late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia part ii kurnalpi terrane abstract the penny dam belches yilgangi white hills granny royal lucky brock and wallaby supersequences of the kurnalpi terrane crop out in synclines that are the erosional remnants of basins that developed as linear zones of subsidence between approximately north trending compartment bounding faults dominant sedimentary facies range from gravel rich and sand rich concentrated density flow deposits to thickly and thinly bedded bouma type turbidites background deposits are black shales or laminated green mudstones and banded iron formation is preserved in some condensed sections periods of non deposition are recorded by bedded chert which is the product of early diagenetic sea floor silicification the basins were separate deep marine canyon to linear fan systems that were linked laterally and distally by mud rich deposits there is only minor evidence for fault anchored canyon margin slope aprons basin development occurred during two stages separated by a period of deformation uplift and erosion sparse palaeocurrents establish sediment dispersal was to the south or southeast indicating that uplift to the north or northwest controlled palaeoslopes stacking of lithofacies is aggradational and consistent with deposition in fault bound basins tectonostratigraphic equivalence with siliciclastic supersequences of the kalgoorlie and gindalbie terranes is established by common depositional systems stratigraphic styles and stratigraphic responses to tectonic forcing consequently the siliciclastic supersequences represent overlap or late stage basins that tied together arc related terranes following terrane amalgamation similar depositional facies aggradational styles of facies stacking and two stage development characterise the syncollisional plio pleistocene coastal range basin of taiwan modern analogues are the deep marine southern longitudinal and taitung troughs off southeastern taiwan that are being filled by synorogenic sediments derived from the taiwan orogen

magnetotelluric imaging of upper crustal convection plumes beneath the taupo volcanic zone new zealand 1 broadband mt magnetotelluric data were recorded that form an array of measurements at the south eastern margin of the tvz taupo volcanic zone in the central north island of new zealand these array data are used to investigate mechanisms by which the tvz s extraordinarily high heat flux is transported to the surface taken together with seismological data these mt data show compelling evidence that support a model of hydrothermal convection within the brittle upper 6 7 km part of the crust both 2 d and 3 d inversion models of these mt data show vertical low resistivity zones that connect surface geothermal fields to an inferred magmatic heat source that lies below the brittle ductile transition

crustal deformation of the eastern tibetan plateau revealed by magnetotelluric imaging the ongoing collision of the indian and asian continents has created the himalaya and tibetan plateau through a range of deformation processes these include crustal thickening detachment of the lower lithosphere from the plate delamination and flow in a weakened lower crust 1 6 debate continues as to which of these processes are most significant 7 in eastern tibet large scale motion of the surface occurs but the nature of deformation at depth remains unresolved a large scale crustal flow channel has been proposed as an explanation for regional uplift in eastern tibet 6 8 9 but existing geophysical data 10 11 do not constrain the pattern of flow magnetotellurics uses naturally occurring electromagnetic waves to image the earth s subsurface here we present magnetotelluric data that image two major zones or channels of high electrical conductivity at a depth of 20 40 km the channels extend horizontally more than 800 km from the tibetan plateau into southwest china the electrical properties of the channels imply an elevated fluid content consistent with a weak crust 12 13 that permits flow on a geological timescale these findings support the hypothesis that crustal flow can occur in orogenic belts and contribute to uplift of plateaux our results reveal the previously unknown complexities of these patterns of crustal flow many previous studies of the indiaasia

continental transform tectonics great slave lake shear zone ca 1 9 ga northwest canada great slave lake shear zone gslsz is a northeast trending dextral mylonite zone across which the coeval north trending thelon and taltson magmatic zones on the west margin of the churchill province are offset 300 700 km it is proposed that the gslsz is a continental transform structure related to oblique collisional indentation of the active margin of the churchill province by the microcontinental slave province this accounts for geologic and geophysical differences between the thelon and taltson zones and explains the localization of a tibetan type queen maud uplift north of the gslsz and the abrupt loss of slip on the gslsz east of the uplift it also provides a new model for the athapuscow aulacogen analogous to the modern bay of bengal indo burman ranges sagaing transform andaman sea situation

terrane amalgamation in the eastern goldfields superterrane yilgarn craton evidence from tectonostratigraphic studies of the laverton greenstone belt abstract tectonostratigraphic evolution of the laverton greenstone belt involves amalgamation of structural relics of sedimentary basins and continental fragments in the burtville terrane to the east prior to their accretion to the kurnalpi terrane to the west the burtville terrane records sedimentation and volcanism in a continental setting between 2758 ma and 2715 ma the kurnalpi terrane is dominated by 2715 2700 ma andesitic volcaniclastic rocks with erosional remnants of uplift of the margaret dome a granite gneiss cored batholith north of the laverton domain is interpreted to have been the main driving force for rapid deposition and burial of conglomeratic and turbiditic siliciclastic sequences in the lake carey basin a switch from submarine canyon deposits to deep water turbiditic rocks with time suggests extension and basin formation was initially driven by the uplift of the dome however basin development switched to a flexural phase with increasing sediment load and ongoing uplift to the north the 2663 ma maximum depositional age for the granny smith sequence indicates that active sedimentation may have been occurring contemporaneous with collisional orogeny the collisional orogeny comprises four main episodes of compressive deformation starting after either 2675 ma or 2665 ma and ending at 2650 ma d 1 involved development of n s upright to inclined km scale folds and axial planar foliation and inversion of pre collision extensional faults as reverse faults during east west directed compression d 2 involved reactivation of d 1 reverse faults as transcurrent faults and development of nw se directed dm scale folds with associated axial planar foliation that overprints the early n s foliation d 2 occurred under a ne sw directed compressive stress field to the west the celia fault records d 2 as its first deformation suggesting a westward migrating deformation front the celia fault zone records its second deformation episode d 3 as sinistral reactivation of the fault zone and locally influenced lode gold development d 3 occurred during wnw ese directed maximum compression the final compressive deformation event d 4 involved overprinting of the d 3 shear fabric by conjugate arrays of kink bands in the celia fault zone during a clockwise rotation of the compressive stress field into a nnw sse orientation the tectonic process responsible for such a rapid uplift and deposition of thick synorogenic sediments lateral to the deformation front and penecontemporaneous with collisional orogeny invites comparisons with modern arc related settings the present suture zone between the indo australian plate and the pacific plate in new guinea records active deposition of fluvial sediments laterally along the accretion zone and in thick deep water fans onto the bismarck sea floor

area selection for diamond exploration using deep probing electromagnetic surveying abstract previously proposed methods of area selection for diamond prospective regions have predominantly relied on till geochemistry airborne geophysics and or an appraisal of tectonic setting herein we suggest that a novel deep probing geophysical technique electromagnetic studies using the natural source magnetotelluric mt method can contribute to such an activity essentially diamondiferous regions must have 1 old lithosphere 2 thick lithosphere and 3 lithosphere that contains high concentrations of carbon deep probing mt studies are able to address all three of these the second and the third of these can be accomplished independently using mt but for the first the geometries produced from modelling the mt observations must be interpreted with appropriate interaction with geologists geochemists and other geophysicists examples are given from the slave and superior cratons in north america with a brief mention of an area of the rae craton and general speculations about possible diamondiferous regions

preliminary results from a geophysical study across a modern continent continent collisional plate boundary the southern alps new zealand abstract the southern alps of south island new zealand is a young transpressive continental orogen exhibiting high uplift rates and rapid transcurrent movement a joint us nz geophysical study of this orogen was carried out in late 1995 and early 1996 to derive a three dimensional model of the deformation the measurements undertaken include active source and passive seismology magnetotelluric and electrical studies and petrophysics preliminary models for the active source seismic measurements across south island confirm in general terms a thickened crust under the southern alps a high velocity lower crustal layer and a major crustal discontinuity associated with the alpine fault the anisotropy in physical properties of the rocks of the plate boundary zone is clearly demonstrated in the preliminary results of laboratory seismic velocity measurements shear wave splitting and resistivity the mid crust under the southern alps coincides with a major electrical conductivity high which possibly corresponds to fluid in the crust the top lies at about 15 km close to the base of shallow seismicity east of the alpine fault offshore the marine reflection data have consistently imaged a reflective lower crust adjacent to south island these data are showing complex structure particularly off western and southeastern south island the complexity in structure high quality data and consistency in results from several techniques indicates that the south island experiment will contribute significantly to our knowledge of transpressive plate boundaries in particular and the continental lithosphere in general

detrital zircon age spectra for late archaean synorogenic basins of the eastern goldfields superterrane western australia abstract the kalgoorlie gindalbie kurnalpi duketon and burtville terranes were amalgamated by 2658 ma into the eastern goldfields superterrane which was later accreted cordilleran style onto the foreland of a pre existing continent late stage synorogenic basins developed within the eastern goldfields superterrane during the early stage of accretion the basins are similar to all late stage basins of archaean provinces developing late in the tectonic cycle in a strike slip regime they developed in two stages during the gap in granite emplacement between 2658 ma and 2648 ma basin analysis suggests that the bulk of sediments was derived from rocks readily identifiable today within the superterrane consistent with the provenance style of all late stage basins sediment derivation from the foreland occurred only during stage ii and is recorded only from basins proximal to the suture detrital zircon geochronology has corroborated those interpretations eleven new shrimp u pb detrital zircon datasets and 14 previously published datasets indicate that the bulk of zircon bearing sediment was derived from the 2715 ma to 2658 ma high ca ttg granites and volcanic rocks that define the terranes moreover primary modes in the detrital zircon age spectra of 2680 3 ma 2673 ma 2666 3 ma and 2660 3 ma coincide with the peaks of emplacement of high ca granites provenance from across the suture has also been corroborated with detrital zircon age spectra of the kurrawang basin a stage ii basin close to the suture having no match to rocks of the eastern goldfields superterrane defining modes of 2730 ma and 3000 ma some as old as 3750 ma and 3540 ma can be matched to an 2735 ma calc alkaline volcanic province and quartzites with 4350 3130 ma detrital zircon in the foreland the study shows that detrital zircon age spectra can be powerful supporters of tectonic analysis but for the study presented here their interpretation has been greatly assisted by prior basin analysis and a considerable geochronological database of source rock equivalents

