palaeoenvironment reconstruction volcanic evolution and geochronology of the cerro blanco subcomplex nevados de chillan volcanic complex central chile nevados de chillan volcanic complex central chile has been active for at least 640 ka a period spanning a number of glacial and interglacial periods geologic mapping radiometric dating and geochemical analysis have identified six new volcanic units and produced four new 40ar 39ar ages for cerro blanco the northern subcomplex of nevados de chillan volcano compositions range from dacite to basaltic andesite and a new geologic map is presented examination of lava fracture structures on both newly mapped lavas and those mapped during previous studies has enabled interpretations of former eruptive environments palaeoenvironment reconstructions combined with 40ar 39ar ages and comparison with the marine oxygen isotope record show that at least three phases of volcanic activity have occurred during the evolution of cerro blanco 1 a constructive pre caldera collapse period 2 a period of caldera formation and collapse and 3 a constructive period of dome growth forming the modern day volcanic centre this style of volcanic evolution whereby large scale caldera collapse is followed by growth of a new stratocone is common at andean volcanoes

the age and origin of the central scotia sea summary opening of the drake passage gateway between the pacific and atlantic oceans has been linked in various ways to cenozoic climate changes from the oceanic floor of drake passage the largest of the remaining uncertainties in understanding this opening is in the timing and process of the opening of the central scotia sea all but one of the available constraints on the age of the central scotia sea is diagnostic of or consistent with a mesozoic age comparison of tectonic and magnetic features on the seafloor with plate kinematic models shows that it is likely to have accreted to a mid ocean ridge between the south american and antarctic plates following their separation in jurassic times subsequent regional shallowing may be related to subduction related processes that preceded backarc extension in the east scotia sea the presence of a fragment of jurassic cretaceous ocean floor in the gateway implies that deep water connections through the scotia sea basin complex may have been possible since eocene times when the continental tips of south america and the antarctic peninsula first passed each other

brazilian and african passive margins of the central segment of the south atlantic ocean kinematic constraints the thinning of passive continental margins is usually explained by models using pure stretching or simple shear these models imply hypothetical extensional structures and large horizontal movements between the two conjugate margins more than 250 km for the brazilian and angolan margins refraction reflection data together with the most recent and tightest pre opening fit using continental and oceanic geological and geophysical constraints show that the substratum of the sag basin is divided into an autochthonous part upper continental crust and an allochthonous part exhumed material the thinning process which evolves in an elevated position of the system until at least the break up seems to be depth dependent and to mainly concern the lower middle crust which we postulate is exhumed this exhumation does not explain the entire thinning of the system horizontal motions cannot alone explain the formation of the huge angolan brazilian basin similar observations are made on the whole central segment of the south atlantic ocean part of the lower continental crust is still missing and it seems most improbable that the continental crust maintains its integrity throughout the thinning process

importance of surface crust strength during the flow of the 1988 1990 andesite lava of lonquimay volcano chile 1 the emplacement of the 1988 1990 andesite lava of lonquimay volcano chile is examined using theoretical expressions for the relevant dynamical flow regimes the surface crust regime where the flow is controlled by the growth of a surface crust rather than by the rheology of the fluid interior is found to predict accurately the entire propagation of the lonquimay lava flow this discovery offers the exciting prospect of being able to predict the propagation of future blocky lavas simply from early observations of both the volume erupted and the flow length as functions of time without the need to know anything about the rheology of the interior lava

mesozoic and cenozoic stratigraphy eastern scotian shelf new interpretations a 320 km long deep seismic reflection line across the central part of the scotian basin provides new insight on the deep stratigraphy of the basin confirms the existence of up to 18 km of mesozoic and cenozoic sedimentary strata and provides additional control on the relationship of proximal and distal stratigraphic units within the basin detailed biostratigraphic and seismic correlation from three outer shelf upper slope wells yields important new information on late cretaceous and tertiary unconformities and identifies a number of third order sequence boundaries the dating of prominent unconformities and correlation with the oceanic seismic markers ac and au results in the reidentification of these horizons in the sohm abyssal plain

evidence for a mazama like tephra deposited ca 10 000 bp at copper lake banff national park alberta mazama tephra is a widespread mid holocene stratigraphic marker dating to ca 6845 bp and is considered useful for the correlation of holocene events in western north america we present evidence from copper lake alberta for a tephra layer that underlies mazama tephra and appears to be a discrete tephra layer but that is indistinguishable from the conventional mazama tephra by microprobe analysis of glass shards three alternative hypotheses are considered to explain this tephra underlying mazama in situ position of an earlier mazama like tephra recycling of tephras and settling of tephra slabs through the gyttja the size distributions of birch pollen grains indicate that the mazama like tephra is in primary stratigraphic position in the early holocene gyttja and that sediment recycling has not destroyed the integrity of copper lake sediment stratigraphy it is concluded that most probably a mazama like tephra was deposited between 9700 and 10 500 bp if this in situ hypothesis is correct the id

basin scale salt tectonic processes of the laurentian basin eastern canada insights from integrated regional 2d seismic interpretation and 4d physical experiments

