impact of inversion tectonics on hydrocarbon entrapment in the baggara basin western sudan abstract the baggara basin of sudan is an ene wsw trending basin located within the west and central africa rift system wcars and nw of the muglad basin the trend of the baggara basin is similar to other basins in eastern chad where oil has been discovered since there are no published studies for the baggara basin the present paper represents an addition to the geology of eastern extension of the wcars in sudan the 2d seismic data interpretation sheds light on the development and evolution of key regional and local structures of the basin the baggara basin is characterized by graben geometry with two boundary faults characterized by dextral strike slip movement and a depocenter at the central part the interpretation reveals the presence of two types of structures within the baggara basin inversion tectonics restricted to the larger basin bounding faults in the western part and en echelon faults in the central part the timing of inversion tectonics is during the eocene and it is related to a brief major compressional event due to the collision of the african and eurasian plates although the inversion created a four way dip closed anticline structure this anticline postdated the initial migration from the source rock accordingly the anticline structural is unsuccessful for hydrocarbon entrapment

the 25 september 2007 eruption of mount ruapehu new zealand directed ballistics surtseyan jets and ice slurry lahars abstract at 20 26 nzdt on 25 september 2007 a moderate gas driven eruption beneath the summit crater lake of mt ruapehu new zealand generated a directed ballistic fallout apron and surtseyan jet that impacted an area of c 2 5 km2 to the north of the vent two climbers were caught in the blast at a hut 600 m from the vent primary ice slurry lahars were generated in two catchments draining the summit region including a flow that entered a commercial ski field the ejecta consists of blocks of andesitic lava and scoria erupted during previous volcanic episodes a variety of mineral cemented lake floor sediments vent fill debris and rare glassy material interpreted to be derived from a fresh batch of magma a minority of clasts contain formerly molten elemental sulphur indicating that vent temperatures at the base of the lake were in excess of 119 c the ballistic deposit is confined to a c 40 wide swath directed northwards that extends up to 2 km from the inferred vent location beneath crater lake the strong directionality of the jetted deposits and ballistic free shadow zones sheltered by intervening topography all point to a relatively low angle directed blast previous similar eruptions at ruapehu also show preferred deposit orientations that are here correlated with an inclined and off centre vent beneath crater lake the 25 september 2007 eruption appears to have resulted from failure of a relatively shallow hydrothermal seal composed of elemental sulphur and other mineral precipitates developed in the vent breccia which breached following overpressurization by a pulse of magmatic gases released from deeper in the conduit this event highlights the potential hazard from relatively minor gas driven eruptions at ruapehu that can occur with little or no warning improved understanding of the eruption mechanism for this style of eruption will inform future risk assessments for ruapehu and other similar volcanoes worldwide

the significance of marine boreal molluscs in the early pleistocene faunas of the mediterranean area abstract repeated migration of boreal guests into the mediterranean during the early pleistocene has been considered a weak paleoclimatic oceanographic tool this attitude is partly held because the plio pleistocene boundary as defined in the mediterranean does not seem to coincide with isotopic events more important than those of the piacenzian an analysis of their present biogeographical distributions suggests that the successful migration of the boreal elements into the early pleistocene mediterranean was caused by the establishment of a climatic phase of high seasonality with winter temperatures lower than 9 10 c and summer temperatures not lower than 20 c such a situation would also have been suitable for the taxa of warm affinity that characterize the present mediterranean moroccan bioprovince this hypothesis applies mainly to the first phase of migration santernian substage which is the best known from a malacological point of view the available paleoclimatic and isotopic record seems in agreement with the hypothesis

present day crustal deformation and geodetic strain in the vicinity of dholavira harappan civilization site kachchh western part of the indian plate abstract to understand the geodynamics in the vicinity of dholavira a harappan civilization site hcs in kachchh western india gps data from 2009 to 2015 were processed and analyzed tectonically this part of northern kachchh is in the influence of island belt fault ibf and allah bund fault abf and is seismically one of the most active intra plate regions of india the motion with respect to indian plate is used for the estimation of deformation and strain rate while fault associated slip rate has been estimated by inverse modeling of observed site motions the analysis reveals that the abf deforms with an average rate of 1 8 mm yr however the segmented ibf has a maximum deformation rate of 2 9 mm yr towards the pachham island and minimum towards the khadir island the computation discloses a maximum seismic moment m0 of 2 1 1024 dyne cm in this part which corresponds to an earthquake of mw 6 0 the calculated maximum strain of 0 04 micro strain year is low but significant in the intra plate region the postseismic deformation after the mw 7 7 2001 bhuj earthquake is estimated to be low in this part the reverse along with strike slip motion of faults builds up stress in the area and accumulating more strain

early cretaceous rifts of western and central africa an overview abstract the structure and evolution of early cretaceous rift basins in western and central africa are described two stages of rift development and fracturing have been identified 1 from neocomian to early aptian roughly e w and nw trending troughs upper benue n cameroon s chad sudan etc opened in response to a submeridian extensional regime in central africa while in western africa the n s trending transsaharian fault zone acted as a sinistral wrench 2 from middle aptian to late albian large northwest trending troughs e niger sudan sirte etc opened in response to a northeast extensional regime while the central african fault zone from benue to sudan exhibited strike slip movements generating pull apart basins these rift and fracture systems delimit three large blocks within the african plate a western block an arabian nubian block and an austral block the arabian nubian block tends to separate from the two other blocks migrating towards the north during the first stage of basin development and then towards the ne during the second stage the opening of the atlantic ocean was the dominant driving force for the western and austral blocks while the arabian nubian block probably moved in response to the opening of the indian ocean and to the evolution of the tethyan margin

differential opening of the central and south atlantic oceans and the opening of the west african rift system abstract plate tectonic studies of the development of the central and south atlantic oceans using seasat and geosat altimeter and magnetic anomaly isochron data now provide quantitative models of seafloor spreading through time such models enable an initial assessment of the differential opening between these two oceanic basins to be determined the equatorial atlantic is an integral part of this oceanic rifting process allowing stresses arising from the differential opening to be dissipated into both the caribbean and africa along its northern and southern boundaries respectively the tectonic model for the west african rift system based on geological and geophysical studies shows a series of strike slip fault zones diverging into africa from the gulf of guinea and dissipating their shear movement into the development of extensional basins orientated perpendicular to these faults zones the development of the west african rift system was contemporaneous with the early opening of the south atlantic continued to develop well after the final breakup of south america from africa and did not cease until the late cretaceous when there was a major phase of basin inversion and deformation santonian 80 ma deformation across the benue trough nigeria is broadly contemporaneous with dextral shear reactivation of the central african fracture system which in turn resulted in renewed extension in the sudan basins during the late cretaceous and early tertiary this paper illustrates the close linkage in both time and space between the history of the african rift basins and the opening of the atlantic both exhibit distinct phases of evolution with the rift basins developing in direct response to the differential opening between the central and south atlantic in order to dissipate stresses generated by this opening the mesozoic tectonic model proposed is therefore one of an intimate interaction between oceanic and continental tectonics

