first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

high resolution diatom record of paleoceanographic variations across the early middle pleistocene boundary in the chiba section central japan abstract we conducted diatom analyses of a core from the pleistocene marine sequence drilled at the chiba section central japan to reveal detailed paleoceanographic variations across the early middle pleistocene boundary diatom valves are most abundant at 4 5 m below the matuyama brunhes magnetic polarity boundary followed by the second highest abundance at 18 6 m above the boundary which is consistent with the sea level changes inferred from planktonic 18 o data the highest abundance is correlated with the marine isotope stage mis 19 3 sea level highstand and the second highest with the mis 19 1 highstand the abundance of the extinct species actinocyclus ingens is quite low around these sea level highstands whereas it is greatest at the core depth with the minimum diatom total valves which is correlated to the mis 19 2 lowstand cold water diatom species that are characteristic of the oyashio current are dominant but coexist with warm water species of the kuroshio current below a core depth just postdating mis 19 2 above this depth warm diatom species of the kuroshio current become dominant the change in dominant diatoms suggests that the kuroshio front shifted northward just after mis 19 2 and remained in the northern area until at least early mis 18 this shift caused a rapid increase in sea surface temperature by 4 5 c within a sediment thickness of 1 3 m representing 300 years

structure of a paleoproterozoic continent continent collision zone a lithoprobe seismic reflection profile across the trans hudson orogen canada abstract an 800 km reflection seismic profile across the trans hudson orogen northern saskatchewan and manitoba images crustal scale tectonic imbrication in an unprecedented picture of paleoproterozoic crustal accretion and continent continent collisional tectonism the profile is crudely symmetric about a crustal scale culmination in the western part of an accreted juvenile collage reindeer zone geologic and isotopic data suggest that this culmination is cored by microcontinental archean basement west of the culmination highly reflective juvenile crustal elements dip westward into the lower crust beneath the wathaman batholith and archean continental crust of hearne craton to the east strong reflections in the juvenile reindeer zone crust and reworked archean foreland of the thompson belt have eastward dips persisting into the middle crust and extending beneath the superior craton a continuous reflection moho well defined for 500 km in the western part of the profile shows marked relief 12 15 s including a prominent root below the crustal culmination these imaged structures give evidence of substantial crustal shortening and thickening via large scale imbrication consistent with collisional orogeny w dipping structures below the wathaman batholith and reworked hearne craton may reflect subduction polarity in this part of the orogen however geological evidence suggests that e dipping structures below superior craton are largely related to late post collisional deformation rather than to prior oceanic subduction polarity

proterozoic plate tectonics glaciation and iron formations abstract glacigenic diamictites and associated rocks are common in early and late proterozoic sequences most of the preserved proterozoic glacigenic rocks formed in marine basins in an extensional tectonic regime at or close to the rift drift transition the reasons for glaciation in this tectonic setting are not known but two possible contributing factors given an initially cool climatic regime are the formation of new seaways by continental fragmentation and uplift of the rift shoulders during early stages of breakup the absence of such glacigenic deposits in the classical miogeoclines of the peri atlantic region may be due to an anomalously warm global climatic regime in the late mesozoic iron formations in the archean and early proterozoic formed in many different environments the critical factor was a dearth of siliciclastic deposition starved basins commonly developed during the transition from miogeocline to foreland basin in the wilson cycle of ocean opening and closure when drainage from the craton was replaced by that from a rising orogen early proterozoic iron rich sediments in the great lakes region in southeast wyoming and in the northwest territories of canada are interpreted as the earliest deposits of such foreland basins they are considered to be chemical precipitates from iron rich sea water late proterozoic iron formations formed under an extensional regime possibly in small red sea type ocean basins where hydrothermal circulation produced metal rich brines these brines were displaced by movement of cold waters derived from sea going glaciers leading to dilution and precipitation of fe and si thus in the late proterozoic there was a critical association of continental rifting glaciation and hydrothermal circulation this combination of environmental parameters led to the production of the unique late proterozoic class of hydrothermal iron formations

aeolian granule ripple deposits namibia granule ripples are a common feature of most dunefields yet they have seldom been recognized in ancient deposits although granule ripples are common in erosional settings such as windward slopes of dunes or scour surfaces in interdunes they nevertheless migrate laterally and leave distinctive deposits that can be recognized in ancient rocks these deposits have characteristics of type b sand sheet deposits including poured in texture curving ripple trough tangential coarse grained foresets irregular silty layers well sorted coarse and fine layers either horizontal or within foresets and fine layers in ripple troughs wind tunnel experiments suggest that under low velocity wind conditions granule ripples grow to a significant degree as parasites dependent on saltation of fine sand grains whose impact moves the larger grains of the granule ripple although the depositional surface of granule ripples is commonly coated with a layer of coarse grains this is in most places only a few grains thick underlying deposits commonly have a poorly sorted or poured in texture this texture results from an admixture of fine grains that fall among the spaces between the larger grains during deposition

allogenic controls on the evolution of storm to tidal shelf sequences in the early proterozoic uncompahgre group southwest colorado usa dominantly coarse grained shallow marine metasedimentary rocks of the early proterozoic uncompahgre group ug record periods of shoaling and drowning on different temporal scales that are attributed to episodic long term oscillations in relative sea level with superimposed shorter duration excursions in relative sea level long term events are probably tectonic whereas short term events are eustatic the 2 5 km thick uncompahgre group consists of 250 600 m thick dominantly coarse grained quartzite units q1 q4 and 200 300 m thick mudstone pelite units p1 p5 five depositional systems comprise the uncompahgre group the outer shelf system oss is composed of bouma type beds and intercalated mudstones that are transitional vertically to parallel laminated to wave rippled sandstones and hummocky cross stratified sandstones of the inner shelf system iss trough cross stratified sandstones comprise the shoreface system shs the tidal inner shelf shoreface system tis shs consists of a complex interlayering of cross bedded sandstones thin bedded conglomerates mudstones and rippled sandstones trough cross bedded pebbly sandstones and thin to thick bedded conglomerates represent the alluvial system alls depositional systems in the ug are associated in transgressive and highstand systems tracts that make up four sequences 1 to 4 sequence boundaries do not correspond with lithostratigraphic boundaries but are defined by subtle unconformities the basal q1 p1 unit lower sequence 1 consists of alls to tis shs to iss comprising a transgressive systems tract a maximum marine incursion is reflected by deposition of oss facies in stratigraphic units p1 p2 shoaling in the transition from p2 to the uppermedial portion of q2 oss iss shs to a thick tis shs alls records the highstand systems tract of upper sequence 1 a subtle disconformity paraconformity delineates a type 2 sequence boundary at the top of the highstand systems tract the drowning to shoaling pattern is replicated in sequence 2 upper q2 to p3 to upper medial q3 sequence 3 upper q3 to p4 to upper medial q4 and an incomplete sequence 4 upper q4 through p5 thinner shoaling intervals of oss iss shs in p3 and in lower q2 q3 and q4 represent parasequences sequences of 107 years duration are attributed to periods of increasing and decreasing subsidence rates due to tectonism marginal to the sedimentary basin parasequences record shorter duration temporal controls of c 104 to 105 years related to eustatic oscillations as a consequence of shoaling and aggradation progradation in the highstand systems tract tis shs and alls overlie and are temporally separated from oss to iss to shs this transition records filling of the basin to sea level leading to a shelf geometry that was conducive to tidal amplification a composite relative sea level curve integrating long term pulsatory subsidence and short term eustasy best explains the stratigraphic evolution of the uncompahgre group