fluid generation and pathways beneath an active compressional orogen the new zealand southern alps inferred from magnetotelluric data 1 forty one wideband magnetotelluric mt soundings were collected in a 150 km long transect across the southern alps of the central south island of new zealand an active compressional orogen decomposed mt impedance tensors vertical magnetic field relations and reconnaissance soundings at two locations off line imply an approximately two dimensional geometry here with average regional geoelectric strike of n40 e similar to surface geologic trends two independent two dimensional inversion algorithms were applied to the mt data and both imply a concave upward u shaped middle to lower crustal conductive zone beneath the west central portion of the island the average conductivity of this zone in the strike direction appears to be much higher than that required across strike and may represent anisotropy or along strike conductive strands narrower than the transverse magnetic cross strike mode mt data can resolve the deep crustal conductor under the southern alps is interpreted to represent mainly a volume of fluids arising from prograde metamorphism within a thickening crust fluid interconnection and electrical conduction are promoted by shear deformation the conductor rises northwestward toward the trace of the alpine fault but attains a near vertical configuration at a depth of 10 km and reaches close to the surface 5 10 km inland of the fault trace itself the transition to vertical orientation at this depth is interpreted to occur as fluids ascend across the brittle ductile transition in uplifting schist and approach the surface through induced hydrofractures the high grade schist becomes resistive after depletion of fluids and continues to extrude toward the alpine fault shallow extensions of the deep high conductivity are coincident with modern hydrothermal veining and gold mineralization interpreted to be of deep crustal provenance to the southeast high conductivity also reaches the surface coincident with a major back thrust fault zone of the doubly vergent southern alps orogen which also exhibits evidence for expulsion of high temperature fluids the higher conductivity inferred along strike possible anisotropy could reflect more efficient fluid interconnection in this higher strain direction as well as possible contributions by sheared fluid deposited graphite conductivity of the uppermost mantle of the south island is low consistent with advection of cold mantle lithosphere into the underlying asthenosphere as suggested by p wave delay studies

tectonic entrapment and its role in the evolution of continental lithosphere an example from the precambrian of western canada new geophysical data acquired over the buried crystalline basement of western canada provide constraints on the history of tectonic assembly of the western canadian shield in the interval 1 75 1 85 ga specifically these data provide new perspectives on the evolution of an archean continental fragment heame province that was trapped in a tectonic vise between coeval orogenic belts that dipped beneath the hearne province the trans hudson orogen developed along the southeastern margin of the hearne province as a series of ocean floor oceanic arc and arc marginal basins were telescoped and thrust obliquely beneath the hearne along the northwest edge of the hearne collapse and subduction of a narrow marginal basin now marked by the subsurface extension of the snowbird tectonic zone led to formation of magmatic arc and collision of older proterozoic terranes the hearne province itself is characterized by regional granulite grade metamorphism and evidence of extensive and pervasive partial melting of the crust the internal character of the hearne province seen on crustal seismic reflection profiles is that of a crustal scale structural fan with reflection fabrics that verge toward the bounding orogens the deformation of the hearne is predominantly of paleoproterozoic age and constitutes a thorough reworking of this formerly archean crustal domain over a distance of more than 600 km across strike entrapment and thermal weakening of the hearne resulted from mechanical coupling of inferred buoyant subduction collision zones and removal of or modification of archean lithospheric mantle that may have originally formed the keel to the hearne long period magnetotelluric profiles show that anomalously conductive mantle lithosphere underlies the present day hearne province which is attributed to metasomatic modification of the subcontinental lithosphere following collisional thickening and delamination convective removal of thickened lithosphere in the proterozoic tectonic entrapment as illustrated by the proterozoic structural and thermal evolution of the hearne province crust and subcontinental mantle may be an example of the lithospheric consequences of opposing collisional polarity during assembly of continents

deep electrical structure of northern alberta canada implications for diamond exploration geophysical studies of upper mantle structure can provide constraints on diamond formation teleseismic and magnetotelluric data can be used in diamond exploration by mapping the depth of the lithosphere asthenosphere boundary studies in the central slave craton and at fort a la corne have detected conductors in the lithospheric mantle close to or beneath diamondiferous kimberlites graphite can potentially explain the enhanced conductivity and may imply the presence of diamonds at greater depth petrologic arguments suggest that the shallow lithospheric mantle may be too oxidized to contain graphite other diamond bearing regions show no upper mantle conductor suggesting that the correlation with diamondiferous kimberlites is not universal the buffalo head hills in alberta host diamondiferous kimberlites in a proterozoic terrane and may have formed in a subduction zone setting long period magnetotelluric data were used to investigate the upper mantle resistivity structure of this region magnetotell

archean neptunian fissures and early history of the destor porcupine fault zone timmins ontario canada neptunian dikes and fissures are intimately associated with a minor archean sedimentary basin near timmins ontario in the southwestern part of abitibi subprovince of the canadian shield these structures are associated with the late archean timiskaming unconformity and were formed by clastic sedimentation in fissures opened by the reactivation and dilation of basement faults along a major crustal shear a pull apart origin is indicated for the sedimentary basin by published township maps and the underground geology of the dome gold mine the neptunian dikes and fissures are discussed in relation to previous stratigraphic and tectonic interpretations of the timmins area

age constraints on recycled crustal and supracrustal sources of archaean metasedimentary sequences eastern goldfields province western australia evidence from shrimp zircon dating abstract shrimp u pb zircon dating of sedimentary volcanic and subvolcanic intrusive rocks from unconformity bounded sequences in the eastern goldfields province defines maximum depositional ages for those sequences the kambalda sequence which records a backarc basin was deposited prior to 2700 ma and from at least 2715 ma the spargoville sequence which records an arc adjacent volcano bound basin was deposited between 2700 and 2683 ma the kalgoorlie sequence which records an intra arc rift basin comprises two tectonic stages with depositional ages of 2681 2670 and 2661 2655 ma deposition of the spargoville and kalgoorlie sequences overlapped with the intrusion of granitoids and subvolcanic porphyries thereby indicating that those granitoids were emplaced during crustal extension younger submarine fan kurrawang sequence and braided fluvial merougil and jones creek sequences deposits of a remnant ocean basin were deposited at when coupled to palaeocurrent data provenance dating of detrital zircon suites fingerprints southeast oriented amalgamation of felsic detritus from multiple sources within longitudinal depositional systems and longitudinal reworking of felsic detritus from single sourced transverse systems most felsic detritus was recycled from coeval volcanoplutonic arcs and the orogen to the remnant ocean basin sequences with zircon ages ranging from 2730 to 2660 ma older felsic sources with age ranges of 3570 3130 3030 2900 2870 2770 and 2760 ma could have been structural enclaves within the volcanoplutonic arcs or orogen but were more likely within a distant cratonic basement when coupled with stratigraphic data it is concluded that detrital zircon age distributions preserve evidence for the sedimentary recycling of coeval volcanoplutonic arc or orogenic sources uplifted basin floor and dissected arc successions and cratonic basement virtually all source terrains have apparently vanished presumably through the combination of tectonic and sedimentary recycling and tectonic severance increased use of detrital zircon suites from archaean sedimentary sequences is advocated in order to quantify the balance between sedimentary recycling of felsic rocks from contemporaneous volcanoplutonic sources which themselves document crustal growth in arc environments and from relict volcanoplutonic and crustal basement sources which document the degree of crustal recycling

the wangkathaa orogeny an example of episodic regional d2 in the late archaean eastern goldfields province western australia abstract the eastern goldfields province egp is a late archaean orogenic belt located on the eastern margin of the yilgarn craton the egp is characterised by a prominent nnw trending tectonic grain developed largely during regional e w oriented shortening a simplistic sequence of largely progressive compressional deformation events d 1 to d 4 has been used by previous workers to explain the development of the egp regional d 2 was thought to be responsible for most of the finite strain and therefore a reliable marker event for correlating deformation histories across the region a re examination of the deformation history has shown that d 2 kalgoorlie orogen of weinberg et al precambrian res 120 2003 219 239 was not a single progressive event rather it was episodic and involved several stages of switching of the tectonic mode compression extension switching between 2665 and 2655 ma the sequence of events during d 2 was an early stage of e w shortening d 2a followed by extension with basin formation and normal fault movement d 2e and finally a second stage of e w compression d 2b we introduce this sequence of events as the wangkathaa orogeny more complex in tectonics and broader in geographical scope than the kalgoorlie orogen features of the wangkathaa orogeny are well illustrated by the geometry and geological relationships that occur in the kurnalpi terrane welcome well area and the kalgoorlie terrane ora banda kambalda and the boorara domains of the central and southern egp available geochronology indicates that this sequence of events was likely to have been diachronous across the entire egp and the eastern southern cross province of the yilgarn craton recognition of these separate episodic stages of e w oriented compression and extension is important in our understanding the tectonic evolution of the egp switching tectonic mode between compression and extension will change the state of mean stress in a rock mass e g from net dilation to net constriction and vice versa and is an effective method in transporting and trapping mineral rich fluids the paradox of the different timings established for d 2 across the province are explained by the wangkathaa orogeny especially if it were diachronous as different workers dated different phases of an episodic orogeny

electric lithosphere of the slave craton the archean slave craton in northwestern canada is an ideal natural laboratory for investigating lithosphere formation and evolution and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes three deep probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures the magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity 30 m at depths of 80 100 km and is interpreted to be due to dissolved hydrogen or carbon in graphite form this geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer the tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630 2620 ma

