces to a fault growth model the vogar fissure swarm of the reykjanes peninsula southwest iceland abstract the vogar fissure swarm is one of four en echelon fracture swarms that connect the reykjanes ridge to the south iceland seismic zone and the western volcanic zone occurring in an area of flat topography this fissure swarm is clearly visible at the surface where it can be seen to affect recent postglacial lavas using remote sensing methods to identify and measure all the faults and fractures in the swarm combined with additional field observations and measurements we measured 478 individual fractures 33 of them being faults and 67 being fissures the fracture lengths show roughly log normal distributions most of the individual fractures belong to 68 main composite fractures seven of which are longer than 2500 m and correspond to the main fault scarps of the fissure swarm we showed that these main faults are distributed along five equally spaced zones 500 m apart and a few kilometers long we drawn 71 across strike profiles to characterize the shape of the fault scarps and 5 along strike profiles to characterize the evolution of vertical throw along the main faults each fault consists of a coalescence of individual segments of approximately equal length fault throws are never larger than 10 m and are smallest at the junctions between individual segments analyses of along strike throw profiles allowed us to determine the early stages of growth after coalescence the earliest stage is characterized by an increase in the throw of the central parts of segments this is followed by a second stage during which the throw increases at the junctions between segments progressively erasing these small throw zones

deformation partitioning inside a fissure swarm of the northern icelandic rift abstract extension in an oceanic rift domain is accommodated by fissuring and faulting processes the fissuring including dike injection involves extension and an increase in crustal volume consistent with magma supply as in oceanic ridge and hot spot contexts the faulting mainly involves thinning and extension the relationship between these two deformation modes is analysed in the northern rift of iceland in the active rift zone deformation and volcanism occur within 2 3 km wide fissure swarms separated by 10 km wide zones where deformation is minor based on topographic and geological mapping the local structure in a major fissure swarm was analysed the fracture density increases near the faults but also beyond their tips the geometrical analysis of structures revealed that for the last 10 000 years block faulting and tilting accommodated about one half of the extension the other half resulting from fissuring this distribution shows that even in an active volcanic zone with widespread magmatism the stretching resulting from normal shear can play an important role as estimated from the ages of eroded surfaces and lava flows the deformation rates range between 1 5 and 15 cm year these values are compared with other rates estimated for different time intervals in the same area

architecture of normal faults in the rift zone of central north iceland this work examines the architecture and propagation of normal faults that accommodate extension in the rift zone of central north iceland it is based on a structural study of portions of three grabens their normal faults and dilational fractures in basaltic lavas of the fremri namur and dyngjufjol volcanic systems we analyzed the shape throw and dilation along faults in order to infer their possible evolution the shape of throw profiles departs from elliptical varying along each fault with throw being lower at the fault and segment tips propagation of fault segments is accommodated by dilational fractures nucleating at their respective tips segments link by lateral propagation of one or both tips or by the development of intervening linking segments two orders of fault segmentation indicate that the present throw accumulated by many increments as the fault scarps changed their geometries faults have large dilational components weakly correlated with fault throw and length

a magnetotelluric transect in the oregon coast range as part of emslab broadband magnetotelluric data have been collected and analyzed at 11 sites along an east west transect in the oregon coast range using one and two dimensional modelling the coast range was found to consist of marine sediments about 4 km deep near the coast and volcanic rocks about 8 km deep farther inland willamette valley consists of surficial conductive sediments to a depth of 1 km which overlie an eastern extension of the rocks that compose the coast range an east dipping conductor at a depth of approximately 20 km is associated with the upper surface of the subducting juan de fuca plate possible mechanisms for the high conductance are water saturated fractured basalts forming the surface of the plate subducted sediments and or the dehydration of the subducting slab trapped by metamorphic layering within the lower crust this conductor correlates with an east dipping reflector 35 40 km below the eastern flank of the coast range defined by cocorp profiling and thought to represent the surface of the subducting juan de fuca plate

interaction and linkage of extension fractures and normal faults examples from the rift zone of iceland abstract field and photogeological studies were made of 90 zones of interacting fracture segments along the rift zone of iceland each zone consists of a pair of extension fractures or a pair of normal faults with lengths from metres to kilometres these zones evolve from an underlapping stage through an overlapping stage the most common configuration and finally to a linkage stage of all the zones only 7 are underlapping whereas 93 are overlapping with hook shaped fracture pairs the length width ratios of the overlapping zones have a mean value of 3 5 the preferred geometry of the overlapping zones depends upon the initial configuration of the interacting fractures length overstep and the development of the interaction in the overlapping zones most fracture pairs show moderate shear strike slip components related to local variations in the extension opening directions vertical displacements on normal faults decrease as the overstep and length of overlapping zones increase both in turn are proportional to the total lengths of the faults forming the pair the geometrical and kinematic features of overlapping spreading centres at mid ocean ridges show close similarities to those reported here these similarities indicate that the architecture and evolution of overlapping zones are scale independent

mesoarchaean palaeoproterozoic crustal scale tectonics of the central witwatersrand basin interpretation from 2d seismic data and 3d geological modelling abstract the witwatersrand basin represents one of the largest exposures of meso neoarchaean rock on earth and hosts the vredefort dome at its geographic centre the southern half of the basin is covered by thin palaeozoic to early mesozoic marginal sequences of the karoo supergroup however visualisation and analysis of this unexposed portion of the basin can be achieved in a 3d geomodelling environment through the integration of geophysical 2d reflection seismics and traditional geological data in this study the 3d structural architecture around the vredefort dome in the witwatersrand basin is evaluated to establish strato tectonic relationships first order scale structures and a new model for the architecture of basement rocks from the geological modelling several strato structural features have been identified in the seismic sections including a well developed listric fault system in the southwest a prominent fold system observed in the transvaal supergroup and a large listric fault in the east that offsets the aforementioned fold system integration of these geophysical and geological data in 3d space has provided 3d constraints on the volumes for the witwatersrand supergroup ventersdorp supergroup and transvaal supergroup to the south southeast and east of the vredefort dome