effects of initial rift inversion over fold and thrust development in a cratonic far foreland setting abstract the effects of initial rift inversion over the geometry of a far foreland intracratonic fold and thrust belt are investigated a total of 25 seismic reflection lines and stratigraphic data from 9 wells are used in association to digital elevation and field geological data to investigate the geometric relationship between shallow thrust development and initial positive inversion of buried rift faults in a cratonic environment where inversion tectonics is not common the study case is conducted in the meeting region of the opposed verging brasilia and aracuai foreland fold and thrust neoproterozoic belts located in central brazil formed in the first stages of west gondwana amalgamation seismic reflection sections are used to map buried paleorift faults and trust ramps the best 5 sections are also interpreted in detail in terms of fold and thrust geometry and their relationship to the inverted rifts is analyzed they show high correlation between shallow fold and thrust geometry and the shortening of synrift strata caused by rift inversion results indicate that initial positive rift fault reactivation exerts a determinant control over the locus of thrust ramp nucleation by causing syn rift strata to overfold and drape fold the passive folding propagates upwards forming gentle folds in the decollement plane where thrust ramps tend to nucleate back thrust forethrust pairs tend to form over elevated horsts where higher topographic plateaus are preserved multiple thrusting and lower relief occur over buried rifts with multiple inverted faults strata over non rifted basement remain practically unaffected by thrusting and folding moreover the development of thrust salients and recesses are demonstrated to be conditioned by the morphology of buried rifts the study also contributes with a more precise mapping of the buried rift system and thrust system in the area and brings new insights about the structural evolution of the brasiliano pan africano orogeny affecting the sao francisco craton basement and covers

quenching of steam charged pumice implications for submarine pyroclastic volcanism abstract huge quantities of silicic pumice have been deposited in intra oceanic convergent margin settings throughout earth s history the association of submarine silicic calderas with thick proximal accumulations of pumice lapilli suggests that these pyroclasts were deposited as a direct result of submarine eruptions yet when first erupted these highly vesicular gas filled clasts had densities significantly less than seawater experiments carried out 1 atm on heated pumice samples whose vesicles were charged with steam the dominant component of magmatic volatiles show that buoyancy of freshly erupted submarine pumice is transient upon quenching the phase change of steam to liquid water creates strong negative pore pressures within the pumice vesicles that accelerate the absorption of surrounding water generating high density pumice and promoting rapid clast sinking variations in the physical properties of steam with temperature and pressure have important implications for submarine pyroclastic eruptions firstly highly vesicular pumice can be deposited on the seafloor at temperatures elevated significantly above ambient if they are erupted at sufficient depths to remain wholly submarine 200 m and either the fluid in which they cool contains heated water and or they only absorb sufficient water to sink secondly the rapid increase in density of the eruption column caused by condensation and the transition from buoyant gas filled to denser water saturated pumice lapilli together with turbulent mixing with the surrounding seawater favour collapse and transport of pyroclasts in water supported gravity currents finally this mixing of the ejecta with seawater and the ease of water ingestion into permeable pumice clasts suggest that water supported transport mechanisms can operate as primary dispersal processes in explosive submarine eruptions

the role of bubbles in generating fine ash during hydromagmatic eruptions the abundant fine ash produced in the 2011 subglacial eruption of grimsvotn iceland highlights the fragmentation efficiency of mafic hydromagmatic eruptions which is considerably higher than for comparable dry eruptions ash from the 2011 eruption can be divided into three morphological components vesicular particles shards and dense fragments distinguished by the size and abundance of constituent vesicles we use the vesicle characteristics to define a new shape factor the concavity index which provides an unbiased way to classify individual ash particles as either bubbly vesicular particles and shards or dense the relative proportion of bubbly and dense particles varies systematically with grain size with the proportion of bubbly grains decreasing as the particle size approaches the modal bubble diameter measured bubble volume distributions are similar to those of rapidly quenched pyroclasts from hawaiian fountains and suggest a comparable degassing history during magma ascent yet concordance between the size distributions of ash and of bubbles in the grimsvotn samples stands in contrast to the size distributions in hawaiian fountains where pyroclasts are orders of magnitude larger than individual bubbles we propose that the grimsvotn ash formed by brittle disintegration of vesicular pyroclasts and that fragmentation efficiency was amplified by residual thermal stresses in glass quenched by glacial water the strong control of resulting particle sizes and morphologies by the size and spatial distribution of bubbles demonstrates that the bubble population cannot be ignored when modeling hydromagmatic fragmentation

mid cretaceous inversion in the northern khorat plateau of lao pdr and thailand evidence is presented from the study area for the occurrence of a regional compressive tectonic event in the mid cretaceous aptian cenomanian this is tentatively attributed to the effects of a distant continent continent collision to the west pre existing structural trends were reactivated parallel to palaeo continental sutures to the northeast song ma and song da and west nan the event interrupted the latest jurassic to earliest palaeocene subsidence and continental sedimentation of the khorat plateau basin the primary effects are shown by seismic data to be regional tilting compressive folding reverse faulting and inversion of the basin subsidence pattern the data do not support the current view that such structuring was initiated during the tertiary however they do show that there was some reactivation of mid cretaceous structures during regional uplift in the tertiary this revision of timing of structuring suggests the hydrocarbon potential of the area may be greater than previously anticipated