correlation between the depth to the lower crustal high conductive layer and heat flow in the canadian cordillera abstract in much of the southern canadian cordillera the top of the lower crustal electrical conductor coexists with the 450 c isotherm a statistical correlation between the depth to the top of the lower crustal electrical conductor heat flow and 450 c isotherm are shown based on mt and heat flow results in southern british columbia canada the depth to the lower crustal conductor and the depth to the 450 c isotherm vary from 20 35 km in western b c insular belt coast plutonic complex intermontane belt to less than 10 km in the eastern part of the canadian cordillera omineca belt the largest depths are in the insular belt 35 km the coexistence of the top of the lower crustal electrical conductor top of the zone of lower crustal seismic reflectors and 450 c isotherm supports the hypothesis that the 450 c isotherm defines the top of the zone over which interconnected fluid can exist in the lower crust

anisotropy and phase splits in magnetotellurics abstract several recent papers suggesting that the electrical conductivity of the lower crust and upper mantle are anisotropic have been based upon magnetotelluric mt observations of phase splits the mt analogue of shear wave splitting the emphasis on the phase rather than the amplitude response results from the distorting effect of conductivity variations in the overlying crust these variations distort the amplitude response from deeper levels and make inferences based on the amplitude difficult at the long periods needed to probe the conductivity at upper mantle depths the mt phase response a tensor is negligibly affected by small scale conductivity variations in the overlying crust however the mt phase response of a half space with a uniform but anisotropic conductivity is independent of polarization direction and no phase split occurs using simple anisotropic layered models we demonstrate that a phase split is produced by the conductivity change at the interface between the isotropic and anisotropic layers the maximum phase difference reflecting the conductivity contrast gradient between the principal conductivities of the anisotropic layer and the isotropic layer s conductivity in the general case where none of the principal axes of the anisotropic conductivity layer lie in the plane of the surface the orientation at which the maximum phase difference occurs i e the principal axis of the phase tensor is determined by the horizontal projection of conductivity tensor s ellipsoid in such cases the orientation of the maximum phase split will not be aligned parallel to any of the conductivity tensor s principal axes mt phase splits are a consequence of spatial differences or gradients in conductivity not the intrinsic bulk properties of the conductivity tensor itself and in this respect are fundamentally different from shear wave splitting

internal structure of the precordilleran fault system chile insights from structural and geophysical observations abstract geological field work and audiomagnetotelluric data from two profiles crossing the west fissure zone in northern chile were used to describe the geometry and structure of the fracture damaged zone surrounding this strike slip fault system the w e profiles show that the width of the west fissure zone based on the fracture density distribution derived from aerial photographs is 4000 m on profile a and 7000 m on profile b the estimated widths correspond roughly to the region where the fault is kinematically uniform the ratio of the fault width to the fault length ranges from 0 024 to 0 041 this ratio compares favourably with the measured ratio of small metre scale natural faults audio frequency magnetotelluric imaging shows low electrical resistivity zones resistivity of 5 30 m coincident with the mapped surface traces of the fault however these zones are very narrow width about 100 m and only 50 m profile a and 200 m profile b deep respectively on profile b the shallow high conductive zone is underlain by a resistive zone 1000 m the conductivity enhancement in these shallow and narrow zones contrasts with the broad process zone revealed by structural analysis we assume that the conductivity enhancement is due to meteoric water entering a zone of ruptured rocks along the fault trace fault core at present we have no indication for seismic slip along the investigated segments of the west fissure zone fault models have shown that during aseismic periods of fault evolution fault healing i e strength recovery due to compaction and cementation is active fluids and fluid transport which are probably responsible for enhanced conductivity are confined to the remaining fractures of the fault core at shallow depth

electromagnetic images of crustal structures in southern and central canadian cordillera data from more than 400 magnetotelluric soundings made since the early 1980 s in the canadian cordillera over a 300 000 km2 area between 49 and 53 5 n are used to image qualitatively regional three dimensional crustal variation in electrical conductivity by means of phase maps phase frequency sections and maps of resistivity at depth two hundred of the soundings were acquired as part of lithoprobe southern canadian cordillera transect activities and their locations were coordinated with the seismic reflection and refraction experiments the lower crust has a generally pervasive low resistivity 1 100 m throughout the cordillera west of the foreland belt within this canadian cordilleran regional conductor the magnetotelluric data reveal both two dimensional structures with highest conductivities along the coast belt and omineca belt and three dimensional variation along geological strike this conductor mapped over a volume in excess of 106 km3 is most probably caused by fluids saline wat

ten turbidite myths abstract during the past 50 years the turbidite paradigm has promoted many myths related to deep water turbidite deposition john e sanders 1926 1999 a pioneering process sedimentologist first uncovered many of these turbidite myths this paper provides a reality check by undoing 10 of these turbidite myths myth no 1 turbidity currents are non turbulent flows with multiple sediment support mechanisms reality turbidity currents are turbulent flows in which turbulence is the principal sediment support mechanism myth no 2 turbidites are deposits of debris flows grain flows fluidized flows and turbidity currents reality turbidites are the exclusive deposits of turbidity currents myth no 3 turbidity currents are high velocity flows and therefore they elude documentation reality turbidity currents operate under a wide range of velocity conditions myth no 4 high density turbidity currents are true turbidity currents reality ph h kuenen 1950 introduced the concept of turbidity currents of high density based on experimental debris flows not turbidity currents high density turbidity currents are sandy debris flows myth no 5 slurry flows are high density turbidity currents reality slurry flows are debris flows myth no 6 flute structures are indicative of turbidite deposition reality flute structures are indicative only of flow erosion not deposition myth no 7 normal grading is a product of multiple depositional events reality normal grading is the product of a single depositional event myth no 8 cross bedding is a product of turbidity currents reality cross bedding is a product of traction deposition from bottom currents myth no 9 turbidite facies models are useful tools for interpreting deposits of turbidity currents reality a reexamination of the annot sandstone in se france which served as the basis for developing the first turbidite facies model suggests a complex depositional origin by plastic flows and bottom currents myth no 10 turbidite facies can be interpreted using seismic facies and geometries reality individual turbidity current depositional events commonly centimeters to decimeters in thickness cannot be resolved in seismic data all turbidite myths promote falsehood and should be abandoned

great slave lake shear zone nw canada mylonitic record of early proterozoic continental convergence collision and indentation abstract great slave lake shear zone in the nw canadian shield is an excellent example of the kinematic magmatic and thermal evolution of a crustal scale shear zone associated with a collisional continental boundary it is a 25 km wide corridor of granulite to lower greenschist facies mylonites and cataclastic fault rocks developed in the deep seated parts of an early proterozoic 2 0 1 9 ga magmatic arc constructed on the upper rae plate at the contact between the archean slave and rae continents the rocks of great slave lake shear zone the thelon magmatic arc and the taltson magmatic zone are all components of the same magmatic arc but their geological histories reflect different aspects of the continental interaction

a magnetotelluric study of the damara belt in namibia 1 regional scale conductivity anomalies abstract the namibian margin is dominated by the late proterozoic to early cambrian fold belts of the damara orogen which wrap around and separate the congo and kalahari cratons this mosaic of relatively soft fold belts and hard cratons apparently controlled the path for the opening of the south atlantic in the early cretaceous the continents split along the coast parallel fold belts of the damara orogen while the inland fold belt damara belt was effected by extension and widespread igneous intrusion but never developed to the rift stage this paper is concerned with the interpretation of magnetotelluric mt data along a 200 km nw se profile across the damara belt in nw namibia the regional two dimensional electrical resistivity model and the induction vector data exhibit three distinctive zones i a generally very resistive upper crust which is typical for the granites and metasediments of the damara belt ii two subvertical conductors in upper to mid crustal levels which correlate with major tectonic zone boundaries and iii a highly conductive middle to lower crust in the southern part of the profile the geometry of the conductive structures could reflect a regional shear system in which upper crustal listric faults pass into a detachment zone in the middle crust we interpret the high electrical conductivity in terms of graphite or other forms of mineralization enrichment along the shear planes this zone of crustal weakness may have originated in pre damara times and had probably experienced several episodes of crustal reactivation before the intrusion of basaltic dike swarms during the cretaceous rifting and magmatism associated with the opening of the south atlantic

the world class wallaby gold deposit laverton western australia an orogenic style overprint on a magmatic hydrothermal magnetite calcite alteration pipe gold mineralisation at the wallaby gold deposit is hosted by a 1 200 m thick mafic conglomerate the conglomerate is intruded by an apparently comagmatic alkaline dyke suite displaying increasing fractionation through mafic monzonite monzonite syenite syenite porphyry to late stage carbonatite in the mine area a pipe shaped zone of actinolite magnetite epidote calcite amec alteration overprints the conglomerate gold mineralisation associated with dolomite albite quartz pyrite alteration is hosted in a series of sub horizontal structurally controlled zones that are largely confined within the magnetite rich pipe the deposit has a current ore reserve of 2 0 moz au and a total resource of 7 1 moz au