shear wave constraints on a deep crustal reflective zone beneath vancouver island analysis of teleseismic receiver functions recorded in the northern cascadia subduction zone show that large p to s converted phases are generated at the boundaries of a dipping layer of deep crustal reflectors the e zone beneath central vancouver island these data provide the first shear wave constraints on this layer which is clearly observed in seismic reflection sections and is coincident with a region of low electrical resistivity interpreted from a magnetotelluric survey modeling of the amplitudes polarities and arrival times of the p to s converted phases demonstrates that the e zone is a prominent low velocity layer with an s velocity 1 0 0 2 km s lower than the regions above and below the e zone dips 7 5 in the direction n10 e 20 and extends from depths of 36 5 to 41 km at alb b on central vancouver island the estimated s velocity contrast when considered with refraction p velocities yields a poisson s ratio of 0 34 for this zone given the uncertainty in vp and vs this value may range from 0 27 to 0 38 the low p and s velocities and high poisson s ratio combined with electrical resistivity measurements suggest a porosity of 0 1 1 9 and crack aspect ratios of 0 001 0 01 values consistent with a region containing thin fluid saturated cracks and thus supportive of the interpretation of this feature as a major shear zone

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

the fissure swarm of the askja volcanic system along the divergent plate boundary of n iceland divergent plate boundaries such as the one crossing iceland are characterized by a high density of subparallel volcanic fissures and tectonic fractures collectively termed rift zones or fissure swarms when extending from a specific volcano volcanic fissures and tectonic fractures in the fissure swarms are formed during rifting events when magma intrudes fractures to form dikes and even feeds fissure eruptions we mapped volcanic fissures and tectonic fractures in a part of the divergent plate boundary in northern iceland the study area is 1 800 km2 located within and north of the askja central volcano the style of fractures changes with distance from askja close to askja the swarm is dominated by eruptive fissures the proportion of tectonic fractures gets larger with distance from askja this may indicate that magma pressure is generally higher in dikes close to askja than farther away from it volcanic fissures and tectonic fractures are either oriented away from or concentric with the 3 4 identified calderas in askja the average azimuth of fissures and fractures in the area deviates significantly from the azimuth perpendicular to the direction of plate velocity as this deviation decreases gradually northward we suggest that the effect of the triple junction of the north american eurasian and the hreppar microplate is a likely cause for this deviation shallow tectonic earthquakes in the vicinity of askja are often located in a relatively unfractured area between the fissure swarms of askja and kverkfjoll these earthquakes are associated with strike slip faulting according to fault plane solutions we suggest that the latest magma intrusions into either the askja or the kverkfjoll fissure swarms rotated the maximum stress axis from being vertical to horizontal causing the formation of strike slip faults instead of the dilatational fractures related to the fissure swarms the activity in different parts of the askja fissure swarm is uneven in time and switches between subswarms as shown by a fissure swarm that is exposed in an early holocene lava nw of her ubrei but disappears under a younger 3500 4500 bp lava flow we suggest that the location of inflation centres in askja central volcano controls into which part of the askja fissure swarm a dike propagates the size and amount of fractures in the kollottadyngja lava shield decrease with increasing elevation we suggest that this occurred as the depth to the propagating dike s was greater under central kollottadyngja than under its flanks due to topography

strain accumulation along an oblique plate boundary the reykjanes peninsula southwest iceland strain accumulation along an oblique plate boundary the reykjanes peninsula southwest iceland

tectonics of the thingvellir fissure swarm sw iceland abstract the thingvellir fissure swarm dissects 9000 year old pahoehoe lava and contains about 100 fractures of average orientation n29 3e the average length of fractures is 620 m the minimum being 57 m and the maximum 7 7 km the maximum width and throw on a single fracture are 68 m and 40 m respectively most fractures are vertical at the surface and must be the result of an absolute tensile stress the geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures it is concluded that most fractures attain depths of the order of several hundred meters or less but that the largest faults attain depths of many kilometers comparison with the vogar fissure swarm on the reykjanes peninsula suggests that the thingvellir swarm may have the greater rate of dilation the total maximum postglacial dilation of the vogar swarm is only 15 m whereas the corresponding figure for the thingvellir swarm is about 100 m

earthquakes stress and strain along an obliquely divergent plate boundary reykjanes peninsula southwest iceland earthquakes stress and strain along an obliquely divergent plate boundary the reykjanes peninsula southwest iceland

neoarchaean tectonic history of the witwatersrand basin and ventersdorp supergroup new constraints from high resolution 3d seismic reflection data abstract first order scale structures in the west wits line and west rand goldfields of the witwatersrand basin south africa were mapped using the high resolution 3d reflection seismic method structural models constrain the magnitude of displacement of thrusts and faults the gross structural architecture and neoarchaean tectonic evolution of the west rand and bank fault zones which offset the gold bearing reefs of the basin the merging of several 3d seismic surveys made clear the gross strato structural architecture of the goldfields a macroscopic fold thrust belt is crosscut by a macroscopic extensional fault array these are dissected eroded and overlain by the transvaal supergroup above an angular unconformity the seismic sections confirm that the west rand group ca 2985 2902 ma is unconformably overlain by the central rand group ca 2902 2849 ma with tilting of the west rand group syn to post erosion at ca 2 9 ga the seismic sections also confirm that an unconformable relationship exists between the central rand group and the auriferous ventersdorp contact reef vcr with an easterly verging fold thrust belt being initiated concomitant to deposition of the vcr at approximately 2 72 ga fold thrust formation included development of the 1 newly identified first order scale libanon anticline 2 tandeka and jabulani thrusts which displace the west rand group and 3 parasite folds the fold thrust belt is crosscut by a macroscopic extensional fault array or rift like system of faults which incepted towards the end of extrusion of the ventersdorp lavas and certainly during deposition of the platberg group 2709 2643 ma when a mantle plume may have heated the lithosphere the west rand and bank fault zones formed at this time and include 1 the west rand and bank faults which are scissors faults 2 second and third order scale normal faults in the immediate footwall and hanging wall of the faults 3 drag synclines and 4 rollover anticlines