pahoehoe and aa in hawaii volumetric flow rate controls the lava structure the historical records of kilauea and mauna loa volcanoes reveal that the rough surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate 5 10 m3 s and the smooth surfaced variety called pahoehoe forms at a low volumetric rate 5 10 m3 s this relationship is well illustrated by the 1983 1990 and 1969 1974 eruptions of kilauea and the recent eruptions of mauna loa it is also illustrated by the eruptions that produced the remarkable paired flows of mauna loa in which aa formed during an initial short period of high discharge rate associated with high fountaining and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate with little or no fountaining the finest examples of paired lava flows are those of 1859 and 1880 1881 we attribute aa formation to rapid and concentrated flow in open channels there rapid heat loss causes an increase in viscosity to a threshold value that varies depending on the actual flow velocity at which when surface crust is torn by differential flow the underlying lava is unable to move sufficiently fast to heal the tear we attribute pahoehoe formation to the flowage of lava at a low volumetric rate commonly in tubes that minimize heat loss flow units of pahoehoe are small usually 1 m thick move slowly develop a chilled skin and become virtually static before the viscosity has risen to the threshold value we infer that the high discharge rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers rapid near surface vesiculation of gas rich magma leads to eruptions with high discharge rates high lava fountains and fast moving channelized flows we also infer that long periods of sustained flow at a low discharge rate which favor pahoehoe result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway

measuring large topographic change with insar lava thicknesses extrusion rate and subsidence rate at santiaguito volcano guatemala lava flows can produce changes in topography of the order of 10s 100s of metres a knowledge of the resulting volume change provides evidence about the dynamics of an eruption using differential insar phase delays it is possible to estimate height differences between the current topography and a digital elevation model dem this does not require a pre event sar image so it does not rely on interferometric phase remaining coherent during eruption and emplacement synthetic tests predict that we can estimate lava thicknesses of as little as 9 m given a minimum of five interferograms with suitably large orbital baseline separations in the case of continuous motion such as lava flow subsidence we invert interferometric phase simultaneously for topographic change and displacement we apply this to santiaguito volcano guatemala and measure increases in lava thickness of up to 140 m between 2000 and 2009 largely associated with activity between 2000 and 2005 we find a mean extrusion rate of 0 437 0 06 m

a continuous holocene glacial record inferred from proglacial lake sediments in banff national park alberta canada abstract sediment cores from three proglacial lakes in northern banff national park alberta preserve a record of holocene glacial activity upvalley which is more continuous and better dated than available surficial records dating of the cores is based on tephrochronology and 16 ams14c ages of terrestrial macrofossils all cores contain a threefold sequence of lacustrine sediments overlying a late pleistocene diamicton basal lacustrine sediments 10 10014c yr old contain little organic matter sediment composition indicates a large glacigenic contribution a sharp increase in organic content marks the beginning of the altithermal interval at all three lakes this transition occurred abruptly at about 10 10014c yr b p at crowfoot lake and possibly more gradually at the other lakes altithermal sediments contain relatively little glacigenic material and during most of the altithermal glaciers may have been absent above crowfoot and bow lakes glaciers draining into hector lake appear to have persisted through the altithermal a subsequent decrease in organic content in each lake reflecting increased clastic sedimentation marks the end of the altithermal and the onset of neoglacial ice advances the transition took place between about 5800 and 400014c yr b p and may be time transgressive beginning earlier in hector lake than in crowfoot lake changing neoglacial clastic sedimentation rates through the neoglacial interval indicate two main periods of increased glacier extent between ca 3000 and 1800 varve yr ago ca 2900 190014c yr b p and during the last several hundred years during the intervening period glaciers were less extensive but much more extensive than during the recessions of the altithermal interval

growth of a young frequently active composite cone ngauruhoe volcano new zealand ngauruhoe cone in southern taupo volcanic zone new zealand has grown rapidly over the last 2 500 years in an alternation of effusive strombolian vulcanian and sub plinian eruptions of andesitic magma at times growth has been staccato in fashion as evidenced in the historical record each historical eruption typically lasted days to months alternating with repose periods of years to decades major historic eruptions occurred in 1870 1949 1954 1955 and 1973 1975 encompassing wide variations in eruptive style over short timescales the early period of cone building appears to have been dominated by a more continuous form of activity characterised by a series of numerous frequent explosive eruptions with associated lava flows the 2 2 km3 cone has grown in a piecemeal sectorial manner reflecting constant modification to the morphology of the summit which has funnelled eruption products to specific sectors of the cone eruption rates can be calculated on several different timescales discharge rates averaged over individual eruptive pulses vary by two orders of magnitude 2 7 280 m3 s 1 reflecting variations in high level magma ascent rates and processes such as degassing which are in turn reflected in contrasting eruptive styles lower rates e g 0 65 m3 s 1 are obtained by averaging the discharge over an entire eruption lasting several months and may correspond to the ascent rate of magma batch es feeding the eruption the long term growth rate of ngauruhoe is 0 9 km3 ky 1 this is an average rate reflecting the long term deep supply rate of magma to crustal reservoirs by looking at eruption rates on these different timescales we are better able to constrain processes occurring at various depths within the plumbing system there are few detailed studies of the growth patterns of young volcanic cones but such data are essential in understanding the dynamics of andesitic systems more than 60 lavas and pyroclastic units mapped on different sectors of ngauruhoe cone have been correlated by flow chronology and their distinctive compositions into five groups although the cone has grown rapidly ngauruhoe shows little evidence for the existence of large crustal magma reservoirs and long lived magma batches instead abrupt and non systematic changes in magma chemistry and isotopic composition between and within the five groups indicate that the volcano has an open system multi process multi directional character and erupts small 0 1 km3 and short lived 100 103 years magma batches with no simple time composition relationships between successive batches