spore pollen evidence for early oligocene high latitude cool climatic episode in northern canada climatic cooling in the eocene leading to markedly lower temperatures in the oligocene has been documented in oxygen isotope studies in the north pacific1 2 new zealand3 south pacific4 and north sea5 these lower temperatures in the oligocene correspond to palaeoclimatic interpretations of north american leaf floras6 7 which suggest a profound cooling occurring towards the end of the eocene indicated also in the southeastern united states from studies on mid tertiary spores and pollen8 recently collinson et al 9 have shown palaeobotanically that in southern england cooling occurred gradually starting in the latest early eocene leading to two major periods of floristic change before the end of the eocene thick tertiary sections in the subsurface mackenzie delta region northwest territories10 11 provide well preserved palaeogene palynofloras12 discussed here which indicate a cooler climate in the oligocene following warm temperate conditions in the eocene this widespread early middle oligocene cool episode is thus represented in both high latitude and lower latitude floras and seems to have been of global extent persisting until towards the end of the oligocene but its precise dating remains a problem in northern canada it is followed by an amelioration of the climate in the late oligocene which persisted probably until the middle miocene the rate of cooling during the eocene and oligocene may have varied depending on locality but once established the cooler oligocene climate exerted a profound influence on biotic development

transient electromagnetic investigations in a tectonic domain of the kachchh intraplate region western india a morphotectonic study of the kachchh mainland fault

mesozoic plate tectonic reconstructions of the central south atlantic ocean the role of the west and central african rift system abstract the mesozoic opening history of the central equatorial and south atlantic oceans is closely linked in time and space to the development of the cretaceous rift system in west and central africa geological and geophysical studies of the west and central african rift system indicate it to be dominated by a set of sinistral and dextral strike slip zones diverging from the gulf of guinea and terminating as extensional rift basins in niger and sudan plate tectonic studies of the opening history of the atlantic basins indicate four stages of development stage 1 the jurassic opening of the central atlantic stage 2 the early cretaceous 130 119 ma opening of the south atlantic with rifting propagating deep into africa via the benue trough stage 3 the opening of the equatorial atlantic 119 105 ma and stage 4 the linkage of these oceanic basins and development of a single opening mid oceanic rift system as seen today the west and central african rift system continued to develop until the end of the cretaceous i e into stage 4 when rifting was terminated by a compressional shear deformation event causing the folding of sediments in the benue trough and reactivation of the central african shear zone and extensional tectonics in the basins of southern sudan this event correlates with an important change in the relative opening of the atlantic basins during tertiary recent times the west african rift system has not been active however during this period domal uplift and volcanism have occurred in central cameroon and western sudan and a very active phase of volcanism has occurred within the last 10 ma along the present day cameroon volcanic line although these uplift and volcanic events indicate thermal processes in the upper mantle there is as yet no clear link between their development and plate tectonic processes

revitalising exploration and re development of deep carbonate targets in the southern apennines thrust belt southern italy reappraising vintage data with modern approaches

the late cretaceous tectonic evolution of the south china sea area an overview and new perspectives from 3d seismic reflection data abstract the late cretaceous is a significant geologic period in the south china sea area as it marks the tectonic transition from the early paleo pacific plate subduction to the subsequent cenozoic rifting related to the opening of the scs however the late cretaceous tectonic evolution of the south china sea area is yet to be clearly defined and it is at present largely debated in the literature in this paper mainly on the basis of 240 000 km2 newly acquired 3d seismic reflection data 94 industrial wells and existing u pb ages of basement granitoids we carried out an integrated interpretation mapping and analysis of the pre cenozoic 66 ma structures within the basement of the cenozoic rift basin in the northern south china sea area that can be crucial for a broad understanding of the late cretaceous tectonic evolution of this region results of this analysis suggest that three late cretaceous fault systems can be identified in the northern south china sea margin 1 the wnw striking thrust system 2 ene striking extensional fault system and 3 ene striking thrust system these three fault systems are considered to have developed in the late cretaceous since they are superimposed on the well dated late jurassic to early cretaceous 161 6 101 7 ma arc related granitoids that define most of the basement of the cenozoic rift basin within the study area this new structural evidence combined with a review of previous studies led us to propose a renewed multi phase geodynamic evolutionary model of the south china sea area during the late cretaceous the first tectonic event that can be recognized generated the wnw striking thrust system and is in this paper interpreted to have formed as a result of a sinistral transpressional event that took place in the south china sea area and adjacent areas around 100 ma at the early late cretaceous boundary this transpressional event was likely related to intense oblique convergence between the paleo pacific ocean plate and the eurasia plate the second tectonic phase that was identified in this paper 100 ma to 72 ma is defined by an extensional event that generated the largely ene striking extensional fault systems and associated basins which is interpreted to be related to the back arc extension that took place in this period in response to the slab roll back and high angle subduction of the paleo pacific plate this extensional phase eventually led to the opening of the proto south china sea on the se margin of the proto south china block and it should be distinguished from the cenozoic rifting phases that are related to the opening of the south china sea the third and last tectonic phase that was recognized in this paper is a compressional event that took place in the late stage of the late cretaceous 72 ma to 66 ma and was responsible for the development of the ene striking thrust system we suggest that this post subduction post yanshanian compressional event can be interpreted to have developed in response to process of ridge push related to the sea floor spreading of the proto south china sea

the influence of a reverse reactivated normal fault on natural fracture geometries and relative chronologies at castle cove otway basin abstract despite the ubiquity of normal faults that have undergone compressional inversion documentation of the structural history of natural fractures around these structures is limited in this paper we investigate the geometries and relative chronologies of natural fractures adjacent to a reverse reactivated normal fault the castle cove fault in the otway basin southeast australia local variations in strain resulted in greater deformation within the fault damage zone closer to the fault structural mapping within the damage zone reveals a complex tectonic history recording both regional and local perturbations in stress and a total of 11 fracture sets were identified with three sets geometrically related to the castle cove fault the remaining fracture sets formed in response to local stresses at castle cove rifting in the late cretaceous resulted in normal movement of the castle cove fault and associated rollover folding and the formation of the largest fracture set reverse reactivation of the fault and associated anticlinal folding occurred during late miocene to pliocene compression rollover folding may have provided structural traps if seals were not breached by fractures however anticlinal folding likely post dated the main episodes of hydrocarbon generation and migration in the region this study highlights the need to conduct careful reconstruction of the structural histories of fault zones that experienced complex reactivation histories when attempting to define off fault fluid flow properties