nature of the low velocity zone in cascadia from receiver function waveform inversion abstract a dipping low velocity zone lvz is a ubiquitous structural element of subduction zones worldwide in this study we map seismic attributes characterizing the lvz beneath the cascadia subduction zone from northern vancouver island to northern california using receiver function waveform inversion throughout this region the lvz is characterized by high v p v s ratios mean 2 77 strong s velocity contrasts 50 and thicknesses averaging 3 37 km the lvz is immediately underlain by a second weaker layer exhibiting moderate v p v s ratios mean 1 85 with mean thickness of 4 62 km we interpret the combined structure in terms of subducting oceanic crust based on classical structural petrological descriptions and constraints from previous studies of ophiolites and ocean drill cores the lvz is identified with pervasively hydrated high porosity pillow basalts and sheeted dikes of layer 2 with possible contributions from sediments layer 1 fluids released from metamorphic dehydration reactions are maintained near lithosphere fluid pressures through an impermeable plate boundary above and a low porosity gabbroic mafic cumulate dominated layer 3 below

nature and origin of carbon in the archaean witwatersrand basin south africa the nature and origin of the carbonaceous matter carbon in the gold bearing reefs of the archaean witwatersrand basin of south africa has long been a subject of controversy and debate the importance of the carbon results from its direct association with the gold grade the studies reported here were directed at resolving the nature and origin of carbon techniques used include stope scale macroscopic mapping mesoscopic hand specimen examination and microscopic studies incorporating the use of optical reflectance measurement macroscopic mapping has demonstrated the braided habit of carbon bodies over tens of square metres with a close parallel to sedimentology mesoscopic observations however show the vein like carbon seams are often cross cutting sedimentary features with the carbon occupying an anastomosing set of bed parallel fractures microscopic observations show a globular form for the carbon and inclusions of uraninite particles in seam carbon the globules are generally modified into elongated spindles oriented perpendicular to bedding optical properties are dominated by generally high reflectance and anisotropy with a swirling liquid crystal structure classifying seam carbon as carbonaceous mesophase it is this fine mesophase texture that has previously been wrongly attributed to plant algal morphology highly significantly the optical anisotropy is bed perpendicular rather than bed parallel as in normal sedimentary burial indicating mesophase formation during fracture opening both reflectance and bireflectance of the seam mesophase show a range of values at all scales no stratigraphic or depth trends were apparent though geographic variation points to localization of the maximum temperature event s given high anisotropies maximum reflectance values in the 4 8 r o range and minima in the 1 2 r o range give arithmetic mean reflectance values of about 4 0 r o kinetic modelling equates these values with short term 50 000 years temperature events of c 400 c or burial temperatures of c 280 c lower reflectance values of c 2 0 r o obtained from the shales suggests thermal detachment of permeable and impermeable beds a characteristic of pulsed hydrothermal flux the included uraninite grains influence the orientation of the surrounding anisotropic mesophase demonstrating that uraninite predated the mesophase formation event regional palaeo stress frozen into the mesophase anisotropy indicates a nw se deviatoric stress the combination of hydrocarbon radiation and hydrothermal fluid flow marks witwatersrand basin pyrobitumens out as being different from conventional oil putative source rocks for the oil are identified in the intra basinal witwatersrand shale units lithological control in carbon seams is explained by the poro perm and mechanical properties of the unconformity sequence of footwall reef conglomerates and hanging wall quartzites favouring fracture along the unconformity surfaces an overpressured oxidizing fluid created the fracture network that hosted the oil and also introduced uranium as uranyl ion on meeting a reducing agent oil uraninite precipitated stabilizing the oil by polymerization further hotter hydrothermal flux then coked the bitumen to often fibrous spindle shaped mesophase continued flow deposited gold in the available pore spaces most notably those formed by lateral contraction between mesophase spindles the speculative role of methane in the reductive precipitation of gold is indicated by the presence of gold in any co eval porosity largely independent of the mineral matrix

throw rate variations within linkage zones during the growth of normal faults case studies from the western volcanic zone iceland abstract this work investigates how throw rates vary within fault bends and sites of fault linkage during the process of normal fault growth in the western volcanic zone iceland through detailed field mapping and field measurements of fault throws normal faults are mapped and along strike throw profiles are constructed in order to understand how the throw rates relate with the local fault geometry along faults at different stages of linkage the results show that throw rates increase within linkage zones and propagating fault bends independently from the stage of maturity of the fault bend this implies that 1 the relationship between the local fault geometry and the along strike distribution of throw rate is driven by the deeper part of the fault where established fault bends start propagating to the surface 2 faults grow first by linkage and coalescence of separate faults and then by accumulation of slip on the resultant fault in agreement with models of fault growth by linkage and coalescence 3 incipient fault bends can produce uncertainty associated with palaeoseismological results if fault bends remain unrecognised moreover this work demonstrates that existing models showing increased co seismic and throw rates within fault bends and sites of fault linkage found in continental extensional settings are valid in a geodynamic context of a mid oceanic rifts

fault and graben growth along active magmatic divergent plate boundaries in iceland and ethiopia recent studies highlight the importance of annual scale dike induced rifting episodes in developing normal faults and graben along the active axis of magmatic divergent plate boundaries mdpb however the longer term 102 105 years role of diking on the cumulative surface deformation and evolution of mdpb is not yet well understood to better understand the longer term normal faults and graben along the axis of mdpb we analyze fissure swarms in iceland and ethiopia we first focus on the simplest case of immature fissure swarms with single dike fed eruptive fissures these consist of a 1 km wide graben bordered by normal faults with displacement up to a few meters consistent with theoretical models and geodetic data a similar structural pattern is found with asymmetric and multiple graben within wider mature fissure swarms formed by several dike fed eruptive fissures we then consider the lateral termination of normal faults along these grabens to detect their upward or downward propagation most faults terminate as open fractures on flat surface suggesting downward fault propagation this is consistent with recent experiments showing dike induced normal faults propagating downward from the surface however some normal faults also terminate as open fractures on monoclines which resemble fault propagation folds this suggests upward propagation of reactivated buried faults promoted by diking these results suggest that fault growth and graben development as well as the longer term evolution of the axis of mdpb may be explained only through dike emplacement and that any amagmatic faulting is not necessary