archean polyphase deformation in the lake johnston greenstone belt area implications for the understanding of ore systems of the yilgarn craton abstract a structural and geochronology study on the 2900 ma lake johnston greenstone belt ljgb has defined multiple deformation events quantitative u pb age constraints have been obtained on the ductile deformation and peak metamorphic assemblages the d 1lj event is associated with the development of fold nappes during interpreted nne ssw shortening u pb shrimp dating on monazite from a dyke folded by the d 1lj event gives an age of 2634 1 ma providing an upper age limit on d 1lj this age could have been reset by the d 2lj metamorphic event a major metamorphic thermal event post dates the d 1lj event and is inferred to be the result of elevated temperatures coeval with the emplacement of granitoid intrusions the d 2lj event is associated with peak metamorphic temperatures and reflects nne ssw to ne sw shortening associated with dextral shearing a dyke that intrudes and crosscuts d 2lj fabrics yield a u pb shrimp age on monazite at 2629 1 ma and provides a lower age limit on the d 2lj event d 3lj is defined by crenulation cleavages developed during e w shortening the d 4lj episode is defined by brittle deformation and a series of n to ne trending steep dipping dextral faults formed during craton scale shortening under a ne sw directed far field stress the brittle d 5lj event is characterized by e w shortening with dip slip reverse n s faults the new d 2lj deformation age is younger than previously constrained ages for major deformation in the 2900 ma greenstone belts deformation within the ljgb was broadly coeval with the waning stage of formation of gold deposits in adjacent greenstone belts most peak magmatic and metamorphic events preserved in the greenstone belts have been constrained to ca 2640 2650 ma the presence of such a high temperature deformation event recorded at ca 2630 ma in the ljgb belt has implications for the thermal history fluid formation and geodynamic context of the development of these gold systems the 2630 ma age correlates with late stage and post gold emplacement of pegmatite dykes in surrounding terranes the ljgb is interpreted to have been a mobile belt associated with high grade metamorphism that was inboard west of and active during the formation of major world class gold systems in the eastern goldfields terranes the d 1lj to d 2lj changes in the kinematics of the system i e d 1lj to d 2lj may correlate with a stress switch linked to the influxes of gold bearing fluids in adjacent greenstone belts

along strike variations in the electrical structure of the san andreas fault at parkfield california magnetotelluric exploration has been used to image along strike variations in the electrical resistivity structure of the san andreas fault at parkfield california a low resistivity wedge extending to a depth of several kilometers is continuous over a horizontal distance of 8 km the base of the wedge is coincident with the shallowest microearthquakes a change in the electrical and fluid connection of the san andreas fault with a low resistivity zone in the franciscan formation is observed along the parkfield segment

proterozoic tectonic accretion and growth of western laurentia results from lithoprobe studies in northern alberta the western canadian shield of northern alberta is composed of a series of continental slivers that were accreted to the margin of the archean rae hinterland ca 1 9 2 0 ga preserving a unique record of continental evolution for the interval 2 1 2 3 ga this part of laurentia owes its preservation to the accretionary style of tectonic assembly south of the great slave lake shear zone which contrasts with indentation escape processes that dominate the paleoproterozoic record farther north the buffalo head and chinchaga domains form the central core of this region comprising a collage of ca 2325 2045 ma crustal elements formed on an archean microcontinental edifice and similar age crust is preserved as basement to the taltson magmatic zone the distribution of magmatic ages and inferred collision and subduction zone polarity are used to indicate closure of intervening oceanic basins that led to magmatism and emplacement of continental margin arc and collisional belts that formed from ca 1998 to1900

electrical properties of fine grained olivine evidence for grain boundary transport 1 to investigate the mechanisms responsible for electrical conduction and deduce the relation between grain size and conductivity the electrical conductivity of synthetic polycrystalline forsterite with grain sizes between 1 1 0 4 and 4 7 2 4 m was measured at temperatures up to 1470 c and at 0 1 mpa pressure the complex impedance plots display one clear arc indicating a single dominant conduction mechanism bulk conductivity is inversely proportional to the grain size of the different samples this relation suggests that the electrical conductivity of the samples is controlled by grain boundary diffusion of the charge carriers the apparent activation energy q for diffusion of the charge carriers between 1180 and 1470 c lies between 315 39 and 323 15 kj mol this resembles previous data on grain boundary diffusion of mg in forsterite a geometrical model of less conducting cubic grains and more conducting grain boundaries agrees well with the experimental data this model is applied to predict the conductivity contrast between fine grained shear zones and less deformed regions in the lithosphere upper mantle shear zones are predicted to have 1 5 2 orders of magnitude higher conductivity than less deformed regions in the lithosphere this means grain boundary transport probably has an important role in the existence of anomalously high conductivity zones in the upper mantle and that fine grained shear zones might be detected using magnetotelluric methods

great slave lake shear zone canadian shield reconstructed vertical profile of a crustal scale fault zone abstract the great slave lake shear zone gslsz of the northwestern canadian shield a 25 km wide early proterozoic transcurrent dextral mylonite zone is a type example of a crustal scale fault zone profile with time 1 1 the metamorphic grade decreased from granulite to greenschist facies 2 2 the locus of high strain narrowed and jumped laterally abandoning relatively older mylonites progressively younger mylonites represent both cooler temperatures and lower pressures thus the sequence of narrowing mylonite belts represents a series of progressively shallower seated sections through the gslsz 3 3 deformation evolved through the brittle ductile transition initially producing non dilational penetrative breccias followed first by discrete non dilational faulting and subsequently by dilational quartz stockworks developing crustal scale shear zones grow to an optimum width dictated by the requirement of minimum geologically reasonable strain rates for given conditions of wall rock rheology and displacement rate independent of variations in relative plate motion mature shear zones change width and migrate laterally as a function of inferred thermally activated rheological changes within the wall rocks

geochemistry and sedimentology of a facies transition from limestone to iron formation deposition in the early proterozoic transvaal supergroup south africa this study deals with the geochemistry and sedimentology of a facies transition from interbedded carbonate shale to banded iron formation in the campbellrand carbonate sequence to the overlying kuruman iron formation of the transvaal supergroup in south africa which is approximately 2 3 ga old four major lithologies are 1 limestone and dolomite 2 shale and interbedded shale carbonate 3 siderite rich banded iron formation and 4 iron oxide rich banded iron formation these rocks are unaltered and essentially unmetamorphosed with a maximum metamorphic diagenetic overprint ranging in temperature from 110 degrees to 170 degrees c and with pressures of not more than 2 kbars the oldest rocks are limestones and lesser dolomite with abundant cryptalgalaminae and intraclastic textures interbedded with the limestones and dolomites are carbonaceous shales some of which are unusually ferruginous or pyrite rich these carbonates and shales are overlain by meso and microbanded siderite chert iron formation which grades upward into magnetite chert and carbonate rich iron formation the averages for major and trace elements and rare earth element ree contents of the limestones dolomites and shales are distinct from those of the two types of iron formation the two most chemically diverse rock types are the shales enriched in alkalis and most trace elements and the two iron formation types siderite and magnetite rich which are almost totally depleted in all elements except si fe mg and ca and co 2 the high al 2 o 3 contents of the shales avg 9 55 wt correlate well with their high organic carbon contents avg 3 91 wt the limestones and dolomites have average al 2 o 3 and organic carbon contents of about 3 0 and 0 7 wt percent respectively the two iron formation types have al 2 o 3 average values of 0 099 wt percent siderite rich and 0 066 wt percent magnetite rich and corresponding averages for organic carbon of 0 080 wt percent siderite rich and 0 012 wt percent magnetite rich the ree are generally concentrated in the shales by a factor of about 10 over the iron formations the limestones and dolomites have intermediate values and the iron formations are most depleted the iron formation ree patterns in a ratio with north american shale composite nasc have pronounced positive eu anomalies and slight negative ce anomalies both of these anomalies are absent in the shales but the limestones and dolomites show slight positive eu anomalies the siderite rich iron formations consist of chert siderite ankerite or ferroan dolomite with traces of pyrite and stilpnomelane the siderite in these commonly microbanded assemblages is very fine grained along well defined bedding planes it is concluded from petrographic study that siderite is a primary precipitate the ankerites and ferroan dolomites in these siderite rich occurrences are commonly euhedral rhombohedral much coarser grained and appear to be of later diagenetic origin the magnetite rich iron formations consist of chert magnetite ankerite or ferroan dolomite or siderite or hematite or stilpnomelane or minnesotaite in these oxide rich iron formations the ankerites and ferroan dolomites are similarly much coarser grained than the finely banded siderite on the basis of the geochemical data and a reconstruction of the depositional basin for the carbonate shale to iron formation transition we conclude that the limestone dolomite shale lithologies originated in a water column quite distinct from that in which the iron formations were precipitated we propose a model with a stratified water column in which the surface waters during a regressive stage in the depositional basin were the site of much organic carbon productivity and the locus of cryptalgal limestones and intraclastic limestone deposition with at somewhat greater depth below the chemocline deposition of pyritic carbonaceous shale our model depicts the deeper waters during a transgressive stage of the basin with the kaapvaal craton more deeply submerged as the site for iron formation deposition these deeper waters were depleted in organic carbon and enriched in dissolved ferrous iron relative to the shallower water mass with continued availability of oxygen along the chemocline separating the two water masses the ultimate source of the iron and probably the sio 2 in the iron formations appears to be a very dilute hydrothermal input in the deep ocean waters as concluded from mixing calculations of ree values for modern atlantic ocean water and hydrothermal solutions from the east pacific rise

winagami reflection sequence seismic evidence for postcollisional magmatism in the proterozoic of western canada a recent lithoprobe seismic reflection transect in the peace river arch region of the western canada sedimentary basin traverses continental crust that was assembled in a series of magmatic and collisional events during the interval 2 0 1 9 ga the seismic data reveal panels of dipping reflectivity at tectonic domain boundaries but they are otherwise dominated by prominent subhorizontal reflections that occur between 3 5 and 18 5 km depth over an area of at least 120 000 km 2 these reflections here named the winagami reflection sequence overlap several tectonic domains and are comparable in geometry depth and areal distribution to sills and discordant intrusive sheets imaged by previous seismic profiling in the baltic shield and the basin and range province of the western united states as many as five distinct reflections are present some of which can be traced for more than 100 km contrasting reflection character in adjacent tectonic domains suggests an intrusive geometry controlled partly by the mechanical properties of the host rocks seismic modeling indicates that observed waveforms are consistent with reflections from tabular mafic bodies 70 m thick the winagami reflection sequence crosscuts the collisional fabric ca 1890 ma but the sequence is offset by a brittle fault with an estimated age of 1760 ma these timing constraints suggest that the winagami reflection sequence is a previously unknown component of a regional magmatic event emplaced during a period of brittle indentation of the western canadian shield by the slave province