pin stripe lamination a distinctive feature of modern and ancient eolian sediments abstract pin stripe laminations are a distinctive feature of modern and ancient eolian sediments in sets of eolian ripple or translatent strata they represent deposition of silt and very fine sand in the troughs of the advancing wind ripples in sets of avalanche strata they probably result from the downward settling of fine sand and silt within the moving avalanche to the interface of moving and unmoving sands wind tunnel experiments suggest that pin stripe laminations can also form in grainfall deposits the textural segregation associated with deposition of the fine layers in most cases leads to early cementation along and near the finest sand and silt comprising the pin stripe lamination the pin stripe effect seen in outcrops is usually due to resistance to weathering along such cemented zones the cementation of the pin stripe laminations can occur early in the history of diagenesis and thus may provide clues to the post depositional history of the rock pin stripe laminations in many instances represent the sequestering of the small population of ultrafine sediment present in most eolian depositional systems they may prove useful in the recognition of ancient eolian sediments

cherts of the barberton greenstone belt interpreted as products of submarine exhalative activity cherts studied from the southern barberton greenstone belt are identified as replacement products of various pillow lavas pyroclastic deposits and sedimentary deposits silica filled fracture systems within the cherts have been found to be associated with this silicification process these systems include 1 silica filled fractures 2 silica vein stockworks 3 silica cemented breccias and 4 pseudoconglomerate bodies some of the silica filled fracture systems can be shown to have formed soon after sedimentation silica vein stockworks can be seen to develop laterally into silica cemented breccias the fragments of which have sometimes become rounded within the fracture systems to form pseudoconglomerate the cherts are interpreted as silicified hemi pelagic sediments that overlie pillow lavas some of which are now also cherts cells of convective sea water interacted with oceanic crust to produce silica rich fluids these fluids silicified lava and sediments on re entering the near surface env

continental lithosphere folding in central asia part i constraints from geological observations abstract geological observations support the occurrence of long wavelength surface undulations in central asia which can be treated in terms of folding of the whole lithospheric plate the characteristic wavelength of the whole lithosphere folding is 360 km in the western part of the gobi sub plate superimposed on the long wavelength deformation two shorter wavelength scales of spatially periodical deformation are also observed an intermediate wavelength of 30 50 km of the intraplate deformation reflects the response of the mechanically strong upper crustal part of the central asian lithosphere small wavelength 4 9 km deformation corresponds to faulting in the uppermost crustal layer and folding and faulting in the sedimentary cover folding of the lithosphere controls the location and geometry of compressional basin formation in the western part of the gobi sub plate a number of folding and faulting characteristics of continental intraplate deformation in central asia presented in this study are strikingly similar to observed styles of intraplate deformation in oceanic lithosphere in the northeastern indian ocean

morphology and genesis of asymmetric adhesion warts a new adhesion surface structure abstract adhesion surface structures have been studied during their formation on a fluvial bar in east greenland two main types occurred adhesion ripples and asymmetric adhesion warts adhesion ripples formed on moist surfaces their crests lay transverse to the wind direction and they migrated by trapping dry wind blown sand on their steep fronts asymmetric adhesion warts new structure formed because of falling moisture content by preferred upwind migration of small protuberances on the adhesion ripples the protuberances were apparently inherited from an initial rain sculpturing of the bar surface the asymmetric adhesion warts here described for the first time were elongate parallel to the wind associated with steep upwind facing fronts and commonly displayed sand shadow tails tapering in a downwind direction a study of devonian flood basin deposits hornelen basin norway revealed the existence of adhesion surface structures very similar to their modern analogues the devonian examples were associated with rain sculptured surfaces which are believed to have controlled the morphology of the adhesion surface structures as in the modern example the orientation of the ancient adhesion surface structures is here used for determination of the palaeowind which blew from the ene

mechanism of subsidence of ancient cratonic rift basins cratonic basins commonly occur over ancient rift zones these inactive rift basins are recognizable by a positive linear bouguer gravity anomaly that may correspond to the axial gravity high found in modern rift valleys many of these basins undergo discrete periods of increased subsidence rates or reactivations long after the mass excess responsible for the linear gravity high was emplaced furthermore the reactivation of many cratonic basins occurs simultaneously with large scale compressional tectonics it is suggested that the driving force for subsidence is the isostatically uncompensated ancient mass excess the subsidence of these basins is modelled by a lithospheric flexure model with a nonlinear maxwell viscoelastic rheology solutions to this model indicate that basins may experience a low subsidence rate throughout geologic time the subsidence of a basin will stop only when isostatic compensation of the mass excess is achieved since ancient rift mass excesses may be uncompensated over long geologic time intervals the early thermal and structural evolution of rifts may not significantly influence later basin subsidence the models suggest that basins may be reactivated by any mechanism which lowers the effective viscosity of the lithospheric material allowing the uncompensated basin to settle toward an isostatic compensation depth faster than normal since viscosity is a strong function of temperature reactivation by a world wide increase in heat flow is suggested as a possible mechanism for the synchroneity of basin subsidence throughout a continent an increase of 15 in the geothermal gradient for example from 16 5 18 9 k km will cause about a 5 increase in subsidence this increase in heat flow however seems unlikely of producing by itself the magnitude of basin subsidence during a reactivation phase that is observed in the geologic record where up to 100 increase in subsidence might occur since the rheology of the lithosphere is nonlinear effective viscosity is also a strong nonlinear function of stress the presence of a regional compressive stress during periods of tectonism of 1 1 108 pa about 2 8 of the buckling strength of the lithosphere produces a short period of reactivated subsidence 105 yr during the reactivated subsidence the newly imposed regional stress relaxes sufficiently in the lower lithosphere to restore the effective viscosity to values similar to that before reactivation this suggests that reactivated subsidence caused by regional compression can be maintained as long as the stress level remains high in the lower lithosphere this may be accomplished by an intermittent application of the regional stress over time