mesozoic diapirism in the pyrenean orogen salt tectonics on a transform plate boundary the mesozoic history of the pyrenean region was dominated by the opening of the north atlantic ocean and the left lateral movement of iberia relative to europe most deformation was tensional or transtensional but this style was interrupted by several short compressional events perhaps related to bends in the transform plate boundary or minor changes in plate movement directions these compressional events were recorded by deformation adjacent to and above salt structures which were the weakest parts of the system and so served as especially sensitive strain barometers the alternation of extension and shortening controlled the location and style of salt structures evaporites were initially deformed during the latest jurassic early cretaceous neocimmerian transpression which formed salt cored anticlines above west northwest east southeast trending basement faults transtension in the aptian albian caused salt domes to pierce to the surface at the intersections of these anticlines with reactivated northwest southeast and northeast southwest trending basement structures transtension was at least locally interrupted by brief periods of transpression at the aptian albian and albian cenomanian boundaries transpression shortened the diapirs causing rotation uplift and erosion of beds near the salt most diapirs were buried during the late cretaceous in some cases postburial hydrothermal circulation dissolved much of the halite causing the diapirs to sag and the basins to form in roof strata finally the entire region was shortened during the late cretaceous tertiary pyrenean orogeny which greatly distorted many of the preexisting geometries

polar emergence and the influence of increased sea ice extent on the cenozoic biogeography of pectinid molluscs in antarctic coastal areas abstract from the paleocene to the pliocene chlamys like genera of scallops pectinid bivalve molluscs inhabited coastal areas around the antarctic continent today all of the chlamys like genera are extinct around antarctica and restricted to lower latitude habitats along the surrounding continents the only remaining scallop in the high antarctic zone is the large greater than 108 mm in height endemic thin shelled adamussium colbecki adamussium has a circumpolar distribution with its highest abundances greater than 50 m 2 and 2 kg m 2 in antarctic coastal areas in contrast to the offshore habitats where its ancestors had predominated since the oligocene shell morphology and substrate data indicate that the chlamys like faunas were largely exposed to high energy conditions associated with open water in the coastal zone while the adamussium faunas have been associated with low energy hydrodynamic conditions modern and mid holocene distributions in the antarctic coastal zone further indicate that adamussium is largely restricted to protected embayments and habitats with extensive sea ice coverage in addition a decadal mark and recapture experiment demonstrates that adamussium has extremely slow growth and a century scale lifespan which complement the life history traits that are considered to be characteristic of species inhabiting predictable and constant environments in the deep sea stabilization of the water column by increased sea ice extent together with stenothermal conditions prolonged darkness and limited primary production would have enhanced the habitat similarities between polar coastal environments and the deep sea after the plio pleistocene climate threshold subsequently the deep antarctic continental shelf would have facilitated faunal shifts between deep sea and coastal habitats as with the polar emergence of the adamussium fauna such biogeographic interchanges across high latitude continental shelves since the end of the cenozoic would have contributed significantly to the latitudinal diversity gradients that have been noted from both deep sea and shallow water marine faunas

inversion tectonics in central africa rift system evidence from the heglig field abstract the mesozoic cenozoic tectonic history of the muglad basin is dominated by extension and inversion tectonics but evidence of the inversion tectonics has not been well documented yet in some other rift basins of cars and wars the phase of the inversion tectonics is well documented by several authors this paper presents a structural study of the heglig field area located on the eastern flank of the muglad basin detailed 3d seismic interpretation allows a better understanding of the structural style of the heglig field the new structural analysis has shown that the heglig field has a complex structural framework reflected in the presence of a combination of two structural styles the extensional structure is influenced by inversion tectonics during the santonian time that creates four way dip anticline structure overprinted by the subsequent extensional movement that creates tilted fault block the presence of inversion tectonics has supported by different means including seismic reflection velocity and source rock maturity data the authors attributed the trapping of oil in the lower bentiu reservoir that requires a horizontal seal to the presence of the four way dip anticline structure created by the inversion tectonics the current interpretation of the heglig field 3d seismic data sheds new light on the development and evolution of a key structure in the muglad basin the results help to resolve long standing discussion concerning hydrocarbon accumulation of the lower part of bentiu formation that lacks horizontal sealing

premixing of magma and water in mfci experiments experimental studies have been performed to evaluate pre explosive water melt mixes with respect to explosive volcanic molten fuel coolant interaction mfci i e phreatomagmatic explosion remolten ultrabasic volcanic rock was used as a magma simulant measurement of the explosion intensity was used to determine optimal premixing conditions a well defined optimal range was found for the hydrodynamic mixing energy differential flow speed of 4 2 m s as well as for the water melt mass ratio 0 03 to 0 04 under experimental conditions the mass flux of water had a minor influence on the explosion intensity additionally transparent mixing experiments with silicon oil and inked water were carried out they indicate a direct dependence of the pre explosive water melt interface area on the explosion intensity the experimental results show that the contact conditions of water and melt required for explosive mfci may easily be established in natural volcanic systems thus explosive mfci is a probable mechanism of explosive volcanism

push up inversion structures v fault bend reactivation anticlines along oblique thrust ramps examples from the apennines fold and thrust belt italy the study of inversion structures in fold and thrust belts that originated as extensional faults is valuable for the development of geometric and kinematic models that are helpful in unravelling the structural style and for detecting structural hydrocarbon traps we document push up inversion structures versus fault bend reactivation anticlines occurring along oblique thrust ramps by examining examples of buried apulian structures in the central southern apennines and outcropping thrust related folds in the central northern apennines of italy the different geometric features between push up inversion structures and fault bend reactivation anticlines are functionally controlled by the mechanical characteristics of the involved sedimentary multilayer the original architecture of the former extensional basin and the amount of thrust displacement the recognition of such inversion structures is important in understanding the anatomy and structural style of foreland fold and thrust belts in addition their identification is critical in interpreting the geometry and types of structural traps that may be expected along oblique thrust ramps this study reveals that push up inversion structures and fault bend reactivation anticlines could coexist along oblique thrust ramps within a curved thrust system and may be detected among analogous foreland thrust belts that include inherited extensional basins

in situ granulation by thermal stress during subaqueous volcanic eruptions some of the most complex volcanic thermodynamic processes occur when erupting magma interacts with water in shallow water surtseyan eruptions are spectacular and they efficiently fragment magma into fine ash particles the aviation hazard from these eruptions depends the amount of transportable fine ash that is generated and whether it is aggregated into particle coatings or accretions to investigate both mechanisms we analyzed ash encased lapilli from the surtseyan eruptions of capelinhos azores 1957 1958 and hunga tonga hunga ha apai tonga 2014 2015 using x ray computed microtomography and electron microscopy we discovered pyroclasts that were not coated sensu stricto but had enveloping ash produced by in situ granulation of the particle surface this was caused by thermal stress as pyroclasts briefly traveled through water and were quenched during eruption in situ granulation is thus an important secondary disruption process in shallow subaqueous eruptions our results imply that ash encasement is not always evidence of particle aggregation and accretion but it may also result from new ash formation shallow water conditions produce the most efficient ash generation conditions leading to the greatest hazard to downwind populations and air traffic