comparison of turbidite facies associations in modern passive margin mississippi fan with ancient active margin fans abstract our comparison of the modern passive margin mississippi fan dsdp leg 96 with ancient active margin fans e g eocene hecho group spain reveals major differences in turbidite facies associations mutti and ricci lucchi scheme and in seismic characteristics in the lower fan area the lower outer mississippi fan is composed of channel facies b and f and non channel facies c and d whereas ancient fans are characterized by non channelized thickening upward depositional lobe facies c and d an absence of depositional lobes in the lower mississippi fan is also suggested by a lack of convex upward mounded seismic reflections with bidirectional downlap continuous seismic reflections of the lower mississippi fan may represent sheet sands but not those of true depositional lobes with mounded character extensive channelization in modern passive margin fans appears to be a product of the lateral shifting of a major sinuous distributary system developed as a consequence of low gradients and the transport of sediment with a relatively low sand mud ratio in contrast channels in active margin fans are short and of low sinuosity as a result of high gradients and the transport of sediment with a relatively high sand mud ratio the turbidite facies association scheme which was developed exclusively from ancient active margin fans should be applied to mature passive margin fans with qualifications because of the differences in spatial distribution of turbidite facies and their associations

the tectonic setting and evolution of the 2 7 ga kalgoorlie kurnalpi rift a world class archean gold province the yilgarn craton results from three major mantle input events at ca 3 0 2 9 2 8 and 2 7 ga that have interacted with 3 0 ga continental crust zircon geochronology and sm nd isotopic data subdivide the craton into an older yilgarn proto craton and the younger more primitive eastern goldfields superterrane egst formation of the kalgoorlie kurnalpi rift kkr within the egst was associated with the 2 7 ga event which exploited weakened crust at the eastern margin of the yilgarn proto craton where thick sequences of komatiite and basalt were erupted between ca 2710 and 2690 ma in the kalgoorlie terrane calc alkaline volcanism in the kurnalpi terrane began at ca 2730 ma and continued to ca 2690 ma overlapping rifting and plume related volcanism in the kalgoorlie terrane deposition of siliciclastic sedimentary rocks within basins at ca 2660 resulted from an intra orogenic extensional event and coincided with the transition from high ca to low ca granite magmatism and peak emplacement of intrusions with a metasomatised mantle source component most aspects of the kkr are satisfied by broadly coincident plume related magmatism in the kalgoorlie terrane and westward subduction to the east of the burtville terrane geochemical characteristics of 2730 2700 ma calc alkaline volcanism and 2685 2630 ma low sio2 and alkali rich intrusions support models for a continental margin subduction zone setting world class gold deposits formed in reactivated margins of the kkr which became flux zones for mantle derived magmas hydrothermal fluids and heat during 2675 2620 ma orogenesis the orogenic gold mineralisation can be subdivided into proximal intrusion related and distal source deposits

potential field signatures of buried precambrian basement in the western canada sedimentary basin an internally consistent levelled compilation of magnetic data is derived for alberta and northeastern british columbia with bouguer gravity data this compilation is used to refine the definition of precambrian basement domains within the western canada sedimentary basin magnetic data are draped at a constant distance above the mapped basement surface to reduce the effects of varying magnetic source depths automated interpretation methods that effectively map outlines of magnetic sources are used to characterize the internal structure of the domains and to aid in their delineation the basement domain map thus derived differs from previous interpretations in the extension of domains further to the southwest due mainly to the availability of new public domain magnetic data and the more precise definition of domain boundaries based on the magnetic source location maps the nahanni hottah chinchaga thorsby vulcan and kiskatinaw domains are weakly magnetic and characterized by magnetic sources tha

paragenesis and composition of ore minerals in the randalls bif hosted gold deposits yilgarn craton western australia implications for the timing of deposit formation and constraints on gold sources the randalls district comprises three individual gold deposits cock eyed bob maxwells and santa craze hosted in the same unit of banded iron formation bif in the southern eastern goldfields superterrane yilgarn craton western australia the iron formation is a silicate oxide facies unit with overprinting sulfides and has undergone metamorphism upper greenschist facies and deformation two generations of folds the gold deposits are hosted in two structural locations hinge zones of anticlinal folds e g santa craze and maxwells and overturned steeply dipping limbs of anticlinal folds e g cock eyed bob gold dominantly occurs as inclusions of native gold and or electrum within or around pyrrhotite magnetite and arsenopyrite the earliest mineral assemblage preserved in the banded iron formation at randalls is banded magnetite quartz magnetite in all forms has elevated mg al ti v mn cr zn and w but very low ni compared to other bifs globally pyrrhotite the most abundant sulfide is restricted to ore zones and occurs as laminae blebs and veinlets all of which have replaced magnetite some pyrrhotite in the hydrothermally altered bifs especially in near surface samples has been re sulfidized to pyrite trace elements associated with pyrrhotite include co ni ag sb and pb large coarse grained arsenopyrite overgrows the pyrrhotite defined foliation it is enriched in co ni sb and te but contains very little invisible gold 0 01 ppm au unlike arsenopyrite in many orogenic au systems globally commonly 0 1 ppm au petrographic studies of several mineralized bifs from randalls reveal that pyrrhotite and arsenopyrite are associated with fe ca amphibole which grew during contact metamorphism caused by granite plutons this spatial sulfide amphibole relationship indicates that gold mineralization likely occurred during granite intrusion consistent with prior studies on randalls the trace element characteristics of the sulfides show that they were not inherited from replaced magnetite further these characteristics suggest that the fluid from which sulfides crystallized was not indigenous to the bifs and that this s rich fluid either contained s in abundance originally or interacted with a lithology lithologies that were rich in s before reaching the bifs black shale containing diagenetic nodular pyrite occurs in the upper black flag group which unconformably underlies the belches supersequence at depth 3 km the trace element composition of nodular pyrite from this unit fits well with the trace element characteristics of the bif pyrrhotite and arsenopyrite and the nodular pyrite contains an average of 0 5 ppm au however the nodular pyrites have an average 34s value of 5 6 n 13 which contrasts markedly with the 34s average of pyrrhotite pyrite and arsenopyrite in mineralized bifs 0 8 n 17 thus a definitive source of gold arsenic silver and tellurium in the randalls system remains elusive although we would contend that the upper black flag group should be considered as a possible metal and s source in areas of the yilgarn where it forms a thick substrate to ore camps

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

westward propagation of the north anatolian fault into the northern aegean timing and kinematics we present new evidence for the propagation processes of the north anatolian fault folding in the dardanelles straits region allows us to document the timing of the deformation preceding and the finite displacement after the passage of the propagating tip of the fault the accuracy of the observations is due to interplay between deformation and the sea level changes in the mediterranean the well known messinian regression followed by the pliocene transgression the long term kinematics around the sea of marmara pull apart total displacement of about 85 km over the past 5 m y is similar to the present day kinematics deduced from space geodesy at a larger scale westward propagation of the north anatolian fault over nearly 2000 km in the past 10 m y appears to be associated with strain recovery suggesting that the continental lithosphere retains long term elasticity

magnetotelluric imaging beneath the taiwan orogen an arc continent collision 1 the taiwan orogen has formed since the late miocene by oblique collision between the luzon volcanic arc on the philippine sea plate and the eurasian continental margin this oblique collision has produced an orogen that decreases in age from north to south and permits study of the temporal evolution of an arc continent collision these factors make taiwan a favorable location to study the process of arc continent collision the first long period magnetotelluric mt measurements were recorded in taiwan as part of the taiwan integrated geodynamics research taiger project in 2006 7 measurements were made at 82 sites on three transects across south central and north taiwan that span the breadth of the orogen and cross all major tectonic boundaries robust remote reference processing of the mt time series data resulted in high quality soundings that were modeled in both 2 and 3 dimensions these mt models support predictions of lithospheric deformation i e thick skinned tectonics beneath the central ranges in south and central taiwan but are inconsistent with predictions of orogen scale thin skinned models the mt resistivity model for northern taiwan is consistent with dewatering of the subducting philippine slab and with deformation described by the subducting indenter tectonic model modeling the taiger mt data has definitively shown that conductive and seismically active crustal structures exist to 30 km beneath the orogen these conductive regions interpreted as interconnected fluid map pervasive zones of collisional deformation that are lithospheric in scale

archaean cratonic roots mantle shear zones and deep electrical anisotropy the extent to which the mantle participated in the growth and stabilization of ancient cratons is central to our understanding of the evolution of the continents1 the detection of seismic anisotropy beneath precambrian north america for example has been interpreted as showing that strain induced orientation of mantle minerals in subcontinental lithospheric mantle can preserve a record of ancient episodes of deformation2 here we present mag netotelluric measurements from the superior province of the canadian shield which reveal pronounced electrical anisotropy in the upper 100 km of the underlying mantle we argue that this anisotropy is best explained by conducting graphite films oriented within fractures or on grain boundaries and associated with metasomatism of the mantle roots of major archaean shear zones which transect the entire superior province the uppermost mantle beneath the canadian shield has therefore remained fixed to the crust and isolated from significant tectonic reworking since the late archaean