seismic reflection imaging of the subducting juan de fuca plate mckenzie and parker s early suggestion1 that the juan de fuca plate is underthrusting north america has since been confirmed by numerous studies evidence for plate convergence along this junction includes a sediment filled trench2 7 intense compressional deformation of the continental slope and shelf sediments2 7 seafloor magnetic anomalies that extend beneath the continental slope2 4 inland andesitic volcanoes8 paired low high heat flow and gravity anomalies9 10 active onshore deformation11 12 onshore crustal earthquakes with compressional axes parallel to the direction of plate convergence13 sinistral strike slip earthquakes along the nootka fault fig 1 14 and a well defined benioff wadati zone6 15 16 here we report the results of a seismic reflection survey that has delineated two slabs of oceanic lithosphere underlying vancouver island one that is currently being subducted and one that is underplated these findings lead us to speculate that successive underplating of oceanic lithosphere may be an important process in the evolution and growth of continents

integration of underground mapping petrology and high resolution microseismicity analysis to characterise weak geotechnical zones in deep south african gold mines abstract a highly stressed shaft pillar is prone to large seismic events falls of ground and rockbursts which may cause injuries and loss of production especially in weak geotechnical zones it is thus important to identify weak geotechnical zones in order to mitigate risks in this study we present integrated studies underground mapping petrology rock mechanics and high resolution microseismicity analysis to understand the different geotechnical zones in the shaft pillar of cooke 4 mine in south africa the footwall of the remnant shaft pillar comprises the upper elsburg reef of the mondeor formation while the ventersdorp contact reef vcr of the venterspost formation and soft weak lavas of the westonaria formation form the hangingwall results from underground mapping and microscopic analysis show that the shaft pillar is composed of quartzites pebbly quartzites argillaceous quartzites and conglomerates underground mapping further shows that the shaft pillar is characterized by several discontinuities which vary from minor to macro scale fractures laboratory uniaxial compressive strength ucs tests indicate that quartzite has the strongest strength followed by pebbly quartzite argillaceous quartzite and lastly conglomerate analysis of high resolution acoustic emissions aes clusters indicates that the majority of aes are associated with the mining stope faces the clusters show the formation of ortlepp shears ahead of the stope which is caused by the excavation induced stress field microseismic data further reveal that the fracture turning point occurs in the soft strata weak hangingwall lavas the integration of these datasets has allowed us to develop the fracture model for different geotechnical zones which concurs with previous models developed for the similar underground environment i e weak soft lava hangingwall and quartzite conglomerate footwall this has major implications for future mining support production and safety

evaluating the structural character and tectonic history of the witwatersrand basin abstract the witwatersrand basin in south africa has been the focus of gold exploitation for more than 100 years and the subject of hundreds of research investigations principally sedimentological and metallogenic and recently geophysical and geochronological surprisingly the basin wide structural character has not been fully investigated despite the fact that gold metallogenic models have been proposed that want structural architectures as conduits for gold mobilisation and or sites of deposition i e the modified placer and hydrothermal models these models which have polarised viewpoints on the source for gold placer versus metamorphic or hydrothermal source assume an underlying structural character to the basin and structural relationships between goldfields but these assumptions have never been tested through evaluation of the existing structural dataset as corollary the published data for the structural character of the free state welkom klerksdorp west wits line west rand central rand east rand and south rand goldfields are summarised and evaluated to search for basin wide similarities and or correlations that would support tectonic models for the witwatersrand basin and explain its metallogenic character unfortunately the large published historical database 90 years proved to be quite limited with respect to geometric kinematic and or relative chronological detail but must be evaluated against an advanced understanding of earth processes tectonics and the structural geology approach the dataset is largely qualitative rather than quantitative but is useful in establishing broad tectonic trends of the formation and tectonic evolution for the witwatersrand basin it suggests that two deformation events umzawami and ukubambana events mark the end of deposition in the witwatersrand and transvaal basins respectively with development of fold thrust belts at ca 2 7 and 2 2 2 0 ga and these can be correlated with supercontinental cycles the intervening period i e 2 7 2 2 ga was characterised by at least 4 basin forming events that culminated in the deposition of the volcanic sedimentary sequences of the klipriviersberg group and the sedimentary sequences of the platberg chuniespoort and pretoria groups basin formation was facilitated by the reactivation of structures formed during the umzawami event or older as feeder dykes normal listric faults or growth faults the limited structural data for the witwatersrand basin unsurprisingly also leads to the conclusion that gold was macroscopically to mesoscopically remobilised from a primary placer source but remobilisation was localised goldfields scale and not is a basin wide phenomena