salt structural styles and kinematic evolution of the jequitinhonha deepwater fold belt central brazil passive margin abstract salt structures in linked extensional contractional systems of passive margin sedimentary basins and deepwater fold belts play an important role in the evolution of rifted continental margins the 100 km wide jequitinhonha basin with its salt detached deepwater fold belt provides insight in the post rift salt deformation processes in deepwater fold belts of the central brazilian continental margin seismic interpretation and kinematic analysis of basin scale 2d sections provide insights into the salt tectonic processes related depositional systems salt basin geometry and kinematic evolution of the deepwater fold belt several salt kinematic domains have been identified the most evident is the long term contractional domain in the distal basin which formed impressive salt cored anticlines salt walls and salt massifs reaching 4 km in thickness the post rift basin evolution was dominated by thin skinned deformation and was initiated by differential loading of early post rift shallow marine carbonates subsequently the system was mainly driven by gravity gliding of the overburden on the aptian salt detachment the depocenters were controlled by early post rift salt mobilization and ranged from asymmetric grabens and rollovers to minibasins with vertically stacked or shifting depocenters and formed the basis of the deepwater fold belt architecture in the jequitinhonha basin the structural styles and kinematic evolution of the deepwater fold belt were strongly controlled by the original salt thickness the distribution of albian campanian depocenters and thickness variations in the overburden sediments early cretaceous short wavelength folds have been overprinted by long wavelength folding owing to increased strength and thickness of the overburden since paleogene times large amplitude salt cored folds developed in the deepwater basin due to contraction of pre existing salt walls and formed margin parallel nnw sse trending narrow structural highs separated by wide depocenters ongoing uplift with reduced sediment accumulation over contractional diapirs or crestal erosion of the deepwater anticlines supported long term strain localization and positive feedback in the active deepwater contractional deformation

cross section restoration of salt related deformation best practices and potential pitfalls abstract cross section restoration typically assumes plane strain deformation and area conservation constraints that are usually invalid for salt because of its characteristic three dimensional flow and possible dissolution thus restoration of salt related deformation provides added challenges and uncertainty in this review paper we summarize the historical development of ideas methods and applications of restoration in salt basins while most published restorations do not maintain salt area constraints on its variation range from arbitrary assumptions to quantitatively incorporating isostatic calculations we illustrate several scenarios in which the presence of salt adds ambiguity to restoration primarily because it can hide deformation diapirs can widen during extension and narrow during shortening translating overburden can move into salt and drive allochthonous advance secondary minibasin subsidence can be accommodated at both shallow and deep salt levels and allochthonous salt can record evacuation of deeper salt although we caution against using restoration to test and validate small scale details of interpretations we emphasize that sequential restoration remains an essential tool in structural and basin analyses however because of the uncertainties a regional three dimensional approach and sound geological reasoning are critical for deriving meaningful and useful results from cross section restoration of salt structures

a new conceptual model for the structural evolution of a regional salt detachment on the northeast scotian margin offshore eastern canada in this study we examine using seismic data in conjunction with numerical modeling a regional scale salt detachment and associated synkinematic sediments from the scotian margin offshore eastern canada this part of the scotian margin is characterized by an up to 4 5 km 2 8 mi thick and approximately 175 km 108 mi long synkinematic wedge of jurassic sediments with internal sigmoidal landward dipping reflectors the synkinematic wedge is laterally extensive encompassing an area of approximately 30 000 km2 11 600 mi2 and soles into an interpreted salt detachment the jurassic synkinematic wedge which is interpreted to have formed as an open ended allochthonous salt nappe was loaded by prograding sediments during the jurassic this loading squeezed the salt seaward and caused the overlying sediments to undergo extension and gravity spreading and gliding detaching on the salt sheet the open ended nappe model provides a mechanism for producing a large amount of extension with very little compensating contraction numerical model results indicate that high rates of extension detaching on even a thin salt layer can result in similar sigmoidal landward dipping strata based on the numerical modeling and seismic interpretation results we propose a new conceptual model for the jurassic paleogene structural evolution of the study area this model may also have implications for other passive margin salt basins with regional salt detachments