structural geometry lower crustal magmatic underplating and lithospheric stretching in the ivrea verbano zone northern italy abstract the ivrea verbano zone is believed to represent an up ended cross section through the lower continental crust as it existed at the end of the hercynian orogeny in the southern alps structurally oriented geological mapping has been carried out in the central and northern parts of the ivrea verbano zone a region some 30 km along strike the geology of the southern half of the zone is dominated by a large basic ultrabasic complex the mafic formation which is in contact with a strongly banded metasedimentary metabasic sequence to the north particular attention was paid to a the structural relationship between the rocks of the mafic formation and their envelope of high grade metasedimentary and metabasic rocks b the geometrical configuration throughout the ivrea verbano zone of high temperature shear zones which accommodated post hercynian crustal extension and c the geometry of late low temperature faulting the effects of which have been removed in order to produce a restoration of the structure as it existed during the post hercynian extensional phase the intrusion of large volumes of basic magma ca 50 of the outcrop area to form the rocks of the mafic formation appears to be coeval with the onset of extension ca 280 ma the main basic body has a laccolithic form which was originally more than 10 km thick overfolding developed at the northern margin of the laccolith and is interpreted in terms of the gravitational collapse of the hot immediately subsolidus or partially molten body incorporating its envelope of hot metasediments into a large originally recumbent fold a geometrical association with a high temperature low angle fault zone suggests that faulting was subsequently localized by the several km of uplift associated with the laccolithic intrusion the ivrea verbano zone may therefore demonstrate at least one particular geometry of lower crustal magmatic underplating which may aid in the interpretation of present day deep seismic profiles it also demonstrates the geometry of a network of conjugate high temperature low angle shear zones in a well layered lower crustal section

high resolution electromagnetic imaging of the san andreas fault in central california although there is increasing evidence that fluids may play a significant role in the earthquake rupture process direct observation of fluids in active fault zones remains difficult since the presence of an electrically conducting fluid such as saline pore water strongly influences the overall conductivity of crustal rocks electrical and electromagnetic methods offer great potential for overcoming this difficulty here we present and compare results from high resolution magnetotelluric mt profiles across two segments of the san andreas fault saf which exhibit very different patterns of seismicity parkfield which has regular small earthquakes and creep events and in the carrizo plain where the fault is seismically quiescent and apparently locked in both surveys electric fields were sampled continuously with 100 m long dipoles laid end to end across the fault from 100 to 0 1 hz the data from both profiles are consistent with a two dimensional 2 d fault parallel resistivity model when both transverse electric and magnetic te and tm mode data are included in the interpretation narrow approximately 300 endash 600 m wide zones of low resistivity extending to depths of 2 endash 4 km in the core of the fault are required at both locations however at parkfield the conductance conductivity thickness product of the anomalous region ismore an order of magnitude larger than at carrizo plain suggesting much higher concentrations of fluids for the more seismically active parkfield segment we also image structural differences between the two segments at carrizo plain resistive presumably crystalline rocks are present on both sides of the fault at depths below 3 endash 4 km in particular we clearly image resistive basement extending approximately 10 km or more east of the saf beneath the elkhorn hills and temblor range at parkfield the situation is quite different with a resistive block of salinian granite west of the fault and an electrically conductive presumably fluid rich franciscan complex to the east it is possible that these structural differences control the difference in mechanical behavior of the fault either directly by affecting fault strength or indirectly by controlling fluid supply copyright 1999 american geophysical union less

multi method 14c 36cl 234u 230th age bracketing of the tschirgant rock avalanche eastern alps implications for absolute dating of catastrophic mass wasting correct and precise age determination of prehistorical catastrophic rock slope failures prerequisites any hypotheses relating this type of mass wasting to past climatic regimes or palaeo seismic records despite good exposure easy accessibility and a long tradition of absolute dating the age of the 230 million m3 carbonate lithic tschirgant rock avalanche event of the eastern alps austria still is relatively poorly constrained we herein review the age of mass wasting based on a total of seventeen absolute ages produced with three different methods 14c 36cl 234u 230th 36cl cosmogenic surface exposure dating of five boulders of the rock avalanche deposit indicates a mean event age of 3 06 0 6 ka 234u 230th dating of soda straw stalactites formed in microcaves beneath boulders indicate mean precipitation ages of three individual soda straws at 3 20 0 26 ka 3 04 0 10 ka and 2 81 0 15 ka notwithstanding potential internal errors these ages provide an older than ante quam proxy for mass wasting this article is protected by copyright all rights reserved based on radiocarbon ages nine sites only it was previously suggested that the present rock avalanche deposit represents two successive failures 3 75 0 19 ka bp 3 15 0 19 ka bp there is however no evidence for two events neither in surface outcrops nor in lidar derived imagery and drill logs the temporal distribution of all absolute ages 14c 36cl 234u 230th also does not necessarily indicate two successive events but suggest that a single catastrophic mass wasting took place between 3 4 and 2 4 ka bp taking into account the maximum age boundary given by reinterpreted radiocarbon datings and the minimum u th ages of calcite precipitations within the rock avalanche deposits a most probable event age of 3 01 0 1 ka bp can be proposed our results underscore the difficulty to accurately date catastrophic rock slope failures but also the potential to increase the accuracy of age determination by combining methods this article is protected by copyright all rights reserved

a summary of electromagnetic studies on the abitibi grenville transect electromagnetic surveys on the abitibi grenville lithoprobe transect have elucidated a number of conductivity signatures that can be genetically linked to precambrian tectonic processes some major fault zones are moderately conductive possibly signalling graphite deposition from a mantle co2 flux along crust penetrating fault systems however conductive graphitic metasedimentary rocks characteristic of foreland basins are apparently absent from the transect area a weak inverse correlation between metamorphic grade and electrical conductivity was observed by following rock units across the grenville front into high grade equivalents within the parautochthonous belt a uniformly conductive mid crustal layer extends across the grenville front apparently without change in character the existence of this ubiquitous mid crustal conductor has been interpreted to mean that electrical conductivity is controlled by the present day pressure temperature and fluid saturation of the lower crust independent o

deep electrical structure of the northern cascadia british columbia canada subduction zone implications for the distribution of fluids long period magnetotelluric data have been used to image the deep electrical structure of the cascadia subduction zone in british columbia canada zones of elevated electrical conductivity were found in both the forearc and backarc regions and are interpreted as a consequence of the fluid release from subducting slab a shallow zone of high conductivity beneath vancouver island is likely due to fluids that are trapped above the subducting plate east of this structure is a conductive 0 03 s m forearc mantle wedge that also exhibits low seismic velocities and may be serpentinized a free fluid phase is required to account for this enhanced conductivity elevated conductivities are observed in the upper mantle throughout the backarc 0 01 s m and strongly support the hypothesis of a shallow convecting asthenosphere this enhanced upper mantle conductivity can be explained by either hydrogen ion diffusion in olivine minerals or by a few percent partial melting 4

introduction to special section preparing for the san andreas fault observatory at depth 7209 seismology earthquake dynamics andmechanics 7230seismology seismicityandseismotectonics 8010structural geology fractures and faults citation hickman s m zoback and w ellsworth 2004 introduction to specialsection preparing for the san andreas fault observatory atdepth geophys res lett 31 l12s01 doi 10 1029 2004gl020688

analyses of crustal structure from bandpass and directionally filtered potential field data an example from western canada application of bandpass and directional filtering to potential field data in northwestern canada allows separation of anomalies due to northwest oriented upper crustal sources that are associated w

u pb zircon ages from athapuscow aulacogen east arm of great slave lake n w t canada athapuscow aulacogen is an early proterozoic intracratonic basin located in the east arm of great slave lake between the slave and northwest churchill provinces athapuscow aulacogen comprises three stratigraphic sequences the wilson island group the great slave supergroup and the et then group new u pb zircon ages provide constraints on the development of the aulacogen the blachford lake intrusive suite consists of an older alkaline phase hearne channel granite dated at 2175 7 ma and a younger peralkaline phase thor lake syenite dated at 2094 10 ma confirming the suggestion that the two phases may not be related a felsite from the wilson island group has an age of 1928 11 ma the wilson island group is intruded by epizonal granites butte island intrusive suite one of which has an age of 1895 8 ma the wilson island group and the butte island instrusive suite are entirely allochthonous with respect to the slave craton rocks of the great slave supergroup overlie mylonitized wilson isl

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia abstract siliciclastic supersequences of the kalgoorlie and gindalbie terranes post date 2660 ma deformation uplift and erosion associated with the amalgamation of arc related terranes but were tectonically buried to greenschist or amphibolite facies during the orogenic shortening that accreted the eastern goldfields superterrane to the yilgarn craton the supersequences are synclinal remnants of late stage basins that were linear zones of subsidence adjacent to north trending strike slip faults fluvial facies merougil navajo jones creek scotty creek and yandal basins are dominated by sheet sandstones that represent the deposits of medial braid plains but proximal channel and bar complexes of cobble and boulder conglomerates are also preserved submarine facies kurrawang basin range from boulder to pebble conglomerates but are dominantly bouma turbidites with rare black shales and record the filling of a proximal canyon that fed a distal submarine fan axial depositional systems are preferentially preserved in all basins but transverse systems are preserved at the base of some fluvial deposits the gross vertical succession in all basins is from proximal up to distal the basins developed during two tectonic stages separated by an intervening period of uplift tilting and erosion with the southern kalgoorlie terrane recording change from a stage i fluvial to a stage ii deep marine basin stage ii fluvial deposits are preserved in the northern kalgoorlie terrane and in the gindalbie terrane sparse palaeocurrents establish sediment dispersal was to the south or southeast implying uplift to the north or northwest controlled palaeoslopes stacking of lithofacies is aggradational and is consistent with limited lateral migration of facies tracts in fault confined basins similar lithofacies and aggradational styles of facies stacking characterise syncollisional sequences of the cenozoic coastal range basin of taiwan and the present day syncollisional basins of southeastern taiwan are good analogues of late stage basins of the kalgoorlie and gindalbie terranes