interpreting structural geometry in fold thrust belts why style matters abstract structural interpretation in fold thrust belts has become reliant on a few idealized geometric models i e fault bend fault propagation and detachment folding and their quantitative methods for section construction and validation we couple historical review with selected outcrops to show that there is a substantially greater range of solutions available for interpreting the geometry and evolution of thrust belt structures than implied by these idealized models examples are documented and lessons drawn from comparing structural interpretations developed in the foothills of the canadian rockies with those in the western alps both show a range of structural geometries with regional variations that reflect variations in the pre kinematic stratigraphic template locally fold thrust development can localize on pre existing structures thus consideration of the precursor geology is essential for structural interpretation using a case study from the papuan fold belt we show that even with seismic data assessing the role of basement in structural development can be uncertain the idealized models offer only a narrow range of possible geometries for constructing cross sections and developing structural understanding in fold thrust systems failure to consider alternatives and the inherent interpretation uncertainty has biased understanding of thrust systems leading in turn to over optimistic risk assessment and repeated drilling surprises

reassessment of extinction patterns of pliocene molluscs from california and environmental forcing of extinction in the san joaquin basin abstract thirty latest miocene through late pleistocene molluscan faunas from coastal california and the san joaquin basin sjb central california were reviewed to better understand the pattern of pliocene mollusc extinction in california and particularly in the etchegoin group informal sjb nomenclature species lists were compiled for each fauna taxonomy reviewed to eliminate synonyms and uncertain identifications and the extinct versus living status of each species determined only 34 of molluscan species 40 of bivalves and 21 of gastropods in the etchegoin group are extant as compared to 61 of molluscs 64 of bivalve and 56 of gastropod species in pacific coastal faunas the etchegoin group was deposited in a marginal basin connected to the pacific ocean through a long and narrow silled strait subject to its connection being cut by eustatic regression and regional tectonism seven major regional extinctions affected the etchegoin group molluscan faunas where 40 species became extinct two in the early pliocene upper etchegoin formation at 4 4 and 4 0 ma and five in the early late pliocene san joaquin formation at 4 0 2 9 2 6 and 2 4 ma and that coincident with the final ocean connection closure at 2 3 ma peak diversities corresponded with periods of highest sea level at 4 5 4 2 3 1 2 7 2 5 and 2 4 ma when immigrant faunas became established during periods of warm climate and normal marine conditions upon sea level fall the basin became cooler brackish and faunas adapted to warmer and normal marine conditions became extinct with slow recovery of diversity afterwards low diversity faunas characterize periods of low and rising sea level when circulation through the connecting strait was insufficient to maintain normal marine conditions thus hindering establishment of most immigrants from coastal faunas restricted circulation with the pacific substantially reduced the nutrient supply to the basin leading to a long term productivity collapse that exacerbated the effects of a deteriorating environment thus leading to the major extinction event observed at the etchegoin san joaquin formations contact at 4 0 ma increasing restriction from the pacific ocean during the pliocene limited immigration of coastal species into the san joaquin basin to those opportunistic species best able to adapt to the environment inside the basin while species unable to adapt to conditions inside the sjb were filtered out in the strait stenotopy of endemic species precluded range expansion through the connecting strait into the pacific ocean thus abrupt regression driven hydrologic change productivity collapse from coincident geochemical and sedimentary change and climatic change led to the major extinction events in the pliocene sjb speciation events following extinctions suggest diversification of surviving faunas into habitats created by changed environmental conditions despite the number and wide geographic distribution of faunas reviewed in this paper only 50 90 of extant mollusc species found in pliocene etchegoin group faunas are also found in coastal california pliocene faunas demonstrating the incompleteness of the california fossil record

fault zone migration by footwall shortcut and recumbent folding along an inverted fault example from the mosha fault central alborz northern iran the mosha fault is a multiply inverted fault in the central alborz field observations and structural data from this fault show that a footwall shortcut is the major mode of response of this fault to contractional deformation although the mosha fault is a basement involved fault there is no evidence of involvement of basement along its footwall shortcuts at least in the study area footwall shortcuts along this fault vary in size from several hundreds of metres to tens of kilometres suggesting that a footwall shortcut can be scale independent it is proposed that footwall shortcuts can also occur as blind thrusts under fault related folds in the terrains near the major inverted faults similar cases also exist in other regions such as japan some large footwall shortcuts may be the causative fault of devastating earthquakes in the active inverted terrains such as the south central alborz incompetent layers acting as detachments may play an important role in the development of footwall shortcuts recu

carbonate platform to foreland basin revised stratigraphy of the table head group middle ordovician western newfoundland carbonates of the table head group and associated strata were deposited along the western margin of a foreland basin during initial stages of the taconian orogeny and record collapse and cannibalization of a long lived carbonate platform the stratigraphy of klappa opalinski and james is here revised to reflect better understanding of lithologic units within this complex tectono stratigraphic assemblage the table head group now contains only three formations the table point and table cove formations as originally defined and the cape cormorant formation redefined and restricted to western port au port peninsula black cove formation shales are removed from the table head distinctive conglomerates once placed in the cape cormorant are now recognized as separate units within the overlying flysch and called the daniel s harbour member the foreland basin developed in three stages 1 fragmentation uplift and erosion of the platform and subsequent deposition of shallow water limestones table point

ecological and paleoenvironmental information using stable isotope profiles from living and fossil molluscs stable oxygen and carbon isotopic profiles from recent specimens of spisula solidissima and placopecten magellanicus collected off the virginia coast are used to calibrate annual temperature controlled cycles in the 18o profile and shell growth characteristics the amplitude of the annual 18o cycles decreases with increasing water depth and may be used as a relative depth indicator the isotopic of profiles of two fossil specimens of pleistocene spisula are very similar in amplitude and character to modern analogues and verify the formation of annual growth increments in these fossil specimens offsets between the 18o profiles of the fossil and modern specimens are related to known temperature and ice volume effects during the late pleistocene and support the utility of mollusc isotope records as a stratigraphic tool carbon isotopic profiles from all specimens show light values in the spring and or fall associated with seasonal phytoplankton productivity offsets in the carbon isotopic composition between infaunal and epifaunal species may be related to sediment porewater geochemistry and oxidation of organic matter trends toward lighter 13c values through ontogeny suggest the effects of metabolic changes from a fast growing juvenile into a slower growing sexually mature adult anomalies in both the oxygen and carbon isotopic profiles identify the formation of minor shell growth increments as corresponding to hydrographic perturbations and to winter cold temperatures