hydrofracturing stress measurements in the iceland research drilling project drill hole at reydarfjordur iceland two independent suites of hydrofracturing stress measurements were conducted in the top 600 m of the iceland research drilling project deep hole at reydarfjordur east iceland as indicated by the continuously extracted drill core the tested section consists of tertiary subaerial tholeiitic lava flows cut by many basaltic dikes the density of the basalt was used to estimate the vertical stress v in megapascals at depth d in meters v 0 027d in the 33 to 270 m depth range the horizontal principal stresses hmax and hmin as calculated from hydrofracturing results increase gradually with depth but at a lower rate than the vertical stress hmin 2 1 0 016d hmax 3 3 0 016d in the 300 to 550 m depth range a major perturbation occurs with both horizontal stress magnitudes rapidly rising between 300 and 400 m and then uniformly decreasing between 400 and 550 m to a level apparently predictable by the relationships obtained in the top 270 m conditions favoring normal faulting v hmax hmin were established in the 540 to 580 m range and are conjectured to persist at greater depths since the vertical stress gradient is higher than that of hmax and hmin this would be in accord with the predominant fault type in east iceland the relative low horizontal stress values and the small difference between them are supportive of ocean floor spreading theories and contradict near surface high compressive stresses previously determined by hast 1973 in east iceland the large increase in horizontal stress in the range of 300 to 500 m depth is not understood a correlation appears to exist between the zone of almost unintruded thick lava flows 200 500 m and that of high stresses an intuitive model suggesting locked in stresses in this zone is proposed the hydrofracturing determined overall mean direction of hmax is n50 e while a more selective sampling of results yields n40 e the measured stress directions appear to be reasonably compatible with the local structure such as dike direction nne and lava dip wsw significantly hmax direction is subparallel to the n20 e trend of the nearest fissure swarms in the axial rift zone this almost coincidence of directions like the measured stress magnitudes is also in agreement with the model of spreading lithospheric plates being pulled apart

crustal spreading due to dikes and faults in southwest iceland abstract a 30 km long profile extending from 3 million year old pleistocene rocks to the holocene rocks of the rift zone in southwest iceland is dissected by 443 dikes and 156 normal faults nearly all the dikes are basaltic with an average strike of n26 e which is subparallel to the trend of fractures in the nearby rift zone their average dip is 69 and average thickness 1 37 m both values much lower than averages for regional tertiary dike swarms in iceland many dikes are seen to change into sills the great majority of the faults are pure dip slip with an average throw of 10 2 m their average strike n37 e is different from the average strike of the dikes in this area and the fractures in the nearby rift zone their average dip is 75 2 which is somewhat steeper than in the tertiary areas but the dips are less in the interbedded sedimentary layers than in the lava flows growth faults are very rare in the area and most of the faults appear to have completed their growth in less than 10 000 years in subsections crustal dilation due to dikes is 0 4 3 7 that due to faults is 0 6 5 3 and that due to both dikes and faults is 1 04 6 44 in these subsections average fault spacing is one fault per 170 563 m spacing decreases toward the active rift zone it is concluded that in typical fissure swarms of the rift zone the dilation at the surface is almost entirely due to extensional fractures and gaping vertical normal faults in the depth range of a couple of hundred metres to perhaps 1 km the non vertical faults contribute most to the dilation at deeper crustal levels however dilation due to dikes becomes dominant

thickness composition and evolution of the indian precambrian crust inferred from broadband seismological measurements abstract we present seismological constraints to model the evolution of indian precambrian crust through receiver function analysis of teleseismic waveforms recorded on 22 broadband seismographs along a n s corridor from southern granulite terrain to delhi on the aravalli craton archean terrains eastern dharwar bastar bundelkhand and aravalli cratons are underlain by crust with felsic intermediate composition vp vs 1 65 1 75 in contrast to mafic composition vp vs 1 78 of crust beneath vindhyan basin godavari basin and the narmada son lineament nsl of proterozoic age the moho depth varies between 33 and 43 km with no definite relationship with age of the crust duplex moho with highly mafic underplated lower crust is characteristic feature of the nsl signifying its paleo rift character present day felsic character of the indian archean crust could be a consequence of post formation modification of the initial mafic lower crust through the process of lithospheric delamination this major tectonic event also created layered lithosphere beneath parts of india imprinted as positive velocity contrast at a depth of 80 km usually referred to as hales discontinuity the upper part of the lithosphere has shear velocity vs 4 52 km s and the lower one 4 77 km s we speculate that such a layered lithosphere could be due to a low density depleted spinel peridotite with archean affinity lying over higher density fertile garnet peridotite of proterozoic age

3d edge detection seismic attributes used to map potential conduits for water and methane in deep gold mines in the witwatersrand basin south africa abstractinrushes of ground water and the ignition of flammable gases pose risks to workers in deep south african gold mines large volumes of water may be stored in solution cavities in dolomitic rocks that overlie the black reef blr formation while there are several possible sources for methane namely coal seams kerogen found in some gold ore bodies or methane introduced by igneous intrusions potential conduits that may transport water and methane to underground workings were mapped using 3d reflection seismic data edge detection attributes successfully identified many faults some with displacements as small as 10 m faults that displace the ventersdorp contact reef vcr and the blr horizons were of special interest as known occurrences of fissure water and methane in underground workings show a good correlation with faults that were imaged on the vcr and blr horizons because there are uncertainties in determining the linkage of faults with aquifers and methane sources it is considered prud

p to s converted wave observations at long valley caldera we report observations of p to s converted teleseismic body waves recorded with seismic stations located on the resurgent dome of long valley caldera we observe strong conversions occurring at boundaries that could be the tops of small magma bodies at depths of 4 9 kilometers beneath the resurgent dome this technique could prove to be an extremely powerful tool in the search for magma bodies for geothermal energy development our results from a two month deployment of triggered digital teleseismic recorders are discussed

field examples and numerical modelling of oversteps and bends along normal faults in cross section abstract oversteps and bends along cross sections through small scale numerical modelling distinct element analysis has been carried out to simulate the patterns of deformation at oversteps and bends area increase pull apart development and localised stress concentration occur at extensional oversteps and bends stress concentration and a tendency for compaction are typical of contractional oversteps and bends such results are compatible with the structures observed in the field a sensitivity analysis of the modelling has been carried out to investigate the effects of variations in mechanical parameters on deformation although models with weaker properties are more deformable the overall geometry of deformation varies little with variations in mechanical parameters