a systematic technique for the sequential restoration of salt structures abstract a method is described for the sequential restoration of cross sections in areas of salt tectonics where deformation is confined to the salt and higher layers the subsurface geometry evolves with time through the interaction of various processes sedimentation compaction isostatic adjustment thermal subsidence if present faulting and salt withdrawal diapirism the technique systematically calculates and removes the effects of each of these processes during specified time intervals defined by the interpreted horizons it makes no assumptions about salt kinematics and generally results in the area of the salt layer changing through time the method is described for restoration of extensional terranes but it is also suitable for areas of contractional salt tectonics with only minor modifications after converting an interpreted seismic profile to depth the top layer is stripped off and the underlying section is decompacted according to standard porosity depth functions a deep baseline unaffected by compaction or deformation is used to restore any isostatic compensation or thermal subsidence isostasy is calculated according to the airy model and differential sedimentary loading across a section is shown to be approximately balanced by changes in salt thickness so that the load is evenly distributed after these processes have been reversed the resulting geometry and the seismic data are used to create the sea floor template for structural restoration fault offsets are removed and the layers down to the top salt are restored to this template while the base salt remains fixed the resulting space between the restored top salt and the fixed base salt defines the restored salt geometry in addition the difference between the sea floor template and a fixed sea level provides a measure of the change in water depth ignoring eustatic changes in sea level the technique is applied to an interpreted seismic profile from the eastern green canyon ewing bank area of offshore louisiana the section is characterized by a variety of salt structures including salt rollers a diapiric massif a remnant salt sheet and a salt weld which are shown to have derived from an originally continuous salt sheet which has been modified by sedimentary loading early loading created vertical basin growth that was accommodated primarily by salt withdrawal and associated diapiric rise through the process of downbuilding once the salt weld formed continued sedimentation was accommodated by a lateral increase in basin size caused by down dip extension on listric growth faults

eruptive history of la poruna scoria cone central andes northern chile new stratigraphic lithological and petrographic analyses of la poruna scoria cone 21 53 s 68 30 w central andes northern chile allow the reconstruction of the eruptive sequence of this monogenetic cone petrographic and lithological characteristics allow us to identify three main lithostratigraphic units at la poruna scoria cone the first unit consists of agglutinated lapilli spatter beds and clastogenic lavas that are related to the construction of the cone the other two units are associated with a lava flow field and consist of flow of andesitic composition which differ both in their degree of weathering and in their development of channel surface and internal structures e g levees ogives and joints with these lithostratigraphic analyses we interpret that the construction of la poruna occurred during four main eruption phases involving strombolian hawaiian and transitional eruptive styles furthermore differences in the degree of erosion alteration and weathering of the lithostratigraphic units in the lava field of la poruna suggest that this flow field was formed during two eruptive events the excellent outcrop conditions and preservation state of the volcanic products of la poruna allow new stratigraphic insights that advance the wider and more general understanding and the dynamics of this important type of volcanism and the potential hazards of a scoria cone eruption the polycyclic style of the eruption needs to be included in the hazards assessment of these centers type especially when the cone is associated with structures that can be reactivated this process could correspond to a second phase of activity involving ash fall bomb and lava emission

factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models 1 the perdido fold belt pfb is a prominent saltcored deep water structure in the northwestern gulf of mexico it is characterized by symmetric kink banded folds of a 4 5 km thick prekinematic layer and its vicinity to the extensive sigsbee salt canopy we use 2 d finite element numerical models to study the evolution of the pfb as a gravity driven fold belt both in a local context and in the context of the larger scale passive margin influenced by adjacent allochthonous salt structures we show that parameters such as overburden strength salt geometry or salt viscosity determine timing extent and location of the modeled fold belt simplified models of the gulf of mexico show that toe of slope folding is a viable mechanism to develop diapirs in the deep salt basin and to delay folding of the distal overburden in this scenario the pfb likely represents the terminal folding of a much larger diachronously formed fold belt system citation gradmann s c beaumont and m albertz 2009 factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models tectonics 28 tc2002 doi 10 1029 2008tc002326

the dynamics of a channel fed lava flow on pico partido volcano lanzarote a short length of channel on pico partido volcano lanzarote provides us the opportunity to examine the dynamics of lava flowing in a channel that extends over a sudden break in slope the 1 2 m wide 0 5 2 m deep channel was built during the 1730 1736 eruptions on lanzarote and exhibits a sinuous well formed channel over a steep 11 slope 100 m long proximal section over flow units comprising smooth pahoehoe sheet flow as well as evidence on the inner channel walls for multiple at least 11 flow levels attest to unsteady flow in the channel in addition superelevation is apparent at each of the six bends along the proximal channel section superelevation results from banking of the lava as it moves around the bend thus causing preferential construction of the outer bank as a result the channel profile at each bend is asymmetric with an outer bank that is higher than the inner bank analysis of superelevation indicates flow velocities of 8 m s 1 our analysis of the superelevation features is based on an inertia gravity balance which we show is appropriate even though the down channel flow is in laminar flow we use a viscosity gravity balance model together with the velocities calculated from superelevation to obtain viscosities in the range 25 60 pa s assuming that the lava behaved as a newtonian liquid estimated volume fluxes are in the range 7 12 m3 s 1 an apparent down flow increase in derived volume flux may have resulted from variable supply or bulking up of the flow due to vesiculation where the channel moves over a sharp break in slope and onto slopes of 6 the channel becomes less well defined and widens considerably at the break of slope an elongate ridge extends across the channel we speculate that this ridge was formed as a result of a reduction in velocity immediately below the break of slope to allow deposition of entrained material or accretion of lava to the channel bed as a result of a change in flow regime or depth