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

evidence for the transition to an oxygen rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern africa abstract strata 2 5 to 1 8 ga in southern africa appear to span the transition from a non oxidizing atmosphere to one characterized by free oxygen red beds in 2 3 to 2 1 ga unconformity bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks ironstones or banded iron formations which evidently were sources of iron for the matrices however red beds in 1 9 ga unconformity bounded sequences not only matrix staining but also hematitic coatings on grains these beds also have far fewer interstratified ferruginous lithologies because red beds with stained clay matrices banded iron formations pyritic witwatersrand type conglomerates and iron depleted paleosols all occur in the 2 2 ga pretoria group formation of these lithologies may not have been time dependent until atmospheric oxygen approached 10 2 atm by 1 9 ga atmospheric oxygen was 10 2 atm causing lateritic weathering and red beds with hematitic coatings on grains

sediments and processes on a small stream flow dominated devonian alluvial fan shetland islands abstract the main conglomerate type of a small devonian alluvial fan in shetland northern britain is an inverse to normally graded framework supported gravel the sedimentological details of these beds and fundamental considerations of the mechanics of movement of highly concentrated flows suggests that neither debris flow nor grain flow were responsible for the deposition of these conglomerates on the contrary these inverse to normally graded conglomerates were deposited by water as thick gravel sheets with little topographic relief but broadly analogous to longitudinal bars they were deposited under high aggradation conditions first on the rising and then on the falling flood the high concentration of material in transport on the rising and peak flood was responsible for the polymodal and unstratified nature of the conglomerates as the flood waned normal grading was developed and a preferred horizontal fabric was produced under low concentrations of sediment other conglomerate types comparatively of lesser importance were deposited from highly concentrated clast dispersions and are notably richer in matrix and locally possess inverse grading the hydrological environment was one of flashy discharges of short duration but of high velocity flow was dissipated rapidly perhaps due to extreme water loss this example from the devonian of shetland provides an interesting alternative to the mass transport dominated fan models currently in vogue

relationship between the montgomery lake and hurwitz groups and stratigraphic revision of the lower hurwitz group district of keewatin recent detailed mapping has resulted in a possible solution to a long standing stratigraphic controversy as to whether the contact between the montgomery lake group age uncertain and the hurwitz group paleoproterozoic is an unconformity in its type area the montgomery lake group consists of mainly immature litharenite and lithic wacke and is separated from underlying archean greenstones and the overlying hurwitz group by angular unconformities in the padlei belt a unit previously referred to as montgomery lake sedimentary rocks montgomery lake group or map unit x consists of mainly submature to mature subarkose to quartz arenite with interbeds of pyritic auriferous and nonpyritic locally red quartz pebble conglomerate these rocks differ from the montgomery lake group in its type area but bear strong lithologic and stratigraphic resemblence to rocks in the lower hurwitz group furthermore interfingering between map unit x and hurwitz group lithotypes suggests that map unit x is an

sandy high energy flood sedimentation some criteria for recognition with an example from the devonian of s w england abstract many modern sand deposits resulting from episodic floods in semi arid zones are dominated by parallel laminations but parallel laminated sands are not a major feature in perennial streams single flood events may deposit over 1 5 m of parallel laminated sand hence ancient parallel laminated sand dominated alluvial deposits may be the product of ephemeral flows the trentishoe formation hangman sandstone group middle devonian of north devon consists in part of laterally extensive beds of parallel laminated fine grained red sandstones that build multistorey sand bodies individual beds are little more than 1 m thick but may extend laterally for hundreds of metres a vertical sequence of erosion surface sparse silt clasts parallel laminations silt drape is often found suggesting high flow stage deposition of plane bedded sand followed by a very rapid waning of flow with silt fallout in the final stages of the flood this sequence bears marked similarity to certain modern ephemeral flow sands

tectonic cycles in southern africa abstract the recurrence of major geological events within the 3600 ma stratigraphic record of southern africa suggests a periodicity of320 25ma for at least the last 3200 ma this periodicity is reflected in major episodes of continental volcanism granitoid plutonism orogeny reciprocity of terrestrial and marine dominated sequences platform carbonate sedimentation glaciation and first order changes in sea level these serve to define 11 completed megacycles each introduced by a major rifting event accompanied by listric faulting alluvial sedimentation in restricted fault bound troughs volcanism and a type granitoid plutonism prior to 3200 ma continental rift magmatism occurred subaqueously due to very high early archean sea levels resulting in pillow lavas and a lack of interbedded terrigenous siliciclastics the long term reciprocity of terrestrial and marine dominated sequences records eustatic first order changes in sea level with highstands characterized by generally humid warm continental climates low relief and tropical afforestation on the other hand lowstand climates were hot and arid with high relief and widespread desertification thick marine sequences accumulate in areally extensive epeirogenic basins without significant volcanism or faulting and commonly are divided into an early phase of open shelf sedimentation followed by deposition in a brackish marine foreland basin within openshelf environments an early phase of biochemical precipitation and or siliciclastic reworking along linear shorelines is separated from a later phase involving rapid deposition of immature siliciclastics along lobate shorelines evolution of the foreland basin is related to orogenic activity and the isolation of an epeiric sea which sometimes developed the characters of a barred basin marine dominated sequences display a deterministic stacking of lithofacies important glacial epochs commonly occur midway through first order regression sometimes with lesser events closely following peak transgression potentially they display 320 ma periodicity but with an unexplained mid proterozoic gap glaciation is related to the combined effect of climatic cooling associated with first order regression and depletion of atmospheric co2 by biological uptake associated with platform carbonate development terrestrial volcanism is believed to be the main contributor of atmospheric co2 climatic change is accompanied by cyclical patterns of cratonic weathering and pedogenesis