formation and growth of normal faults at the divergent plate boundary in iceland the divergent plate boundary in iceland is characterized by 40 80 km long and 5 10 km wide swarms of tension fractures 102 m long and normal faults 103 m long the upper part of the crust is mainly composed of lava flows with abundant columnar joints that are mostly perpendicular to the lava contacts the lava flows are horizontal at the surface of the rift zone but become tilted at the rate of 1 for every 150 m depth in the crust at the surface of the rift zone the joints are vertical and parallel to the vertical principal stress because of tilting of the lava pile the columnar joints become oblique to this stress hence becoming potential shear fractures and form echelon sets at greater depths in the crust theoretical considerations suggest that normal faults start to nucleate on sets of en echelon columnar joints and or large scale tension fractures at crustal depths of 0 5 1 5 km the width depth must be the smallest controlling dimension of many faults nevertheless there is a positive linear relation r 0 91 between maximum throw and length of the holocene faults if the faults grow as self similar structures the throw length relationship can be explained by a similar relation between fault length and width

array analysis of the large aperture array of the 1988 89 passcal basin and range passive source seismic experiment summary the 1988 89 passcal basin and range passive source seismic experiment consisted of two array deployments the large and small aperture arrays the large aperture array consisted of seven stations with three component mid period seismometers and independently triggered prototype passcal data recorders with goes clocks the array spatial distribution and the synchronized data have permitted us to employ simple beamforming operations to improve receiver function estimation from single teleseismic events the beamforming reinforces the coherent teleseismic p wave and the deterministic portions of the p wave coda primarily generated by interactions at major lithospheric interfaces microseismic background noise is incoherent across the array and is attenuated by beamforming beamforming also diminishes those portions of the coda due to scattering by inhomogeneities that appear random on the scale of the array aperture the enhanced signal to noise ratio of the teleseismic recording dramatically improves the single event receiver function deconvolution results

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

the ventersdorp contact reef model in the kloof gold mine as derived from 3d seismics geological mapping and exploration borehole datasets abstract a model of the ventersdorp contact reef vcr orebody at kloof gold mine was derived by integrating 3d reflection seismic data with information derived from underground mine mapping and exploration drilling the study incorporated the depth converted prestack time migrated pstm seismic cube the mine geomodel faults and dikes mapped in excavations mine development infrastructure and intersections of the vcr by surface and underground exploration drilling the 3d seismic data provide an accurate geometric model of the vcr orebody and its offsets the underground mapping datasets help to define minor faults and dikes that are below seismic resolution limits the integration of the these datasets allowed 1 for better mapping of fault architectures and distributions within the lease area 2 definition of the likely zones of difficult ground conditions around seismically imaged dikes and faults and 3 better predictions of the number and spacing of faults that offset the vcr within minable blocks the model is useful in mitigating both economic and safety risks of deep mining

geophysical constraints on the crustal structure of the southern rio grande rift abstract evidence of geologic activity still occurring in the rio grande rift rgr includes quaternary faulting seismicity and widening at a small rate we map the crustal thickness and seismic velocity ratio to generate constrained crustal model cross sections that highlight the regional extension of the southern rgr srgr specifically we computed receiver functions and receiver function stacks for 147 stations from earthscope usarray transportable array and previously collected data in the southwestern united states and interpolated the crustal and velocity results using a kriging interpolation scheme we include a new optimized gravity inversion approach using the receiver function results as a constraint for inverted density profiles we produce constrained crustal models that characterize the srgr as showing a shallower moho 30 km in the region of the srgr which is thicker than previously suggested the crust appears to be delaminated west of the rgr in the basin and range province and thicker east of the rgr toward the great plains with a denser lower crustal layer we also find that the delaware basin shows a significant geophysical anomaly likely due to thick saturated sediments given that there is no deep mantle anomaly determined from other studies in this region we suggest that rifting could be the result of small scale convection of the uppermost mantle and we conclude that the rgr is at a minimum the boundary between the basin and range province and the great plains perhaps reflecting the terminus of flat slab subduction

continental margin tectonics forearc processes recent studies of convergent plate margins and the structural development of forearc terranes are summarized in a critical review of u s research from the period 1987 1990 topics addressed include the geometry of accretionary prisms coulomb wedge taper and vertical motion in response to tectonic processes offscraping vs underplating or subduction the response to oblique convergence fluids in forearc settings the thermal framework and the effects of fluid advection and serpentinite seamounts also included is a comprehensive bibliography for the period

structural controls on witwatersrand gold mineralisation abstract structural controls on the distribution of gold in the witwatersrand basin can be seen at scales ranging from that of thin sections up to regional seismic data sets at the thin section scale gold occurs largely in fractures with hydrocarbons these fractures are associated with pulses of thrusting deformation that occurred in the latter stages of the basin s development the distribution of thrust fracture networks and therefore gold is controlled by the mechanical attributes of the host stratigraphy thrust displacements are generally very low with structures exploiting depositional contacts and mesoscale sedimentary bedforms many of the small faults are isolated from each other with bimodal vergence between larger scale thrusts of the same age early formed faults and fractures become folded and faulted by subsequent propagation of larger scale thrusts these characteristics are consistent with formation of the mineralised thrust fracture networks in the frontal low displacement parts of a thrust system in the carletonville goldfield the prospectivity of the ventersdorp contact reef vcr has been evaluated by using underground observations to pinpoint the structural habitat of gold and therefore characterise the size of prospective zones around structures intersecting the vcr mapped within 3d seismic data in the welkom goldfield the controls on gold distribution at the tshepong mine have been characterised from 3d seismic data drilling and underground observations and used to develop an ore body model that aids reserve estimation and production planning