neogene mass extinction of western atlantic molluscs there have been many hypotheses for the global mass extinctions that occurred during the phanerozoic history of the earth and although we cannot study the course of such a mass extinction in progress it is possible to evaluate major pulses of extinction that have taken place recently in geological time explanations of the unusually high rates of extinction within late cenozoic marine faunas of the tropical western atlantic have focused on possible blocking of nutrient laden waters by the origin of the isthmus of panama1 2 and on pleistocene cooling2 3 5 6 or regression2 4 5 of seas here we report that major episodes of molluscan extinction occurred at least as far north as virginia and we conclude that these episodes were caused primarily by refrigeration associated with the late pliocene and pleistocene growth of continental glaciers

the structure of mountain fronts commonly the part of a mountain front which is visible at the surface consists of foreland dipping thrust belt rocks elevated above their regional structural position and overlain more or less conformably by molasse several explanations for their geometry are possible 1 major detachments exist within or beneath the molasse resulting in transport of the foreland basin examples of this geometry come from the swiss molasse plain the southern pyrenees and the mackenzie mountains of canada 2 displacement is lost on major backthrusts beneath the frontal monocline examples cited here are the rockies of alberta the sulaiman ranges of pakistan the mackenzies and the andes in peru 3 thrust sheets travelled over an old land surface and syntectonic molasse contemporaneously offlaps the topographic high of the thrust front this phenomenon occurs along the alpine thrust front in haute provence 4 the frontal fold represents deformation above a large scale thrust tip no unequivocal example of tip line strain at this scale has been recorded although this type of deformation may occur in the brooks range of alaska in many areas mountain fronts show a combination of these idealized geometries

structural inheritance in mountain belts an alpine apennine perspective abstract the geological structure of continental lithosphere shows complex variety that is inherited into orogenic belts and influences the localization and amplification of contractional structures during mountain building in the alpine apennine sector together with other sectors of the tethyan orogenic system the pre orogenic crustal template can include arrays of extensional faults other faults can form adjacent to the evolving mountain belt and subsequently become incorporated as the thrust belts migrate into their forelands while in some areas these inherited features may simply reactivate under inversion more commonly faults show complex partial reactivation structures in volumes of distributed strain faults may serve to nucleate large scale buckle folds for example along basement cover interfaces these different patterns of basement reactivation may reflect spatially varying strength depth profiles in continental lithosphere that are themselves inherited from spatially distinct geological histories even when not themselves reactivating basement faults can control deformation in the overlying sedimentary cover by offsetting otherwise regionally extensive detachment horizons the 3d form of thrust systems can be strongly compartmentalized by pre existing cross faults such as the oblique lineaments of the apennines on a large scale the distribution of pre existing faults and other weaknesses may affect the propensity for orogenic contraction in basement and therefore directly control larger scale tectonic processes in the central mediterranean the evolution of slab roll back and the related growth of overlying extensional basins e g tyrrhenian sea may be strongly modulated by the distribution of rift related weak zones in the adjacent continental crust the subduction of continental crust will strongly depend on the inherited structure of this crust specifically the distribution of deep crust of basic composition this develops relatively higher densities associated with eclogite metamorphism which act in turn to reduce the buoyancy of thickened continental crust that otherwise serves to inhibit further shortening investigating all these aspects from the scale of bulk crustal compositions to the geometry timing and strength of earlier fault zones preserved in orogenic belts requires the integration of substantial multidisciplinary geological data sets the extent to which continental orogenic belts represent the amplification of inherited geological heterogeneities as opposed to self ordered phenomena modulated by the syntectonic environment remains unclear

the neogene origin adoption evolution and controversy abstract some stratigraphers have recently insisted that for historical reasons the neogene miocene pliocene should be extended to the present however despite some ambiguity in its application by moriz hornes in the 1850s the neogene was widely adopted by european geologists to refer to the miocene and pliocene of lyell but excluding the diluvium later to become the pleistocene and alluvium later to become the holocene during the late 19th and early 20th centuries the ends of the neogene tertiary and pliocene evolved in response to the progressive lowering of the beginnings of the quaternary and pleistocene this evolution was a logical result of the widespread views that the most recent ice ages were worthy of recognition as a formal unit of the standard geologic time scale and that the structure of this time scale must be strictly hierarchical motivations for the extension of the neogene to the present include the desire to establish a monopoly for marine biochronology in the definition of standard global chronostratigraphic boundaries this agenda would also eliminate the tertiary quaternary and holocene these changes are unnecessary there is every reason to retain the traditional hierarchical structure of the cenozoic time scale

thick skinned tectonics and basement involved fold thrust belts insights from selected cenozoic orogens defining the structural style of fold thrust belts and understanding the controlling factors are necessary steps towards prediction of their long term and short term dynamics including seismic hazard and to assess their potential in terms of hydrocarbon exploration while the thin skinned structural style has long been a fashionable view for outer parts of orogens worldwide a wealth of new geological and geophysical studies has pointed out that a description in terms of thick skinned deformation is in many cases more appropriate this paper aims at providing a review of what we know about basement involved shortening in foreland fold thrust belts on the basis of the examination of selected cenozoic orogens after describing how structural interpretations of fold thrust belts have evolved through time this paper addresses how and the extent to which basement tectonics influence their geometry and their kinematics and emphasizes the key control exerted by lithosphere rheology including structural and thermal inheritance and local regional boundary conditions on the occurrence of thick skinned tectonics in the outer parts of orogens

thermal barriers and the fate of perched faunas thermal barriers provide an explanation for the geologically sudden extinction of benthic faunas of epeiric seas when these seas disappeared by contracting to the open ocean biotic interactions could not have caused the sudden extinctions and neither could reduction of the regional area of seafloor because substantial areas of shallow seafloor remained along neighboring continental shelves when epeiric seas drained instead temperature contrasts must have been responsible epeiric seas had strongly seasonal climates and when some receded to continental margins many of their species would have encountered waters that failed to provide the maximum or minimum temperature required for reproduction when epeiric seas receded poleward equatorward or into coriolis driven currents many species faced lethal temperatures the history of the jurassic sundance sea provides a striking example of the fate of a warm adapted fauna driven westward into an area dominated by a cool coriolis driven current