iceberg drop dump and grounding structures from pleistocene glacio lacustrine sediments scotland abstract dropstone dump and grounding structures resulting from the melt out of debris contained in floating and grounded icebergs are described from a pleistocene ice marginal glacio lacustrine sequence in scotland dropstones display bending penetration rucking and complete rupture of stratum occurring beneath them and onlap above them with the degree of deformation varying as a function of the size shape and axial disposition of the clast and of the sediment type into which they fell dump structures are conical mounds of gravel or diamict formed by the break up and overturning of dirt laden icebergs and the consequent release of large quantities of debris to the lake floor grounding structures are caused by the grounding of icebergs the down warping of underlying lake floor se iment and the subsequent in situ melt out of contained debris to form isolated troughs of diamict

development of the late proterozoic to mid paleozoic intracratonic amadeus basin in central australia a key to understanding tectonic forces in plate interiors the intracratonic amadeus basin in central australia is a complex composite basin covering 17 000 km with at least nine distinct episodes of evolution between 900 and 300 ma these have been identified in neighboring central australian basins and are characterized by intervals of renewed subsidence followed by intervals of erosion and changes in basin shape several unconformities separating the tectonostratigraphic sequences represent periods of mild regional uplifts and correlate with major episodes of compressive tectonism at the evolving margin of the early australian plate in the absence of direct geological evidence for thermal events in the amadeus basin after the initial period of subsidence ended possibly at about 800 ma the correlations found between events in the amadeus basin those in the neighboring interconnected basins and tectonic events at the continental or plate margins emphasize that regional horizontal stress fields contribute significantly to primary basin forming mechanisms this suggests that stresses generated at continental or plate margins may propagate to the plate interior with the lithosphere acting as a stress guide throughout the history of the amadeus basin both periods when compressional stresses were dominant and when extensional tectonics controlled subsidence can be identified there were also transitional periods when neither compression nor extension dominated so that other processes masked the effects of weak horizontal stresses the most recent compressional thrust belt concentrated at the northern margin between 300 and 400 ma and has partly obscured the record of earlier basin shape and size

intraplate stresses and the subsidence history of the sirte basin libya abstract results of quantitative analysis of subsidence data from 20 wells covering six major basement blocks from the sirte basin libya are presented following the late cretaceous onset of extension and associated collapse of the sirte arch the tectonic history post dating this event can be separated into four distinct phases 1 1 minor subsidence during the late cretaceous 2 2 differential fault tectonics and reactivation of basement faults during latest cretaceous to paleocene 3 3 regional uplift and subsidence during the eocene 4 4 minor subsidence during the early oligocene and early miocene a late cretaceous paleocene stretching phase of finite duration can explain the overall patterns of observed tectonic subsidence during the syn and post rift evolution of the sirte basin strong short term deviations from thermally induced long term subsidence observed during the eocene can be explained by intraplate compression and subsequent stress release related to changes in atlantic flowline geometry modelling of the observed tectonic subsidence for the sirte basin yields estimates of extension in excess of estimates inferred from displacements along high angle normal faults the effect of late stage intraplate compression on tectonic subsidence may cause part of the observed discrepancy between estimates of crustal extension

origin of cratonic basins tectonic subsidence curves show that the illinois michigan and williston basins formed by initial fault controlled mechanical subsidence during rifting and by subsequent thermal subsidence thermal subsidence began around 525 ma in the illinois basin 520 460 ma in the michigan basin and 530 500 ma in the williston basin in the illinois basin a second subsidence episode middle mississippian through early permian was caused by flexural foreland subsidence in response to the alleghanian hercynian orogeny resurgent permian rifting in the illinois basin is inferred because of intrusion of well dated permian alnoites such intrusive rocks are normally associated with rifting processes the process of formation of these cratonic basins remains controversial past workers have suggested mantle phase changes at the base of the crust mechanical subsidence in response to isostatically uncompensated excess mass following igneous intrusions intrusion of mantle plumes into the crust or regional thermal metamorphic events as causes of basin initiation cratonic basins of north america europe africa and south america share common ages of formation around 550 to 500 ma histories of sediment accumulation temporal volume changes of sediment fills and common dates of interregional unconformities their common date of formation suggests initiation of cratonic basins in response to breakup of a late precambrian super continent this supercontinent acted as a heat lens that caused partial melting of the lower crust and upper mantle followed by emplacement of anorogenic granites during extensional tectonics in response to supercontinent breakup intrusion of anorogenic granites and other partially melted intrusive rocks weakened continental lithosphere thus providing a zone of localized regional stretching and permitting formation of cratonic basins almost simultaneously over sites of intrusion of these anorogenic granites and other partially melted intrusive rocks

anorogenic granites magmatic underplating and the origin of intracratonic basins in a non extensional setting abstract we present an active rift model for cratonic basin formation with crustal kinematics governed by basaltic underplating a case study of a deeply eroded continental rift belt in south finland shows close association of anorogenic intrusives and remnants of cratonic basins lower crustal thinning took place during anorogenic magmatism whereas the upper crust experienced vertical block faulting in a laterally neutral deformation facies combined analyses of field data petrological constraints and geophysical evidence allow the reconstruction of the kinematic scenario of crustal thinning this model involves the formation of an incipient crustal dome by asthenospheric upwelling associated with basaltic underplating and migration of induced lower crustal partial melts to upper crustal levels in this model light lower crustal diorite is replaced by a denser mafic layer the consequences for the post magmatic crustal history are evaluated by numerical modelling of the effects of crustal densification subsidence occurs in the absence of crustal extension suggesting that the magmatic underplate is the key element for the formation of an intracratonic basin subsidence curves for this model show an incipient nearly linear fast downwarp related to crystallization of gabbro at the moho followed by slow contraction induced by solid state cooling the predicted subsidence is similar to the pattern of the vertical movement induced by passive extension in the proposed model of continental rifting the effects of extension and underplating largely cancel out concurrent crustal extension will cause the removal of a magmatic underplate by promoting basaltic extrusion the model provides a consistent explanation for a number of key aspects of cratonic basins including their circular shape crustal configuration and evolution and their spatial distribution on continents