propagation of normal faults to the surface in basalt koae fault system hawaii normal fault scarps of the koae fault system on kilauea volcano consistently display locally breached monoclines underlain by prominent cavities deep gaping fissures on the footwall finer fissures on the hanging wall and buckles at the scarp base elastic analyses reveal that this assemblage forms as a fault propagates up towards the surface rather than down from it models of a planar blind normal fault with a dip exceeding 60 yield a monocline with a tensile stress concentration at the surface where gaping fissures occur a stronger subsurface tensile stress concentration near the blind fault tip where cavities occur and a compressive stress concentration at the surface where buckles occur the footwall fissures grow down from the tensile stress concentration at the surface and link with a fault as its scarp grows in contrast the cavities initiate at depth near the fault tip and propagate with it up towards the surface the hanging wall fissures apparently open in response to slip on late forming blind antithetic faults near the surface stoped blocks derived from footwall fissure walls help prop the footwall fissures open as a normal fault breaches the surface the fissures cavities and scarp rubble provide highly conductive hydraulic pathways

irrefutable detrital origin of witwatersrand gold and evidence of eolian signatures toroidal shaped gold particles extracted from a sample of the basal reef orebody in the welkom gold field that contained crossbedded gold have been recognized in thin section demonstrating that the form in question is not an artifact of extraction but is a genuine detrital form similar forms have been recovered in great abundance from an eolian paleosurface in yakutia and from a recent storm beach placer in new zealand indicating the likelihood that eolian processes produced these forms this has been confirmed by means of wind tunnel experiments toroids in various states of preservation have been extracted from a number of other witwatersrand paleoplacers where there is an association with ventifacts indicating that wind deflation might have played a role in concentrating gold from placer sand deposits

crustal deformation at the oblique spreading reykjanes peninsula sw iceland gps measurements from 1993 to 1998 in 1993 and 1998 a 38 point gps network was surveyed on the reykjanes peninsula sw iceland according to the nuvel 1a plate motion model the spreading rate of the north american and eurasian plates in sw iceland is 18 9 0 5 mm yr toward n102 7 1 1 e highly oblique to the plate boundary instead of oblique spreading the measurements indicate left lateral shear strain accumulation parallel to the reykjanes peninsula seismic zone n76 e at the rate of about eyx 0 2 strain yr tensor shear strain subsidence is generally observed toward the seismic zone a local maximum subsidence of 60 mm was measured in the svartsengi geothermal area subsidence in this area has previously been detected with geodetic measurements and is considered a result of geothermal usage expansion was observed in the area of the hengill triple junction this is probably a result of magma accumulation beneath mount hromundartindur as previous seismic and geodetic measurements have indicated using a simple screw dislocation model we fit the majority of the data assuming a left lateral shear zone at depth along the seismic zone we estimate locking depth of 6 5 km and deep slip rate of 16 5 mm yr the maximum left lateral displacement predicted by the screw dislocation model 11 85 0 06 mm yr is consistent with the observed value of 11 9 0 5 mm yr if the hengill area is excluded little extension is observed across the peninsula the discrepancy between the nuvel 1a oblique spreading and the observed transcurrent motion is thought to be caused by lack of magma intrusion into the crust during this time period

propagation of brittle failure triggered by magma in iceland abstract the architectures of normal faults at a divergent plate boundary in iceland are examined by combining surface fault observations with cross sectional studies at different structural levels to constrain a model of failure propagation the structures of holocene faults defining graben are analyzed to characterize the upper most parts of ruptures the shapes of faults resulting from growth and interaction of separate segments are used to better understand failure propagation inferred to occur in the intervening stages of displacement accumulation and lateral propagation pleistocene faults in volcanic sequences exhumed from 800 to 1000 m are analyzed to characterize deeper portions of failure that occurred beneath the central rift zone tertiary dikes exhumed from depths of 1300 1500 m are studied to infer how magma controls the failure initiation field studies in combination with a literature review indicate that the planar ruptures are likely to initiate at depth as magma filled vertical fractures and lengthen upward and laterally as failures propagate to higher crustal levels they are likely to change into inclined normal faults at near surface levels faults link with cooling joints and dilational fractures propagating downward from the surface it is suggested that the inferred stages of fault propagation are characteristic for normal faults developed at spreading ridges

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

overlapping faults intrabasin highs and the growth of normal faults normal fault systems bounding extensional basins are typically adjoined by a series of subbasins separated by intrabasin highs the strata within these basins form syndepositional anticlines and synclines whose axes are transverse to the strike of the main bounding fault one possible explanation for these intrabasin highs is that they result from persistent along strike deficits in fault displacement such deficits are incompatible with scaling relationships observed between fault displacement and length based on large populations of faults we present data from active normal faults within the basin and range province and from inactive normal faults of the newark basin of eastern north america demonstrating a clear correlation between the along strike position of overlapping splay faults and the location of intrabasin highs as well as syndepositional transverse folds summed displacements for all faults within an intrabasin high are comparable to the displacements on faults bounding flanking subbasins o

propagation of blind normal faults to the surface in basaltic sequences insights from 2d discrete element modelling abstract a discrete element model is used to investigate the progressive deformation of a thin basaltic cover overlying a pre existing blind normal fault as it propagates to the surface the cover materials representing basalt are homogeneous strong and display elastic brittle material behaviour cover deformation is seen to evolve through a series of distinct stages initial displacement on the underlying fault produces a very gentle monoclinal flexure with continued displacement open fractures develop at the monocline surface and propagate downwards whilst the deeper fault propagates upwards simultaneously a series of fractures in the future hanging wall of the main fault develop in the upper part of the cover the monoclinal flexure is then cut by these structures producing a surficial fault and fracture bounded wedge finally a prominent surface fracture and the upward propagating fault link cutting the entire cover sequence this fault is dilatant in the upper c 100 m of the cover has a significant surface aperture and forms a prominent fault scarp many of the key model results are strikingly similar to those seen in natural settings and emphasise that the occurrence of dilatant faults open fractures and cavities caves in extensional settings is not necessarily restricted to the very shallow section but can extend to several hundred metres depth therefore the results have implications for permeability and fluid flow in such settings comparison is also made with a weak cover experiment using granular materials with no cohesion or tensile strength similar to the dry sand used in many analogue modelling studies