integrated crustal structure across the south central california margin santa lucia escarpment to the san andreas fault the continental margin of south central california has undergone different styles of deformation over the past 30 m y as the north america pacific plate boundary has evolved from subduction to transform tectonics results presented here identify strike slip and compressional structures in the lower crust that probably developed during this period of tectonic transition an integrated model of crustal structure along deep seismic line pg e 3 crosses the entire continental margin west of the san andreas fault near the latitude of san luis obispo a compressional wave velocity model was developed using coincident marine multi channel seismic and onshore offshore wide angle reflection refraction seismic data and was tested independently with gravity modeling a high velocity layer 6 8 7 0 km s about 6 km thick in the lower crust that extends east possibly as far as the san andreas fault is interpreted as tectonically underplated oceanic crust the upper and middle crust of the patton and sur obispo franciscan terranes extends to depths of 14 to 20 km the velocity and density modeling indicate lower average velocities and densities for the patton terrane supporting earlier interpretations that it is a distinct terrane within the franciscan complex abrupt crustal thickening associated with broken and downdropped oceanic crust beneath the santa lucia escarpment probably results from active transform faulting that occurred along the continental slope from about 22 to 16 ma following the subduction of the pacific arguello ridge an imbrication within the oceanic crust centered beneath the coast is evidence for about 15 km of shortening within the oceanic crust convergence between the monterey and arguello microplates during the final stages of farallon subduction about 24 to 16 ma probably caused the imbrication

striding athabasca mylonite zone implications for the archean and early proterozoic tectonics of the western canadian shield study of the northern saskatchewan district of mackenzie segment of the snowbird tectonic zone suggests that fragments of relatively stiff mid archean crust possibly arc related have controlled the localization shape and complex kinematics of the multistage striding athabasca mylonite zone during the archean as well as the geometry of the early proterozoic rifted margin of the western churchill continent by the late archean the striding athabasca mylonite zone was located in the interior of the western churchill continent well removed from the contemporaneous plate margins except for the alberta segment the snowbird tectonic zone was not the site of an early proterozoic plate margin we suggest that the geometry of the archean early proterozoic boundary in the western canadian shield represents a jagged continental margin composed of a pair of reentrants defined by rifted and transform segments these segments were inherited from early proterozoic breakup and controlled by the archean structure

tectonic setting of the taltson magmatic zone at 1 92 0 ga a granitoid based perspective the paleoproterozoic taltson magmatic zone is one of the key tectonic features of western laurentia the existing tectonic model for the belt envisions its formation by subduction of oceanic crust beneath a continental margin followed by direct collision between formerly separate crustal blocks we tested this model by comparing the large geochemical and isotopic database available for taltson magmatic zone granitoids with similar databases for phanerozoic granitoid suites from different tectonic environments the comparison reveals that the early granitoid suite of the taltson magmatic zone which had been ascribed to the subduction phase of orogenesis lacks the mantle signature apparent in granitoids of phanerozoic continental margin arc settings instead both early and late suites appear to have an intracrustal origin similar to mesozoic and cenozoic granitoids of the cordilleran interior of western north america which formed in the distant hinterland of a convergent plate margin in light of thes

correlation of electrical conductivity and structural damage at a major strike slip fault in northern chile 1 large scale strike slip fault zones are often imaged as electrically conductive structures in the brittle crust however the relationship of conductivity and internal architecture of the fault zone remains largely unclear this paper presents results of a study designed to compare the record of structural deformation across a fault zone with its electrical conductivity image two high resolution magnetotelluric profiles trend perpendicularly across the west fault a branch of the precordilleran fault system in northern chile the magnetotelluric and geomagnetic response functions in the frequency range from 1000 hz to 0 1 hz clearly image a fault zone conductor fzc about 350 m wide and 1500 m deep trending along the surface trace of the fault the position of the fzc and its geometric properties width dip correlate with a region of intense fluid alteration and the orientation of fault related damage elements minor faults fractures according to estimates of fluid salinity and rock porosity the conductivity anomaly results from meteoric water penetrating into a permeable zone this suggests that the increase in electrical conductivity is causally related to a mesh of minor faults and fractures acting as a pathway for fluids the fzc reflects the central and most fractured part of the damage zone in view of similar published magnetotelluric studies we infer a dependency of a fault s state of activity and the characteristics of the fzc such as its conductance width and depth extent ongoing deformation is the controlling factor for maintaining a permeable and electrically conductive fracture network

electromagnetic images of the trans hudson orogen the north american central plains anomaly revealed magnetotelluric studies of the trans hudson orogen over the last two decades prompted by the discovery of a significant conductivity anomaly beneath the north american central plains nacp from over 300 sites yield an extensive database for interrogation and enable three dimensional information to be obtained about the geometry of the orogen from southern north dakota to northern saskatchewan the nacp anomaly is remarkable in its continuity along strike testimony to along strike similarity of orogenic processes where bedrock is exposed the anomaly can be associated with sulphides that were metamorphosed during subduction and compression and penetratively emplaced deep within the crust of the internides of the orogen to the boundary of the hearne margin a new result from this compilation is the discovery of an anomaly within the upper mantle beginning at depths of 80 100 km this lithospheric mantle conductor has electrical properties similar to those for the central slave craton mantle conductor

density modified grain flow deposits from an early paleozoic passive margin abstract thick to very thick beds of sand to gravel matrix limestone clast conglomerate constitute an important lithology of the richmond slice of the allochthonous hamburg klippe of southeastern pennsylvania sedimentary structures in the conglomerates can be explained by both debris flow planar clast fabric nondeforming plugs and grain flow reverse grading mechanisms however the lack of a cohesive mud matrix and the great thickness of the beds rule out either one of these sediment gravity flow types previous studies have illustrated that thick flows of poorly sorted gravel size debris can move over relatively low slopes by density modified grain flow a cross between true unimodal grain flow and debris flow the coarse grained matrix conglomerates of the richmond slice can be exp ained as ancient density modified grain flow deposits in addition the presence of layers of normally graded level and parallel to cross laminated sand size material on top of some of the conglomerates is suggestive of increased turbulence toward the top of the flow this is probably related to either backward shearing of sediment and water in front of the moving sediment mass creating turbulence on top of the flow or expulsion of pore fluid during movement of the main sediment mass

proximal to distal sedimentological variation in a linear turbidite trough implications for the fan model determination of geographically dependent sedimentological variation proximality in ancient flysch deposits formed an important part of early turbidite studies attempts to quantify this variation highlighted anomalies which were neatly resolved by application of vertical sequence analysis and the use of fan models however there are many turbidite formations such as the lower cretaceous cumberland bay formation cbf of south georgia which cannot be described in terms of existing fan models but show strong proximal to distal sedimentological changes the cbf is a thick sequence of volcaniclastic sandstone turbidites deposited in a linear back arc basin principally by currents flowing wnw parallel to the basin margin four lithofacies associations are recognized on the basis of sandstone shale ratio the two finergrained associations are constant in character across the cbf outcrop in the coarse grained associations there is a change in character wnw down the palaeocurrent direction this is brought out by decreasing sandstone bed thickness and percentage amalgamation but these changes are not always marked or consistent in contrast the internal character of the sandstone beds changes strongly with a marked proportional increase in tb and tc divisions within the bed downcurrent the evidence suggests that the system was aggradational rather than progradational tectonic control of the basin margins prevented major migration of the depositional system and most areas remained in the same position relative to source through time comparison of the cbf with other turbidite formations suggests two end member states which will produce radically different vertical sequences progradational systems will produce strong vertical facies changes where beds deposited in distal environments are overlain by beds deposited in environments progressively nearer source however there will be no lateral change in the character of any particular facies type in contrast in aggradational systems the major sedimentological variation will be lateral rather than vertical

late archaean volcanic arcs and their accretionary history eastern goldfields superterrane plate tectonic models inferred from tectonostratigraphic analysis

unstable graphite films on grain boundaries in crustal rocks abstract the origin of high electrical conductive anomalies in the lower crust is a long standing and controversial problem although it has been proposed that saturated saline water or partial melt increases electrical conductivity graphite film has also been recognized as a potential cause of high conductivity since the discovery of fine graphite films on the grain boundaries of high grade metamorphic rocks to investigate the stability of graphite film on grain boundary of silicate minerals under lower crustal conditions electrical conductivity of graphite film on synthetic grain boundaries of quartz bicrystals was measured by means of impedance spectroscopy at 1 gpa and up to 1200 k in a multianvil apparatus at first heating the electrical conductivity of the thin graphite film with thickness less than 100 nm was initially very high but decreased with time during annealing under high temperature conditions 1000 k the conductivity of a thin carbon film rapidly decreases and approaches the quartz conductivity this indicates that graphite film on a grain boundary between two quartz crystals is not stable at high temperatures optical microscopic observation of the run products suggested a disconnected feature of graphite on a quartz grain boundary disconnection of graphite film can be caused by higher interfacial energy between graphite and silicate minerals than that of the grain boundary energy therefore a thin graphite film is not a likely candidate to account for high conductivity anomalies in the middle and lower continental crust