initiation of 2 45 2 1 ga intracratonic basin sedimentation of the hurwitz group keewatin hinterland northwest territories canada continental to shallow marine rocks of the hurwitz group exposed as outliers across the hearne province in northern canada were deposited between 2 45 and 2 11 ga on a 3 3 2 6 ga granite greenstone basement initiation of the hurwitz basin was characterized by low relief and rates of subsidence that never progressed beyond the intensity of regional sagging during stage 1 polymictic conglomerate lower noomut formation was deposited in palaeovalleys cut into archaean basement and subarkose to quartz arenite quartz pebble conglomerate upper noomut formation was deposited on a low relief sand plain abundant first cycle quartz rich sandstones are attributed to intense humid climate weathering in low relief source areas and at sites of temporary storage as well as to repeated fluvial and eolian reworking interbedding of pyritic palaeoplacers and continental redbeds denotes fluctuating levels of atmospheric oxygen disappearance of pyritic palaeoplacers upsection signifies ultimate changeover to an atmosphere with sustained free oxygen stage 2 padlei formation was marked by a climatic shift toward cold glacial conditions as indicated by dropstones greywacke till pellets and possible ikaite pseudomorphs in lacustrine rhythmites stage 3 kinga formation represents a return to wet warm conditions with fluvial eolian deposition of subarkose and quartz arenite on a low relief sand plain maguse member and deposition of supermature quartz arenite in a vast 100 000 km2 minimum shallow water lake or series of lakes whiterock member local chert beds at the top of the kinga formation hawk hill member formed by spring discharge due to regional gravity driven convective groundwater flow an intracratonic setting is implied by 1 a symmetric basin centred fill 2 absence of features indicating a shelf slope break 3 absence of rift rocks 4 a prolonged slow subsidence history 5 likely correlation with other continental to shallow marine deposits scattered across the keewatin hinterland and 6 positioning 1000 2000 km away from closest coeval passive margin sequences we speculate that hurwitz basin formed due to lithospheric stretching related to a large scale mantle upwelling during the protracted breakup of an earliest palaeoproterozoic supercontinent kenorland

hydrological disturbances of the conductive heat flow in stable continental crust abstract geophysical data generally support the presence of free aqueous fluids in the crust to at least the depth of the 700 750 c isotherm where granulite facies conditions are reached which are presumed to have no free water the fluid phase distribution and transport of fluid in rocks strongly depend on the rheological conditions in the lithologically heterogeneous crust changes in rheological regime can produce quite complex sandwich like fluid transport layering including domains of upward migration very impermeable layers or thin reservoirs with nearly horizontal fluid transport in the present study two two dimensional thermal models of the stable continental crust with different spatial configurations of fluid systems have been calculated the results show that such systems redistribute heat in the crust and give rise to zones of concentrated heat energy storage and transfer the pattern of the disturbances to the conductive heat flow depends on the arrangement of the fluid systems which is a function of the crustal structure temperature and regional stress field and shows significant lateral and vertical variations even at very low velocities of the fluid percolation a few millimetres per year the disturbances to the conductive heat flow may reach 30 60

metamorphic alteration of a precambrian 2 2 ga paleosol from south africa revealed by backscattered electron imaging abstract like many precambrian paleosols the 2200 ma old waterval onder clay has been metamorphosed to the greenschist facies of regional metamorphism the extent of metamorphic alteration of the fine grained uppermost horizons of the paleosol is assessed here using optical and backscattered electron microscopy in hand specimens and under low power optical examination the paleosol shows surficial lamination cumulic horizon of soil terminology rounded clasts coated with opaque sesquioxides manganoferriargillans cracks defining a fitted breccia of blocks that were evidently soft in places blocky angular peds cracks filled with multiple generations of material from above illuviation argillans and zones of unusually highly birefringent clay climobimasepic plasmic fabric with backscattered electron microscopy however at a resolution of less than about 10 m the fine structure of originally colloidal parts of the paleosol mangans argillans matrix and plasma separations is shown to be extensively recrystallized to illite and berthierine original soil structure has been obscured at this resolution by crystal growth at random across original microscopic discontinuities preferred orientation of metamorphic crystals can still be seen along what are presumed to be highly birefringent zones of oriented clay similar to those in modern soils but the matrix itself shows random clusters of crystals suggestive of an advanced stage of illitization by ostwald ripening the style of recrystallization of laminae in the surface horizon is similar to that seen lower in the profile where recrystallized beds are thicker and coarser grained thus the paleosol probably formed on an upward thinning and fining sequence of sandstone and shale rather than on basalt on this assumption the profile has gained iron compared with its shaly parent material rather than lost iron compared with basalt this may be an indication of greater oxygenation of the atmosphere than previously thought possible microbial traces can be seen in thin section but because of pervasive recrystallization these remain dubiofossils

nonglaciogenic deltaic deposits in the early proterozoic gowganda formation cobalt basin ontario the gowganda formation in the eastern part of the cobalt basin comprises the coleman member and the conformably overlying firstbrook member the firstbrook member differs from the coleman member in that its lithofacies display i marked lateral continuity ii abundant diagenetic red colouration iii evidence of tidal deposition and iv a lack of glaciogenic features the firstbrook member is here divided into a mud dominated lower facies a silt dominated middle facies and sand dominated upper facies these are respectively equated with the prodelta delta slope and delta foreslope subenvironments of a prograding river dominated deltaic wedge composed of coalescing deltas probably braid deltas the deltaic wedge was likely fed by a fluvial system analogous to the braidplain system that prevailed during deposition of the overlying lorrain formation uninterrupted delta progradation is suggested by a single coarsening upward sequence up to 535 m thick deposition probably occurred within a marine

the fluvial geochemistry and denudation rate of the guayana shield in venezuela colombia and brazil abstract the guayana shield is composed of early to mid precambrian igneous and metamorphic basement rocks with a quartzitic platform cover the complete absence of limestones and evaporites allows a clear chemical expression in the stream data of the primary weathering of the basement in a humid tropical environment total erosion rates are extremely slow 10 m m y with equal contributions from the dissolved and suspended loads however the former is largely silica with ratios of si to total cation equivalents si tz ranging to in excess of three weathering is complete to kaolinite and gibbsite i e the environment is one of active laterisation with a penetration rate of the weathering front into the fresh substrate about twice the denudation rate in basins of relatively homogeneous lithology rb sr isochrons constructed from the river data agree with the whole rock ages from the drainages thus all the common refractory rb containing minerals k feldspar mica are completely dissolved the thick lateritic regolith that is accumulating as a result of this intense weathering is a common relict feature on other southern hemisphere shields in the absence of active tectonics or greatly accelerated mechanical erosion the weathering rates of these basement rocks must be quite insensitive to environmental change