magnetotelluric observations across the juan de fuca subduction system in the emslab project a magnetotelluric mt transect has been obtained near latitude 45 n from the active juan de fuca spreading center across the subduction zone and cascades volcanic arc and into the back arc deschutes basin region this paper presents the mt data set and describes its major characteristics as they pertain to the resistivity of the subduction system in addition we discuss the measurement and processing procedures employed as well as important concerns in data interpretation broadband audiomagnetotelluric amt mt soundings approx 0 01 500 s period were collected on land with considerable redundancy in site location and from which 39 sites were selected which constrain upper crustal heterogeneity but sense also into the upper mantle fifteen long period mt recordings about 50 10 000 s on land confirm the broadband responses in their common period range and extend the depths of exploration to hundreds of kilometers on the juan de fuca plate offshore 33 out of 39 sea floor instruments at 19 locations gave good results of these locations five magnetotelluric soundings plus two additional geomagnetic variation sites covering the period range 200 105 s approximately constitute the ocean bottom segment of our profile the feature of the land observations which probably relates most closely to the subduction process is a peak in the impedance phase of the transverse magnetic mode around 30 50 s period this phase anomaly with a corresponding inflection in the apparent resistivity is continuous eastward from the seacoast and ends abruptly at the high cascades it signifies an electrically conductive layer in otherwise resistive lower crust or upper mantle with the layer conductance decreasing eastward from the coast to a minimum under the coast range but increasing suddenly to the east of the central willamette basin the higher conductance to the east is corroborated by the vertical magnetic field transfer function whose real component shows negative values in the period range 100 1000 s over the same distance the transverse electric mode apparent resistivity and phase on the land display a variety of three dimensional effects which make their interpretation difficult conversely both modes of the ocean floor soundings exhibit a smooth progression laterally from the coastal area to the spreading ridge indicating that the measurements here are reflecting primarily the large scale tectonic structures of interest and are little disturbed by small near surface inhomogeneities the impedance data near the ridge are strongly suggestive of a low resistivity asthenosphere beneath resistive juan de fuca plate lithosphere approaching the coastline to the east both impedance and vertical magnetic field responses appear increasingly affected by a thick wedge of deposited and accreted sediments and by the thinning of the seawater

geometry formation and development of tectonic fractures on the reykjanes peninsula southwest iceland abstract a study was made of 903 fractures most of which are of postglacial age on the reykjanes peninsula some 120 fractures belonging to the vogar fissure swarm he completely within the same postglacial lava flow and were studied in great detail for the vogar fractures 75 are pure tension fractures and 25 are normal faults the average length of the vogar fractures is 611 m the average width is 0 6 m the average spacing measured perpendicular to the average strike n54e of fractures is 456 m the estimated average depth is of the order of several hundreds of meters and the average throw of the faults is 2 3 m all major fractures on the peninsula appear to have formed by coalescence of smaller fractures as a result of absolute tensile stress the fractures group into four fissure swarms each being a part of a volcanic system which it is proposed is fed by an elongated partially molten magma reservoir located at a depth of about 8 km beneath the peninsula it is suggested that a slight magmatic pressure increase in these reservoirs and resulting uplift and tensile stress at the surface is the main cause of postglacial fracture formation

spatial and temporal variations in growth rates along active normal fault systems an example from the lazio abruzzo apennines central italy abstract the geometry kinematics and rates of active extension in lazio abruzzo italian apennines have been measured in order to gain a better understanding of the spatial and temporal variations in fault growth rates and seismic hazards associated with active normal fault systems we present fault map traces throws throw rates and slip directions for 17 parallel en echelon or end on active normal faults whose 20 40 km lengths combine to form a soft linked fault array ca 155 km in length and ca 55 km across strike throw rates derived from observations of faulted late glacial features and holocene soils show that both maximum throw rates and throw rate gradients are greater on centrally located faults along the strike of the array total throws and throw gradients show similar spatial variations but with weaker relationships with distance along strike when summed across strike throw rates are increasingly high towards the centre of the array relative to summed throws we interpret the above to suggest that throw rates have changed in the recent past ca 0 7 ma from spatially random fault growth rates initiating at 2 5 3 3 ma to growth rates that are greater on centrally located faults we interpret this as evidence for fault interaction producing throw rate variations that drive throw profile readjustment on these crustal scale soft linked faults the results are used to discuss seismic hazards in the region which are quantified in a second paper in this issue

verification of five magnetotelluric systems in the mini emslab experiment given the degree of complexity of modern magnetotelluric mt instrumentation comparison of the total performance for two or more systems is an important verification test this paper compares the processed data from five mt systems which were designed and constructed separately and which employ different electrode types electrode separations magnetometers and methods of signal processing the comparison shows that there is a high degree of agreement among the data from the different systems the study also demonstrates the compatibility and reliability of the mt systems employed as part of emslab juan de fuca electromagnetic sounding of the lithosphere and asthenosphere beneath the juan de fuca plate this project proposed by a consortium of institutions involves not only magnetotellurics studies but also studies of magnetic variation on land and on the sea bottom the project calls for the real time mt systems to occupy stations along segments of a profile in oregon a composite profile will be created from the segments prior to commencing the main mt profiling phase one week was set aside in august 1984 for all groups to record and process mt data sequentially at six sites in diverse geologic terrains this experiment was called mini emslab

surface effects of triggered fault slip on reykjanes peninsula sw iceland three large earthquakes mw 4 5 were triggered within 5 min 85 km west of a mw 6 5 earthquake in the south iceland seismic zone sisz we report on surface effects of these triggered earthquakes which include fresh rupture widespread rockfall disrupted rockslides and block slides field data confirm that the earthquakes occurred along n striking right lateral strike slip faults field data also support the conclusion from modeling of insar data that deformation from the second triggered event was more significant than for the other two a major hydrological effect was the draining of water through an open fissure on a lake bed lowering the lake level by greater than 4 m field relationships suggest that a component of aseismic slip could have been facilitated by water draining into the fault zone d 2003 elsevier science b v all rights reserved