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia part iii signatures of tectonic escape in an arc continent collision zone abstract siliciclastic supersequences in the eastern goldfields superterrane are remnants of basins that were linear zones of subsidence between intra terrane faults fluvial deposits record proximal to medial braid plains deep marine canyon and fan deposits were linked across intra terrane compartments only by mud rich deposits axial depositional systems dominate with only minor evidence for slope aprons although transverse and axial systems are preserved in some fluvial deposits basin development occurred during two stages separated by an inversion event with the kalgoorlie terrane recording change from a fluvial to a deep marine basin and the kurnalpi terrane change from distal to proximal deep marine facies tracts sparse palaeocurrents establish that uplift to the north controlled sediment dispersal lithofacies stacking is aggradational whereas abrupt upwards change in facies tracts is back stepping within depositional sequences and between depositional sequence sets but forward stepping between depositional sequences the stratigraphic architecture is atypical of synorogenic basins but consistent with a strike slip regime characterised by pulsed uplift subsidence steady state basin lengthening and subsidence punctuating catastrophic subsidence and lengthening and tectonic anchoring the basins are interpreted to have been similar to graben and half graben of the western anatolian extensional province of turkey with linear zones of subsidence developing adjacent to strike slip oblique slip or normal dip slip faults controls on stratigraphic patterns are considered to have been regional rather than at the scale of each basin the envisaged tectonic setting is similar to projected collision between the philippine archipelago west of the philippine fault and continental crust of the south china sea but the orogenic architecture is similar to that of accreted superterranes in the canadian cordillera interpreted north or northwest convergence between the burtville terrane to the east and the ancestral yilgarn continent to the northwest produced a south propagating orogen with the kalgoorlie gindalbie and kurnalpi terranes trapped between in a tectonic escape corridor tectonic escape to the south between bounding strike slip faults produced back stepping of facies tracts that simulated basin lengthening whereas forward stepping of facies tracts was the response to south propagating uplift

age origin and significance of nodular sulfides in 2680 ma carbonaceous black shale of the eastern goldfields superterrane yilgarn craton western australia abstract sulfide nodules composed of pyrite occur within black shales in the lucky bay area of the bulong domain eastern goldfields superterrane yilgarn craton western australia that correlate with the neoarchean black flag group detailed petrography reveals a variety of shapes and textures from spherical to ovoid and zoned to uniform some nodules have recrystallized resulting in a later generation of pyrite all have been affected by post depositional metamorphism as evidenced by pressure shadows containing quartz mica carbonate assemblages la icp ms analyses show that these nodules are enriched in a range of trace elements including co ni cu ag sb te au tl pb and bi gold and te concentrations range from 0 3 to 1 ppm and 10 to 50 ppm respectively the black flag group equivalents stratigraphically and structurally underlie a younger basin east of lucky bay that contains turbidites sandy debris flows and bif the latter of which hosts several gold deposits at randalls 60 km se of kalgoorlie it is likely that the lateral equivalents of the nodular sulfide bearing black shale originally occurred stratigraphically beneath what is now the ore horizon at randalls given its anomalous au and te content this lithology is a potential contributor of gold to the bif hosted gold deposits of the randalls goldfield as well as other similar gold resources elsewhere in the eastern goldfields superterrane

the arrowsmith orogeny geochronological and thermobarometric constraints on its extent and tectonic setting in the rae craton with implications for pre nuna supercontinent reconstruction abstract geochronological and thermobarometric data for two widely spaced and previously unstudied areas of the rae craton canada provide important temporal and spatial links between previously investigated areas in the northern and southern rae craton that had been interpreted to have been reworked during the ca 2 5 2 3 ga arrowsmith orogeny on southern boothia peninsula upper amphibolite to granulite facies metamorphism dated by in situ shrimp u pb analysis of monazite occurred at ca 2 64 2 60 2 54 2 50 2 49 2 44 and 2 37 2 34 ga with retrograde monazite growth at 2 27 ga shrimp u pb ages of zircon in two metasedimentary rocks provide a further indication of metamorphism at ca 2 54 and 2 50 ga which also corresponds to the age determined for a k feldspar augen granite textural relationships indicate that the regional foliation formed between ca 2 54 ga and 2 50 2 49 ga peak metamorphic conditions of 700 c and 5 6 kbar are interpreted to have been reached during the 2 37 2 34 ga event in the mccann mcarthur lake region of the southwestern rae craton upper amphibolite to granulite facies metamorphism is dated by in situ shrimp monazite ages of ca 2 53 2 47 and 2 37 2 28 ga textural relationships indicate deformation initiated by ca 2 37 ga and terminated by ca 2 30 ga metamorphic conditions of 5 kbar and 700 735 c were quenched at ca 2 3 ga following peak conditions of 7 kbar and 800 c the evidence for regional deformation at 2 54 2 49 ga on southern boothia peninsula supports an andean type accretionary margin setting for the arrowsmith orogeny a collisional event may have terminated high grade metamorphism in the northern region by ca 2 34 ga while metamorphism associated with continental arc magmatism continued in the south until at least ca 2 28 ga these combined data demonstrate the continuity and broad extent of the arrowsmith orogen along the western flank of the rae craton extending 1500 km from northern saskatchewan to northern baffin island the broad extent of this globally rare orogenic event provides an important piercing point for supercontinent reconstructions which are discussed in a companion paper the inferred convergent tectonic activity between ca 2 54 and 2 28 ga is interpreted to have occurred on the flank of a growing supercontinent a setting that could account for an apparent slowdown but not shutdown in global tectonics

crustal structure beneath the western canada sedimentary basin constraints from gravity and magnetic modelling regional gravity and magnetic anomalies that originate from crystalline basement rocks extend over many parts of the western canada sedimentary basin although these potential field anomalies provide a basis for tectonic subdivisions of the basement crust most previous interpretations of these features have been largely qualitative in nature this study focuses on numerical simulation and quantitative interpretation of five regional potential field anomalies in alberta canada for which independent constraints on crustal structure are available from lithoprobe seismic and electromagnetic studies the kimiwan high 55 n 116 w is a roughly 250 km long linear magnetic high seismic profiles across this anomaly provide evidence for a crustal scale extensional fault system that offsets the winagami reflection sequence a series of mid crustal sills we find that the magnetic anomaly can be modelled as either a 15 40 km wide zone of moderate positive susceptibility 4 5 10 3 si units in the hanging wall

tectonostratigraphy of the late archean dharwar supergroup dharwar craton india defining a tectonic history from spatially linked but temporally distinct intracontinental and arc related basins abstract the dharwar supergroup comprises the unconformity bound bababudan and chitradurga groups the bababudan group which is best preserved in the western dharwar craton records a divergent margin comprising a basalt dominant intracontinental rift sequence a shale bif drift sequence and a sandstone shale thermal subsidence sequence the rift stage evolved from 2765 ma to 2720 ma whereas the succession was folded uplifted and eroded during development of a convergent continental margin from 2680 ma ocean opening was to the east or southeast the chitradurga group records a two stage back arc basin behind an east facing continental arc stage i evolved between 2609 ma and 2582 ma and stage ii between 2582 ma and 2540 ma the two stages are separated by a subaerial unconformity stage i comprises siliciclastic fluvial shallow marine and deep marine sedimentary rocks and bimodal volcanic rocks stage i is best preserved in the western dharwar craton but there are equivalents in the eastern dharwar craton stage ii in the western dharwar craton comprises deep marine turbidites of siliciclastic and volcaniclastic provenance and basaltic and felsic volcanic rocks volcanic stage ii sequences in the eastern dharwar craton define an arc adjacent position tectonically driven cyclic uplift and erosion were responsible for the mismatch between preserved stratigraphic thickness and time particularly for stage ii sequences a remnant of a

structure of the altyn tagh fault and daxue shan from magnetotelluric surveys implications for faulting associated with the rise of the tibetan plateau magnetotelluric measurements in the foreland of the daxue shan have imaged low angle 20 thrust faults that extend to a depth of 3 km the surface projections of the faults at depth coincide with the mapped traces of the chang ma m 7 6 1932 and north hills thrusts minimum cumulative offsets of 7 km on the chang ma thrust and 14 km on the north hills thrust are estimated from the horizontal extent of underthrust sediment assuming that regional thrusting began at 5 6 ma this corresponds to minimum convergence rates of 1 3 mm yr and 2 5 mm yr on the chang ma and north hills thrusts respectively these slip rates correspond to 3 mm yr of sinistral slip on the altyn tagh fault between 96 e and 97 e this is consistent with the 4 2 mm yr of slip estimated by geological studies assuming comparable slip rates and similar fault geometry in the neighboring ranges this requires a minimum of 11 mm yr of shortening parallel to the altyn tagh fault between the dang he nan shan and qilian shan both the style of thrusting and rate of shortening are in agreement with geologic studies that favor a relatively high rate of slip on the altyn tagh fault this in turn implies that lithospheric extrusion contributes significantly in accommodating the ongoing convergence between india and asia farther west the altyn tagh fault is imaged on four magnetotelluric profiles as a vertical resistivity contrast extending to a depth of at least 8 km two strands of the north altyn tagh fault are imaged east of the asperity near subei 39 30 n 95 e

the effect of map scale on geological complexity for computer aided exploration targeting mineralization in hydrothermal ore systems has proximity association and abundance relationships with fluid pathways e g faults and lithological contacts based on the concept of deformation induced permeability being localized along such features to empirically calibrate the abundance relationship between these features and gold mineralization a measure known as geological complexity has been quantified a fractal dimension representing the degree of geological complexity is evaluated using a box counting method on the combination of faults and lithological contacts on a series of maps over the kurnalpi terrane of the archaean yilgarn craton using multi scale geological datasets from the kurnalpi terrane we show that geological complexity and its relationship to known occurrences of orogenic gold mineralization varies depending on scale and degree of interpretation of the maps analyzed in terms of scale stronger correlations are observed for higher resolution outcrop geological and structural maps even when the percentage of outcrop was taken into consideration the higher resolution maps still showed greater correlations between geological complexity and gold mineralization differing solid geological interpretations at the same scale significantly affect the degree of quantified geological complexity and its correlation with known gold mineralization the study results also illustrate a negative correlation between gold mineralization and gradients in the geological complexity this outcome is contrary to results obtained by previous workers and indicates that geological complexity rather than complexity gradients is useful as an input predictor map for prospectivity analysis and exploration targeting in the kurnalpi region and other regions containing similar orogenic gold systems furthermore the results indicate that interpreted solid geology maps while critical for manual conceptual targeting may actually be less effective than outcrop maps as a predictive layer in automated conceptual and empirical prospectivity analysis