tectonic cycles and the history of the earth s biogeochemical and paleoceanographic record an integrated approach to earth history for the last 2 5 b y reveals the strong interdependence of all geologic variables three earth history components are identified and interrelated secular trends as the earth ages recurring events of a previously proposed supercontinent megacycle worsley et al 1984 1985 and nonrecurring environmental shifts declining secular trends are endogenic heat production atmospheric carbon dioxide levels and surface temperatures continental differentiation enzyme efficiency atmospheric oxygen levels and solar luminosity are increasing secular trends four phases of a supercontinent cycle supercontinent fragmentation maximum continent dispersal continental assembly and supercontinental stasis recur with a 0 5 b y period and are correlated with tectonism cratonic sediment preservation atmospheric and hydrospheric evolution and the distribution of marine platform stable isotopes these correlations reflect coupling among the various oceanic and continental constituents during consecutive supercontinent assemblies and fragmentations nonrecurring shifts are unique and irreversible events in earth history e g development of photosynthesis occurrence of banded iron formations and deposition of detrital uraninite that represent periods of rapid geochemical adjustment to biospheric evolution earth s tectonobiogeochemical history is then synthesized by integrating secular trends and nonrecurring events with the supercontinent cycle emphasizing the preferentially preserved periods of fragmentation and continental dispersal paleoceanographic consequences of the megacycle are considered to reflect the stepwise coupling of secular trends and nonrecurring events to a biotically controlled phosphorus organic carbon climate linkage required to maintain earth s post archean surface temperature

geothermal mineralization i the mechanism of formation of the beowawe nevada siliceous sinter deposit the siliceous sinter deposit at beowawe nev has a volume of 7 71 x 10 sup 7 m sup 3 and contains 1 28 x 10 sup 11 kg silica this silica precipitated from geothermal solutions that welled up along the malpais fault a graben bounding fault sinter formation involved three steps 1 geothermal solutions saturated with quartz in the reservoir moved to the surface where they cooled and became supersaturated with amorphous silica 2 amorphous silica particles nucleated heterogeneously to produce a colloidal suspension 3 the amorphous silica particles agglomerated and were cemented together by amorphous silica precipitating in the embayments between the particles at least 5 58 x 10 sup 14 kg of geothermal solution must have been produced over time to account for the mass of silica in the beowawe deposit this fluid would have carried at least 4 95 x 10 sup 17 kj of heat to the surface 21 references 6 figures 1 table

comparison of the bedform distribution and dynamics of modern and ancient sandy ephemeral flood deposits in the southwestern karoo region south africa abstract during january 1981 unusually high rainfall patterns in the semi arid southwestern karoo region of south africa gave rise to catastrophic floods in the area the most devastating of which was the ephemeral flash flood that partially destroyed the town of laingsburg flood waters at laingsburg spread from a dry river bed rising to a depth of 12 m within 7 h washing away houses and burying the town with up to 3 5 m of sand and silt facies sequences and bedforms in the laingsburg flood deposits are related to specific subenvironments and flow conditions during the flood event two large scale sedimentary units in erosive contact with each other were deposited by two major discharge pulses within the same flood event vertical and lateral variations in the bedform distributions reflect fluctuations in flow velocity and turbulence comparison is drawn between the laingsburg flood deposits and fluvial sandstones of the permian lower beaufort in the southwestern karoo the multistoried sandstone bodies composed mainly of a horizontally laminated lithofacies are interpreted as the products of a complex series of ephemeral flooding events individual storey units in the palaeochannel sequences are generally envisaged as the deposits of different floods however evidence from the modern flood shows that separate pulses of a single flooding episode can also produce a sequence of storey units bounded by erosive scours spectacular palaeosurfaces in the lower beaufort also have modern analogues in the laingsburg overbank deposits discussion on the preservation potential of delicate sedimentary structures and biogenic traces on these surfaces emphasizes the necessity for pulsatory deposition of bedload and suspended load material over short intervals of time by comparing facies profiles and surface bedforms the ephemeral fluvial facies model for the lower beaufort is critically appraised

ikaite caco36h2o precursor of the thinolites in the quaternary tufas and tufa mounds of the lahontan and mono lake basins western united states the thinolites in the quaternary tufas and tufa mounds of the lahontan and mono lake basins of the western united states are identified as being pseudomorphous after crystals of the mineral ikaite caco36h2o ikaite is a metastable mineral in the sedimentary realm it crystallizes from solution at temperatures close to zero celsius but rapidly decomposes at warmer temperatures and becomes pseudomorphed in calcite by in situ redistribution of the calcium and carbonate ions the likely implication is that the thinolites record ice cold episodes during deposition of the tufas

eolian action and the distribution of cambrian shales in north america prolonged eolian action on the barren prevegetation craton provides a better explanation for the absence of shale in the cambrian and lower ordovician orthoquartzite carbonate suite of east central and arctic north america than does marine bypassing it is hypothesized that paleo trade winds transported much of the missing silt and clay to the southern and western united states and southwestern canada while the remainder diffused radially off the craton under the influence of variably directed storm winds the distribution of shale in the sauk sequence supports this and indicates that the orthoquartzite carbonate suite of upwind regions is replaced by a shale carbonate suite in downwind areas

upper mantle reflectivity beneath the williston basin phase change moho and the origin of intracratonic basins cocorp consortium for continental reflection profiling deep reflection profiles crossing the northern williston basin show 1 dipping reflections within the crust of the underlying trans hudson orogen that extend downward into the upper mantle and 2 no clearly defined reflection moho in contrast lithoprobe canada9s national geoscience project profiles crossing the trans hudson orogen north of the basin show dipping reflections within the crust that terminate at a sharply defined subhorizontal reflection moho a reflection character typical of the interior of collapsed phanerozoic orogens we suggest that a crustal root produced during hudsonian collision was incompletely removed by normal postorogenic extension in the region now underlain by the williston basin this remnant crustal keel much later underwent eclogite facies metamorphism which overprinted a new nonreflective moho across the root and induced the subsidence of the williston basin

opaline cherts associated with sublacustrine hydrothermal springs at lake bogoria kenya rift valley an unusual group of cherts found at saline alkaline lake bogoria in the kenya rift differs from the magadi type cherts commonly associated with saline alkaline lakes the cherts are opaline rich in diatoms and formed from a siliceous probably gelatinous precursor that precipitated around submerged alkaline hot springs during a holocene phase of high lake level silica precipitation resulted from rapid drop in the temperature of the spring waters and possibly ph lithification began before subaerial exposure ancient analogous cherts are likely to be localized deposits along fault lines