late quaternary history of the lake o hara region british columbia an evaluation of sedimentation rates and bulk amino acid ratios in lacustrine records lake o hara subalpine and opabin lake alpine are situated directly adjacent to a high section of the continental divide in the central canadian rocky mountains core samples recovered from the lakes show a consistent stratigraphy comprising gyttja and underlying inorganic clastic sediments the gyttja contains bridge river 2350 years bp and mazama 6800 years bp tephras and is separated from the lower clastic sediments by a sharp conformable contact radiocarbon dates obtained from conifer needles extracted from directly above the contact indicate that deglaciation had proceeded upvalley from the o hara basin priorto ca 10 100 years bp preliminary palaeobotanical and macrofossil data suggest that a pinus abies forest with lesser picea was established in the vicinity of lake o hara by this time consequently the minimum age of moraine systems situated downvalley from lake o hara is late wisconsinan mean annual sedimentation rates were derived from sediment thickness data from 14 lake o hara an

growth fault rollover systems birth growth and decay growth fault rollover systems are extremely common structures of thin skinned extensional systems resulting from gravity gliding above salt their development is studied using laboratory experiments based on brittle ductile models where the sedimentary cover and synkinematic sediments are simulated using brittle sand and the underlying decollement layer by viscous silicone putty the experiments are compared to natural examples from seismic lines the experimental results are used to present the evolution of growth fault rollover systems these being the result of a finite amplitude instability with three distinct stages birth growth and decay each of these stages is identified with typical structures which can be recognized on seismic sections a discussion is presented on the interactions in time and space between synthetic and antithetic listric faults parallel and transverse to the direction of displacement

spatial variations in lava flow field thermal structure and effusion rate derived from very high spatial resolution hyperspectral mivis data 1 high spatial resolution hyperspectral measurements of volcanic thermal anomalies allow for an unconstrained solution of a two component thermal model this can be used for identification of lava flow emplacement style and the calculation of lava flow heat and volume flux the multispectral infrared and visible imaging spectrometer mivis is an airborne sensor equipped with 72 bands in the short infrared range and 10 bands in the thermal infrared region of the spectrum we used mivis acquired for mount etna italy during the july august 2001 eruption to solve the dual band equations in an unconstrained fashion using three bands of unsaturated data our results suggest a complex thermal structure for etnean lava flows this is characterized by a downflow transition from a lightly crusted active channel to a more heavily crusted distal section both surrounded by zones of stagnant cooling flow where exposed molten material is absent and maximum temperatures are thus lower the total flow field effusion rate obtained for 29 july 2001 0700 local time of 8 16 m3 s is in excellent agreement with that obtained from ground based measurements and advanced very high resolution radiometer data flow by flow effusion rates obtained from the mivis data vary depending on whether the vent is linked to the central conduit or the dyke that was injected from greater depth as well as vent elevation with lower elevation vents experiencing higher effusion rates

salt tectonics driven by sediment progradation part i mechanics and kinematics using conceptual reasoning and results from physical models we describe the mechanical and kinematic characteristics of sediment wedges spreading seaward above a viscous evaporitic layer spreading can occur if the distal sediment overburden is thin and weak or if it comprises preexisting salt bodies spreading is accommodated by proximal extension and an associated rise of diapiric ridges by midslope seaward translation and by distal shortening rapid sediment progradation can lead to the reactivation of older distal folds by later extension either retrogradation or renewed sedimentation following a long depositional hiatus can reactivate older grabens and diapirs in shortening

salt structures and hydrocarbon accumulations in the tarim basin northwest china the tarim basin is one of the most important hydrocabon bearing evaporite basins in china four salt bearing sequences the middle and lower cambrian the mississippian the paleogene and the neogene have various thickness and areal distribution they are important detachment layers and intensely affect the structural deformation in the basin the kuqa depression is a subordinate structural unit with abundant salt structures in the tarim basin salt overthrusts salt pillows salt anticlines salt diapirs and salt withdrawal basins are predominant in the depression contraction that resulted from orogeny played a key function on the formation of salt structures growth strata reveal that intense salt structural deformation in the kuqa depression occurred during the himalayan movement from oligocene to holocene with early structural deformation in the north and late deformation in the south growth sequences also record at least two phases of salt tectonism in the yingmaili tahe and tazhong areas low amplitude salt pillows are the most common salt structures and these structures are commonly accompanied by thrust faults the faulting and uplifting of basement blocks controlled the location of salt structures the differences in the geometries of salt structures in different regions show that the thickness of the salt sequences has an important influence on the development of salt cored detachment folds and related thrust faults in the tarim basin salt sequences and salt structures in the tarim basin are closely linked to hydrocarbon accumulations oil and gas fields have been discovered in the subsalt intrasalt and suprasalt strata salt deformation has created numerous potential traps and salt sequences have provided a good seal for the preservation of hydrocarbon accumulations large and small scale faults related with salt structures have also given favorable migration pathways for oil and gas when interpreting seismic profiles special attention needs to be paid to the clastic and carbonate interbeds within the salt sequences because they may lead to incorrect structural interpretation in the tarim basin the subsalt anticlinal traps are good targets for hydrocarbon exploration