capelinhos 1957 1958 faial azores deposits formed by an emergent surtseyan eruption the 1957 1958 eruption of capelinhos faial island azores involved three periods of surtseyan hydromagmatic activity two in 1957 and one in 1958 deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally 1 km from the vent five lithofacies are identified lithofacies i is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout lithofacies ii consists of beds with internally discontinuous lenses and has sand wave structures that increase in abundance toward the outer margins of the tuff cone this lithofacies is interpreted as having been deposited from pyroclastic surges lithofacies iii is composed of mantle bedded deposits with laterally discontinuous internal stratigraphy this lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges in the outer flanks of the tuff cone lithofacies iii grades laterally into fallout beds of lithofacies i lithofacies iv consists of alternating beds of coarse ash aggregates and non aggregated fine ash and is particularly well developed in distal regions some of this facies was formed by fallout alternating beds also occur plastered against obstacles up to 2 km from the vent indicating an origin from wet pyroclastic surges the orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated the distinction between surge and fallout in distal regions is uncertain because wind drifted fallout and decelerating surge clouds can generate similar deposits lithofacies v consists of scoria lapilli beds interpreted to be fallout from hawaiian style fire fountaining in the later stages of the eruption juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular 25 60 of clasts 30 vesicles however on both micro and macroscopic scales there is a wide range in clast vesicularity up to 70 vesicles indicating that although fragmentation was predominantly hydromagmatic vesiculation and magmatic volatile driven fragmentation operated simultaneously

regional biotic turnover dynamics in the plio pleistocene molluscan fauna of the wanganui basin new zealand sampling gaps and variable preservation greatly complicate attempts to interpret patterns of biotic change from the fossil record this study investigates whether observed patterns of first and last occurrences of mollusc species can be explained by poor or uneven sampling of a diverse 800 species plio pleistocene marine fauna from the wanganui basin north island new zealand estimates of sampling completeness are high ranging from 78 to 98 mean 88 despite several sampling gaps in the succession stratigraphic permutation tests suggest that most of the apparent peaks in the local first and last appearances of species are caused by uneven sampling however peaks of last occurrences of species during the late pliocene 2 5 2 0 ma and of first occurrences in the middle pleistocene 0 75 0 5 ma cannot be classified as artefacts and appear to represent real biological events a wide early pleistocene gap in the stratigraphic section in the basin prevents the precise estimation of the magnitude duration or timing of these biotic transitions mechanisms for the biotic change remain elusive better palaeoenvironmental control and detailed ecological study of selected molluscan clades are required to test alternative causal hypotheses however similar turnover episodes have been documented from the temperate and tropical western atlantic suggesting a global biotic perturbation on shallow shelves in response to plio pleistocene global environmental change

basin development and inversion at the appalachian structural front port au port peninsula western newfoundland appalachians in the humber zone of the newfoundland appalachians cambro ordovician shelf and foreland basin successions are affected by middle ordovician taconian orogeny and devonian acadian orogeny deformation on port au port peninsula the presence of the late ordovician to late silurian long point clam bank succession allows these episodes to be separated the taconian foreland basin stratigraphy on port au port peninsula is highly variable on the west coast platform carbonates are overlain by megaconglomerates of the cape cormorant formation which record progressive exposure of 1 km of the platform succession the conglomerates are restricted to a narrow zone consistent with derivation from a fault scarp originally immediately west of the outcrops in palinspastic restoration farther east at victors brook the cape cormorant formation is absent but the overlying almost undeformed goose tickle group contains conglomerate derived both from the upper part of the platform succession and from the tacon

late jurassic half graben control on the siting and structure of hydrocarbon accumulations uk norwegian central graben abstract the late jurassic structure of the central graben is discussed both in terms of the extensional features developed at this time and their subsequent controls on younger inversion structures the regional kinematics of the north sea rift suggest that during the late jurassic the central graben opened approximately orthogonally to the main nnw sse fault trend within the basin some evidence that triassic extensional faults were reactivated at this time is seen towards the basin margins and a significant triassic extensional history is inferred the late jurassic basin comprises the axial c 100 km of the central graben s l the basin axis is offset progressively to the se by both transfer zones and discrete transfer faults footwall uplift of the basin margins during extension provided the source for the upper jurassic reservoir sands encountered by exploration drilling these sands were probably preferentially fed into the basin at transfer zones in the margins salt structures are ubiquitous within the central graben and their siting is everywhere controlled by the underlying fault block topography salt structures form preferentially towards the crests of half graben or buttressed against their boundary faults inversion structures in the norwegian sector reactivated with a reverse sense of displacement late jurassic extensional faults a model is proposed which shows the two main inversion structures feda graben norway tail end graben denmark to be detached on antithetic reverse faults riding in the salt these faults terminate at tip folds within major salt walls the geometry of the inversion structures can be understood only when considered together with the geometry of the underlying late jurassic half graben

the petroleum potential of inversion anticlines in the banda arc the banda arc in eastern indonesia and newly independent timor leste east timor is the zone of collision between the northern margin of the australian continent and an oceanic island arc system bordering the eurasian plate structurally the banda forearc is a fold and thrust belt consisting of the imbricated outer edge of the australian continent overlain locally by fragments of the precollisional oceanic forearc the banda forearc is an established petroleum province in the island of seram but the remainder of the forearc has so far yielded no further exploration success despite close geological similarity to seram a major disincentive to exploration in the banda forearc has been a common perception of structural complexity this complexity may however have been overstated and this paper presents a simple structural model for the evolution of the banda forearc key to this model is the recognition of basement involved inversion structures in the deeper parts of the collision complex the inverted graben basins are filled with permianjurassic continental margin sequences including high quality source rocks of late triassicearly jurassic age and contemporaneous potential reservoir sequences sealed by middlelate jurassic shales these jurassic shales also act as an important decollement horizon separating shallow level structural complexity from a simpler structural style beneath characterized by large inversion anticlines the oseil field in seram may be located in such an inversion anticline and comparable prospective structures are identified elsewhere in the banda forearc particularly in timor and the tanimbar islands