proterozoic mineral deposits near plate margins of the archean wyoming province usa abstract proterozoic mineral deposits located near the margin of the archean wyoming province can be interpreted in a plate tectonic context the southern margin of the archean wyoming province is exposed in southeastern wyoming where early proterozoic island areas have been sutured to an early proterozoic passive margin developed during rifting of the archean wyoming craton mineral deposits include uranium thorium gold bearing quartz pebble conglomerate deposited during rifting stratiform sulphide in black shale of the foredeep basin anomalous platinum group element values in layered complexes of the island arcs and a variety of mineral deposits that formed during closure of the arcs and passive margin the eastern margin of the wyoming province is probably located under phanerozoic cover just east of the black hills of south dakota the black hills is a segment of the early proterozoic trans hudson orogen which is interpreted here as an area exhibiting evidence of opening and closing of an intracratonic basin in the black hills domes underlain by archean rocks are overlain by an early proterozoic clastic successions that preserves evidence of rifting these clastic successions are succeeded by marine quartzite which is in turn overlain by a eugeosynclinal succession containing iron formation graywacke and a variety of volcanic units the basal early proterozoic clastic succession contains uranium and gold bearing quartz pebble conglomerate formed during rifting the eugeosynclinal succession contains the homestake iron formation which is host to the early proterozoic homestake type gold deposits geologic and geochemical evidence can be marshalled to support either an epigenetic or syngenetic origin for the homestake type gold deposits if the gold deposits are syngenetic they may have formed in a foredeep basin developed during closure of the intracratonic basin

deep seismic profile across a proterozoic collision zone surprises at depth collisions between continents generate the world s most striking mountains the deep structure of these collision zones inferred from seismic reflection profiles1 3 can be equally dramatic if less well understood improved knowledge of the structure of ancient collisional belts now known to have developed through plate tectonic processes4 5 can provide important new insights into the mechanisms of crustal growth although many such belts have been partly destroyed by severe post collisional deformation6 7 or magmatism8 some have been preserved in the stable interiors of continents9 one example is the early proterozoic trans hudson orogen9 10 in western canada here we present seismic reflection images across the entire orogen which reveal a broadly symmetric structure with reflections linked to island arc rocks dipping beneath both bounding archaean cratons the observed reflection geometries imply a doubling of crustal thickness during the collision along crust penetrating faults the presence of a significant amount of archaean crust at depth the extent of which could not have been determined from surface mapping suggests the need for caution in inferring crustal growth rate from the surface area of juvenile crust11 12

rifting of africa and pattern of mantle convection beneath the african plate abstract the mesozoic and cenozoic tectonic setting of the african continent is characterized by an extensional stress regime as is evident from the evolution of the west central and east african rift systems the opening of the indian and atlantic oceans and the development of tethys domain as well as from the results of geophysical investigations the most prominent feature of the bouguer gravity anomaly map of africa is a great belt of negative bouguer anomalies which extends across the entire african continent from ethiopia in the east to benguela in the west in conjunction with the red sea spreading zone this belt appears to encircle a centre in cameroon it probably corresponds to a distinct zone of lithospheric thinning and extension the entire african continent is characterized by higher than normal elevations it is located inside the african residual geoid high earthquake fault plane solutions confirm the extensional character of the present tectonic regime of eastern and southern africa where normal faulting predominates the average orientation of t axes is n s in south africa nw se in zambia mozambique and madagascar e w in kenya and ethiopia and ne sw in the red sea region from north to south the orientation of t axes rotates systematically in a clockwise sense by 130 and describes a radial pattern centred on equatorial west africa this indicates that the african lithosphere has been subject to continent wide radial extension around a hypothetical west african spreading centre referred to as the african spreading centre a asc a which coincides with the geographical centre of the african plate shear traction exerted by the convecting mantle on the base of the lithosphere is considered to be a major driving force of plate movements in the case of africa the continent wide regularities in the pattern of lithospheric extension the elevated position of the continent as well as the evolution of the mid oceanic ridge system surrounding africa provide distinct boundary conditions for the pattern of large scale mantle flow beneath the african plate it is proposed that relatively warm and less dense mantle material rises below africa forming a single mega plume or a number of plumes at the base of the african lithosphere the ascending material diverges and flows radially away from the centre of ascent corresponding to the asc a this ascending flow forms part of a very large mantle convection cell which occupies the eurafrican hemisphere the descending branch of this convection cell is located at a distance of 80 90 from the asc a the mid oceanic ridges surrounding the african continent are located between the upwelling and the downwelling branches of this convection cell at a distance of 50 60 from the asc a upper mantle flow below the sea floor spreading axes is uni directional and horizontal and is directed away from the asc a

silica diagenesis origin of inorganic and replacement cherts abstract silicification of originally non siliceous sediments affects a wide variety of rock types and materials and ranges from minor to pervasive partial and minor chertification occur mostly in phanerozoic carbonates carbonate bearing sandstones evaporites and fossil wood the source of the silica is predominantly biogenic in petrified wood the silicification mechanism is a permeation or void filling process not a replacement in this example the sequence of silica phase transformation is the same as that in deep sea siliceous sediments in many silicified rocks particularly in certain carbonates the transformation sequence is different from that in radiolarites or diatomites the chemical environment and conditions of early diagenetic chert formation in shallow water carbonates are delineated by the general mixing model of knauth 1979 but remain unknown for most other types an exception are the flint nodules and bands of the english chalk a detailed geochemical study of the paramoudra flint structures by clayton 1986 provided remarkable insight into the replacement process seven different recurring silica fabrics have been recognized in chert replaced carbonates including equigranular microcrystalline quartz or microquartz and megaquartz and fibrous types chalcedony quartzine or length slow chalcedony lutecite zebraic chalcedony and microflamboyant quartz among the latter quartzine and microflamboyant quartz are common in but by no means restricted to chert replaced evaporites for which milliken 1979 recognized a sequence of seven quartz fabrics as a single criterion only anhydrite inclusions in megaquartz quartzine or microflamboyant quartz provide unequivocal evidence for an evaporite precursor the relative timing between silicification and well established diagenetic carbonate reactions shows that virtually all theoretically possible sequences occur chertification of carbonate host sediment thus may take place during early intermediate or late diagenesis and even during anchimetamorphism pervasive to complete silicification has been described in lacustrine pedogenic and hydrothermal volcanogenic rocks and occurs on the scale of individual beds members or entire formations it may affect some of the aforementioned rock types too for example carbonates the source of the silica is predominantly inorganic chert formation in these environments includes direct chemical silica precipitation from solution through a gel stage magadi type cherts result from the conversion of the hydrous sodium silicate magadiite into microcrystalline quartz which has occurred in east african rift valley lakes such as lake magadi in late pleistocene time they are closely related to directly precipitated inorganic cherts which have been observed in alkaline environments of playa lakes silcretes originate from weathering and soil forming processes under climatic and environmental conditions conducive to the formation of duricrusts and laterites in more humid climates however non weathering profile silcretes occur which have been distinguished from weathering profile silcretes four to five different silcrete fabric types have been established including the 1 grain supported or quartzitic 2 floating terrazzo 3 matrix albertinia and opaline and 4 conglomeratic types in volcanic edifices silicification related to hydrothermal activity occurs 1 along the ascent routes of the fluids vents within the volcanic complexes 2 in isolated ponds and depressions on the sea floor where the fluids discharge mostly in the median rift valley of mid ocean ridges and 3 in geothermal areas on land associated with spreading lineaments volcanic islands arcs transform faults or continental hot spots attention has focussed on the associated base metal concentrations and less on the silicification and accompanying iron enrichment processes the precambrian banded iron formations may be indirectly or directly related to hydrothermal volcanic processes the origin of these cherty iron formations certainly requires more than one genetic model and at present remains a major unresolved problem despite massive efforts to tackle it