mesozoic break up of sw gondwana implications for regional hydrocarbon potential of the southern south atlantic abstract this work provides new palinspastic palaeofacies reconstructions of sw gondwana incorporating rotation of a falkland malvinas microplate we discuss the implications of this for the tectonic evolution of the southern south atlantic and hence for the regional hydrocarbon potential existing gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents in most continental reconstructions the falkland malvinas plateau was assumed to be a rigid fragment of pre permian south american crust however it has been suggested on the basis of palaeomagnetic data that the falkland malvinas islands were rotated by 180 after 190 ma this rotation hypothesis has been successfully tested on the basis of devonian stratigraphy and palaeontology permian stratigraphy and sedimentology and late palaeozoic and early mesozoic structure making it unlikely that the plateau behaved as a rigid structure during breakup we have explored the consequences of accepting this hypothesis for the tectonic evolution of sw gondwana by compiling new palaeogeographic maps for the permian cretaceous of the southern atlantic area to achieve a realistic close fit we have devised a pre rift proxy for the ocean continent boundary for the south atlantic in order to produce the best fit it is necessary to subdivide south america into four plates the consequences of this are far reaching our work suggests that although sedimentary basins were initiated at different times three major tectonic phases can be recognised in regional terms these can be thought of as pre syn and post rift during the pre rift time until the late triassic the area was dominated by compressional tectonism and formed part of the gondwana foreland the falkland malvinas islands lay east of africa the falkland malvinas plateau was 33 shorter and patagonia was displaced east with respect to the rest of south america in part along the line of the gastre fault system potential source facies are dominantly post glacial black shales of late permian age deposited in lacustrine or hyposaline marine environments these rocks would also be an effective regional seal sandstones deposited in the late permian would be dominantly volcaniclastic with poor reservoir qualities triassic sandstones tend to be more mature there was significant extension from about 210 ma end triassic until the south atlantic opened at about 130 ma early cretaceous in the early syn rift phase extension was accompanied by strike slip faulting and block rotation later extension was accompanied by extrusion of large volumes of lava early opening of the south atlantic was oblique which created basins at high angle to the trend of the ocean on the argentine margin and resulted in microplate rotation in ne brazil intermittent physical barriers controlled deposition of upper jurassic cretaceous anoxic sediments during breakup some of these mudrock units are effective seals with likely regional extent during crustal reorganisation clastic sediments changed from a uniform volcaniclastic provenance to local derivation with variable reservoir quality in the late rift and early post rift phase continental extension changed from oblique to normal and basins developed parallel to the continental margins of the south atlantic this change coincides with the main rifting in the equatorial basins of brazil and the early impact of the santa helena plume it resulted in widespread development of unconformities the abandonment of the reconcavo tucano jatoba rift and the end of ne brazil plate rotation which remained attached to south america there was extensive deposition of evaporites concentrated in but not restricted to the area north of the rio grande rise walvis ridge widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements our main conclusion is that the regional hydrocarbon potential of the southern south atlantic has been constrained by the tectonic evolution

40ar 39ar ages for dykes from the falkland islands with implications for the break up of southern gondwanaland 40 ar 39 incremental heating spectra analyses were made of five microgabbro dykes from the falkland islands two of which yielded an acceptable plateau and a third a close maximum estimate all three indicate an age of dyke intrusion of about 190 ma supporting a single published k ar date this provides further support for the geological continuity of the falkland islands and the eastern karoo basin where dyke swarms of similar petrological affinities were intruded at about the same time the age constrains the previously published palaeoreconstruction of the falkland islands based on palaeomagnetic data which places the falklands immediately to the east of southern africa to be valid until at least the late early jurassic

latest pleistocene and early holocene fluctuations of glaciers in the canadian and northern american rockies in the canadian and northern american rockies small scale fluctuations of glaciers during and after late wisconsinan deglaciation have been identified using stratigraphy of superimposed glacial deposits geomorphology and records of glacial advance in lake and peat sediments several authors have proposed that valley glacier readvances of relatively modest scale followed the late wisconsinan maximum evidence for some of these is difficult to evaluate from the literature and no deposits recording such readvances have been numerically dated several minimum limiting radiocarbon dates for deglaciation suggest ice had retreated to within a few tens of km of present limits by ca 12 ka bp and to very near present limits by ca 11 ka bp one or more minor glacial advances or stillstands during recession evidently occurred in valley heads prior to 11 ka bp part of the evidence consists of moraines a few km downvalley from subsequently emplaced little ice age and crowfoot moraines the piper lake moraine in the mission mountains of montana is overlain by glacier peak g tephra and thus has a minimum age of 11 200 14c years bp the crowfoot advance a minor advance of about the same magnitude as little ice age advances occurred between ca 11 300 and 10 000 14c years bp and is thought to be related to the younger dryas climatic reversal crowfoot moraines have been found from jasper national park on the north to the mission mountains on the south crowfoot elas were an average of 40 m lower than modern elas around the canada u s border there have been past proposals of significant early holocene glacial advances in the rockies i e after crowfoot time but before the lia but recent evidence indicates that glaciers then were less extensive than at present