the solubility and stabilization of ikaite caco3 6h2o from 0 to 25 c environmental and paleoclimatic implications for thinolite tufa we determined the solubility of ikaite from 0 to 25 c to model its saturation state in natural waters and test the hypothesis that it is the precursor of the calcite pseudomorphs in thinolite tufa of quaternary lake lahontan reversible solubility at buffered p co 2 yields the following expression for the dissolution constant of ikaite log k ikaite 0 15981 2011 1 t where t circ k and 0 circ c leq t leq 25 circ c derived standard state properties are delta g ikaite circ 2541 9 kjmol 1 pm 0 66 delta h ikaite circ 2973 4 kjmol 1 pm 1 02 s ikaite circ 306 6 jt 1 mol 1 pm 1 2 modeling shows that ikaite is undersaturated at all temperatures in seawater and in alkaline lakes but that it rapidly approaches saturation near 0 c its precipitation in near freezing marine sediments requires large additions of hco 3 to pore fluids from the diagenetic decomposition of organic matter its crystallization in tufas of alkaline lakes howeve

the subsidence of the williston basin a detailed comparison between the pre cretaceous stratigraphic record in the williston basin and the available information about phanerozoic sea level changes indicates that most stratigraphic breaks in the basin reflect eustatic control the basin appears to have been subsiding continuously and steadily about the same centre for most of the phanerozoic above a thick 55 km continental crust deposition kept up with subsidence and most of the sedimentary record was laid down close to sea level erosional breaks in the stratigraphic record may represent periods of removal of strata during episodes of low sea level a possible explanation of this essentially steady nonexponential subsidence record is that a mafic subcrustal body is undergoing transformation to eclogite

epithermal sinters of paleozoic age in north queensland australia silica sinters deposited from hot springs have been found associated with volcanic rocks of late devonian to late carboniferous age in north queensland australia these deposits have many features in common with modern hot spring sinters such as those of the taupo volcanic zone in new zealand they occur associated with subaerial volcanic rocks and are locally found with air fall tuffs and fluvial and lake deposits in which hydrothermal eruption breccias are common the associated rocks are extensively hydrothermally altered and are cut by silica veins showing characteristic epithermal vein textures the silica veins contain low salinity fluid inclusions trapped at epithermal temperatures and there is evidence of boiling textures preserved in the sinters are identical to those found in modern sinters they include columnar structures similar to bacterial stromatolites described from yellowstone national park as well as striated surfaces apparently resulting from silica deposition on filamentous algae plant fossils including oxroadia gracilis are abundant criteria applied to identify these ancient sinters may be applied to other possible sinter deposits and may provide evidence indicating the level of exposure of the former geothermal system which may be an important guide in exploration for epithermal precious metal deposits

early history of the michigan basin subsidence and appalachian tectonics geometries of cambrian to silurian stratigraphc sequences in the michigan basin record discrete episodes of basin centered subsidence separated by periods of regional tilting backstripping reveals irregular subsidence rates that argue against a simple thermal contraction model depositional facies architecture also reflects episodic subsidence patterns basin centered facies tracts dominating during subsidence reactivations these three lines of evidence indicate that subsidence cessations and reactivations characterize the early history of the michigan basin periods of episodic subsidence correlate temporally with orogenic events in the appalachians suggesting that reactivation of basin subsidence is related to tectonic activity we propose that appalachian orogenic activity caused the episodic subsidence of the michigan basin possibly through weakening of the lower crust and reactivation of a preexisting upper crustal isostatic imbalance

the 25 november 1988 saguenay quebec earthquake source parameters and the attenuation of strong ground motion the saguenay earthquake of 25 november 1988 occurred close to the southern margin of the saguenay graben in southern quebec it was caused by almost purely dip slip faulting centered at a depth of 26 km with a p axis oriented northeast southwest this faulting mechanism is similar to those of the larger historical earthquakes in eastern north america but the focal depth is substantially greater than all but one of these events the seismic moment estimated from regional pni waves and teleseismic long period body waves is 5 10 24 dyne cm corresponding to a moment magnitude of 5 8 the source duration of the earthquake is estimated to be 1 8 sec corresponding to a stress drop of 160 bars which is not significantly higher than the average stress drop of 120 bars estimated from previous large earthquakes in eastern north america in order to simultaneously match the recorded ground motion amplitudes of strong motion acceleration strong motion velocity and teleseismic short period and long period body waves it is necessary to use a source function having a complex shape that implies the presence of asperities and larger local stress drops the large set of strong motion recordings of the saguenay earthquake has been used to validate a procedure for estimating strong ground motion attenuation based on a simple wave propagation model the most important feature of the recorded strong motions is that their peak amplitudes do not decay significantly with distance inside 120 km but then decay abruptly beyond 120 km profiles of recorded accelerograms with absolute times indicate that at distances beyond 64 km the peak ground motions are due to strong postcritical reflections from velocity gradients in the lower crust the principal shear wave arrivals and the variation of their peak amplitudes with distance were reproduced in synthetic seismograms generated using a regional crustal structure model the critical distances for the postcritical reflections were short because of the deep focal depth of the event causing the elevation of ground motion amplitudes out to 120 km similar studies of earthquakes in other regions of eastern north america indicate that the strength of the postcritical reflections and the distance ranges over which they are dominant are controlled by the focal depth and crustal structure regional variations in crustal structure thus give rise to predictable regional variations in strong ground motion attenuation