on the variations of flow rate in non explosive lava eruptions abstract a physical model is developed to determine factors which influence the dynamics of non explosive lava eruptions in the model magma rises in a laminar regime from an overpressured chamber surrounded by elastic rock and erupts at the surface where the accumulation of lava may alter the vent pressure in a flow rate versus volume diagram an eruption follows a path which is sensitive to relative changes in flow variables such as viscosity conduit dimensions and thickness of lava over the vent the path does not depend on the unknown elastic properties of the country rock or the chamber dimensions with appropriate volcanological data an observed flow rate versus volume path can be interpreted the approach is applied to three well documented eruptions in each case the eruption rate was partially influenced by decreasing chamber pressure the 1988 1990 eruption of lonquimay chile exhibits an early phase of conduit shrinkage which lasted about 100 days probably due to magma solidification against country rock the 1979 eruption of la soufriere de saint vincent west indies was initially affected by lava dome growth and later passed through a phase possibly caused by constriction of the conduit the 1943 1952 eruption of paricutin mexico was affected by changes of lava level in the cinder cone and magma properties in these three eruptions the pre eruptive chamber overpressures were similar 10 20 mpa even though the erupted volumes differed by as much as two orders of magnitude

new angles on south atlantic opening existing models of relative motion between the south america and africa plates significantly misrepresent the azimuth of lower cretaceous seafloor spreading in the south atlantic ocean an improved model is derived from fits of fracture zones magnetic reversal anomalies and the edges of a rifted large igneous province at the northeast georgia rise and agulhas plateau an absolute date for this rifting event is not known but by assuming least change in spreading rates during magnetic chron c34 rifting can be estimated to have occurred at 100 ma this modelling demands a plate divergence history that involves diachronous opening of the south atlantic consistent with published estimates of the ages of breakup from sedimentary basins on the south american and african passive margins the diachronous opening lasts approximately 40 million years during which time it must be accommodated by significant intracontinental deformation a reconstruction using the intra c34 rotation also illustrates the earliest possibility for direct deep water connection between the central and south atlantic oceans one further consequence of this model is that a total reconstruction derived from it and closure of the central atlantic between north america and africa suggests that the venezuelan and gulf of mexico basins may be conjugates formed during the earliest opening of the central atlantic and not separate marginal basins

morphological structural and textural variations in the 1988 1990 andesite lava of lonquimay volcano chile the 1988 1990 eruption of lonquimay volcano chile 38 s formed a 10 2 km long andesite lava with a volume of 0 23 km 3 over a period of 13 months the lava extrusion rate decreased with time as chamber pressure and vent dimensions decreased the velocity of the flow front decreased exponentially with distance from vent as a consequence of cooling and the increase of apparent viscosity at the flow front the lava developed a central channel which decreased in width and depth with time three prominent lava levees were formed on each margin and resulted from abandonment as the channel decreased in width as a result of a rapid decrease of flow rate over the first 100 days of activity a fourth major levee developed in february during a brief period of flow rate increase down the main channel but its walls were gradually exposed as the lava depth again decreased due to declining flow rate the structure of lava levees depended on their age and longevity of the flow in the adjacent channel initial levees were formed in the first few days as the lava spread laterally and then retreated leaving levees of massive lava more mature rubble levees were formed during the next month by the lava pushing and then shearing aa and blocky breccia which formed on the cooling flow margin fragmentation and abrasion formed a characteristic zonation in the levees a basal zone consists of very poorly sorted matrix rich breccia with very rounded vesicular clasts and bimodal grain size distribution the basal breccia zone strongly resembles block and ash flow deposits this zone passes up into a zone of clast supported clinker breccia which becomes increasingly matrix poor and coarser with clasts becoming more angular upwards the crest of the levee is composed of large 10 100 cm angular to subangular blocks with no matrix the zoned levees form after the active lava channel suddenly narrows lava depth initially increases and breccias are deposited on the channel margins and acquire the zoned structure by progressive shearing and accretion of clinkery aa breccia the lava level then drops exposing the steep inner scarp of a levee the most mature levee type formed in a long lived channel over several months the outer wall of the levee consists of zoned breccia but the inner wall consists of a massive curving wall of strongly foliated lava with well developed horizontal striations and ductile reidel shears the massive foliated facies is a consequence of prolonged flow which coats strongly sheared lava onto the inner levee wall scanning electron microscopy shows that the aa clinker clasts and foliated lava from the levee walls form at low melt fractions 15 in the last three months of the eruption the flow front ceased to advance but thickened as lava drained from proximal regions and intruded into the interior of the distal lava the last stages of lava movement were characterized by updoming in the central channel a lava surface feature named here armadillo structure was formed by deformation of the cooler but still ductile lava crust the deformation caused by underflow produced reidel shears dipping upstream and doming of the lava due to rotation of the shear planes the study demonstrates that lava morphology structure and texture are strongly influenced by variations of effusion rate local flow rate channel topography and thermal maturity of the lava which is reflected in downstream changes in viscosity

reply to comment by d aslanian and m moulin on a new scheme for the opening of the south atlantic ocean and the dissection of an aptian salt basin summary in their comment aslanian moulin argue that our model of south atlantic opening is incompatible with several kinematic and geological constraints as recently evaluated in moulin et al they also claim that we have not appropriately referenced their work we strongly disagree and in addressing their points will argue that our model is compatible with the most important constraints that have been assembled by the many scientists who have worked on the south atlantic and that instead the interpretation by moulin et al suffers from serious flaws

late quaternary environmental change in the lake o hara region yoho national park british columbia abstract palaeoenvironmental reconstructions for the upper cataract brook valley are derived from radiocarbon and tephra dated cores recovered from subalpine lake o hara and alpine opabin lake located near the continental divide in the central canadian rocky mountains late quaternary vegetational and climatic changes are based on pollen and macrofossil data and are supplemented by preliminary diatom data the postglacial colonizing vegetation prior to ca 10 100 yr b p at lake o hara and before ca 8530 yr b p at opabin lake was a shrub herb community dominated by artemisia gramineae and alnus pioneering forests at both sites were composed of pinus cf albicaulis flexilis and abies with lesser abundances of picea and pinus cf contorta timberline remained above the elevation of opabin lake at least 90 m above modern timberline elevation during the period ca 8500 yr b p to ca 3000 yr b p in response to warmer climatic conditions associated with restricted glacial activity forest compositions resembling the modern subalpine picea abies forest had developed by the end of this period the period ca 3000 yr b p to present was marked by deteriorating climatic conditions associated with renewed glacial activity in the opabin cirque and declining timberlines to below the elevation of opabin lake glacigenic sediments in the opabin record from the post bridge river ca 2350 yr b p interval are associated with reduced diatom production and a change in dominant diatom taxa to those characteristic of turbid water conditions and unstable catchment areas

pahoehoe and aa lavas a discussion and continuation of the model of peterson and tilling abstract the graphical method of peterson and tilling for representing the behaviour of a lava flow is considered in terms of simple rheological theory assuming that a lava may be approximately described as a bingham fluid this approach helps resolve an apparent anomaly in the original treatment of peterson and tilling with respect to the representation of the pahoehoe to aa transition it also suggests further methods for quantitatively defining the limits of the pahoehoe and aa fields and the transition threshold zone

architecture and neogene to recent evolution of the western calabrian continental margin an upper plate perspective to the ionian subduction system central mediterranean the western calabria continental margin forms the transition between the pliocene to recent marsili spreading center and continental calabria all parts of the upper plate of the ionian subduction zone integrating high resolution and crustal seismic images constrained by gravity modeling we provide a detailed reconstruction of the architecture of the margin and develop a new scheme for its miocene to present evolution this time span encompasses the continent continent collision between africa and eurasia subsequent orogenic collapse and rifting apart between the two continental masses and the pliocene to recent emplacement of oceanic crust in the vavilov and marsili basins the crust of the margin thins from the calabria coast 25 km to the marsili continent ocean transition 12 km on the whole upper and lower crusts thin proportionally with pure shear geometry the continental margin is covered by an oligocene to present sedimentary succession reaching a maximum thickness of 6 0 km in the paola basin during the miocene the continental margin experienced regional shortening accommodated by a large number of mainly west vergent thrusts possibly associated with the late stages of the kabilo calabrian chain shortening continued through pliocene to recent but was accommodated by a limited number of west vergent thrust faults located in the western part of the margin and by a few tens of kilometers wide syncline located in the eastern part of the profile the accommodation space created in the syncline core hosted a 4 5 km thick plio quaternary sedimentary succession the paola basin no significant extensional fault is observed along the profile miocene to recent subsidence was controlled by 1 a short wavelength component related to shortening and responsible for the formation of the paola basin syncline and possibly contributing to the uplift of onshore calabria and 2 a long wavelength component responsible for the regional subsidence and oceanward tilting of the calabria margin short wavelength subsidence ended in the late pliocene but long wavelength downward movements persisted and even accelerated during late pliocene to quaternary times when the present day bathymetry was achieved both horizontal deformations and vertical movements are difficult to explain in the context of a normal back arc basin without taking into consideration patterns of secondary mantle flow generated by the retreat of the subducting slab

salt geology and tectonics of selected brazilian basins in their global context this work addresses the geological and geophysical interpretation of salt structures in selected brazilian sedimentary basins from intracratonic palaeozoic evaporites in the amazon and solimoes basins to divergent margin evaporite basins formed during the mesozoic break up of gondwana there is an intriguing correlation between evaporite basins and hydrocarbon accumu lations in all the selected basins discussed the solimoes and amazonas basins developed eva porite depositing environments as the middle carboniferous sea was closing during a plate convergence phase the salt basin along the eastern brazilian and western african margins devel oped along the mesozoic rifts of the south atlantic regional seismic interpretation and potential field gravity and magnetic data over the eastern brazilian and west african margins suggest a very thick autochthonous salt layer deposited over rifted continental crust and particularly above the thick sag basin sediments over the hyperextended crust that marks the transition from continen tal to oceanic crust most of the hydrocarbon discoveries in the eastern brazilian and western african margins are in post salt turbidite and carbonate reservoirs but recent discoveries in the deepwater salt basins along the southeastern brazilian margin indicate that pre salt plays will rep resent an important contribution to hydrocarbon production in the near future