a chronology of pliocene sea level fluctuations the u s middle atlantic coastal plain record abstract the sequence of marine and marginal marine silts and sands of the u s middle atlantic coastal plain preserves a relatively complete record of pliocene sea level highstands a general chronology for the deposition of these units has been developed previously from studies of the microfossil biostratigraphy complemented by a limited number of paleomagnetic and radiometric dates within this broad temporal framework it is currently difficult to assign ages to individual transgressive events with a resolution much better than 0 5 ma the age model proposed here attempts to refine the depositional history of the atlantic coastal plain by correlation with the more continuous deep ocean record the model assigns probable ages to the pliocene transgressions onto the atlantic coastal plain based on evidence for sea level highstands inferred from high resolution deep ocean 18 o records at least five major transgressive regressive events can be correlated with presumed eustatic fluctuations the temporal resolution for the timing of the transgressive events is greatly improved over that of the biostratigraphy alone many aspects of the proposed chronology require verification by more detailed field study but it is offered as a testable working hypothesis

fault kinematics and localised inversion within the troms finnmark fault complex sw barents sea abstract the areas bounding the troms finnmark fault complex are affected by complex tectonic evolution in this work the history of fault growth reactivation and inversion of major faults in the troms finnmark fault complex and the ringvassoy loppa fault complex is interpreted from three dimensional seismic data structural maps and fault displacement plots our results reveal eight normal faults bounding rotated fault blocks in the troms finnmark fault complex both the throw depth and displacement distance plots show that the faults exhibit complex configurations of lateral and vertical segmentation with varied profiles some of the faults were reactivated by dip linkages during the late jurassic and exhibit polycyclic fault growth including radial syn sedimentary and hybrid propagation localised positive inversion is the main mechanism of fault reactivation occurring at the troms finnmark fault complex the observed structural styles include folds associated with extensional faults folded growth wedges and inverted depocentres localised inversion was intermittent with rifting during the middle jurassic early cretaceous at the boundaries of the troms finnmark fault complex to the finnmark platform additionally tectonic inversion was more intense at the boundaries of the two fault complexes affecting middle triassic to early cretaceous strata our study shows that localised folding is either a product of compressional forces or of lateral movements in the troms finnmark fault complex regional stresses due to the uplift in the loppa high and halokinesis in the tromso basin are likely additional causes of inversion in the troms finnmark fault complex

catastrophic extinctions and the inevitability of the improbable the earth as we know it today has been shaped by natural processes of the past uniformitarianism emphasizes the cumulative effects of processes operating at uniform rates catastrophists single out the importance of rare and unusual events this article proposes a scale to measure the magnitude of rare events and cites the inverse magnitude frequency relation of naturally occurring events to formulate the philosophical basis for actualistic catastrophism mass extinction at the end of the cretaceous is held up as an example that a catastrophe such a bolide impact could influence the course of biologic evolution improbability of such an event is no argument against such a hypothesis because the improbable even those with odds of once every billion years must have taken place a few times in the geological history the immensity of geological time is not only important because of the immensity of the cumulative effects but also because the long duration provides a chance for the improbable to take place

selkirk fan structure southeastern canadian cordillera tectonic wedging against an inherited basement ramp a revised cross section through the selkirk fan structure provides the basis for a new model for the middle jurassic tectonic evolution of the southern omineca belt canadian cordillera palinspastic restoration of this cross section shows that the southwest verging structures along the west flank of the selkirk fan structure formed as a result of tectonic wedging of distal north american strata clachnacudainn complex beneath more proximal north american strata and that the selkirk fan structure developed outboard from a crustal ramp dogtooth high inherited from late proterozoic early paleozoic rifting along the western margin of north america the first episode of mesozoic deformation in southeastern british columbia occurred between 187 and 173 ma and involved the northeastward juxtaposition of the intermontane superterrane over the outer part of the north american continental terrace wedge it resulted in deep burial 20 25 km of the outer margin of north america a crustal ramp localized along the western edge of the late proterozoic early paleozoic dogtooth high impeded the northeastward propagation of the orogenic wedge comprising the intermontane superterrane and the imbricate underlying northeast verging thrust sheets of north american supracrustal rocks tectonic wedging involving southwest verging deformation occurred within the orogenic wedge and the resulting crustal thickening established sufficient topography and gravitational potential to drive the propagation of the deformation eastward into the dogtooth range and the rocky mountains the southwest verging structures along the west flank of the selkirk fan developed between approximately 173 and 168 ma concurrent with synorogenic extension and 10 km of exhumation the initial subsidence of the foreland basin during kimmeridgian time ca 154 ma provides the first indication of tectonic loading and lithospheric flexure of the north american plate it is interpreted to mark the time at which the orogenic wedge overrode the crustal ramp of the dogtooth high and advanced onto relatively thick and rigid continental lithosphere the tectonic model proposed for the selkirk fan structure illustrates how the configuration of the rifted margin influenced the style of crustal thickening during subsequent compressional deformation

structural and tectonic evolution of the humber zone western newfoundland 1 implications of balanced cross sections through the appalachian structural front port au port peninsula the appalachian structural front in western newfoundland which is principally a submarine feature that comes ashore on port au port peninsula is interpreted to be a structural triangle zone or tectonic wedge rocks within the triangle zone which are therefore transported include the taconian middle ordovician humber arm allochthon taconian foreland clastic sediments and the cambro ordovician platform succession the transported clastic sediments and platformal rocks as well as structurally involved grenville crystalline basement exposed east of the peninsula compose the acadian siluro devonian port au port allochthon which carried the humber arm allochthon as a high structural slice acadian transport of tens of kilometers westward is suggested this interpretation contrasts markedly with the traditional and widely accepted interpretation of the taconian and acadian orogenies in western newfoundland here we reinforce our earlier arguments for an allochthonous interpretation by presenting a series of six closely spaced balanced cross sections across the port au port peninsula these sections illustrate our interpretation of complex and noncylindrical structures within the triangle zone as well as the envisioned structural linkage between exposures on port au port peninsula and the geometry of the triangle zone interpreted from offshore seismic data 23 km along strike to the northeast the southeast vergent tea cove thrust which formed an early upper detachment to the triangle zone is offset and overturned by the north northwest vergent round head thrust the round head thrust is interpreted to flatten into a second southeast vergent thrust the red brook detachment which formed a late upper detachment to the triangle zone some map scale structures are interpreted as fault bend folds above oblique ramps on the red brook detachment allochthonous crystalline basement within the triangle zone beneath its attached platformal cover is interpreted as a result of thrust reactivation of a preexisting basement cutting normal fault which may have formed during rifting of the early paleozoic iapetus ocean this interpretation is suggested by the stratigraphy and geometry of spatially restricted coarse conglomeratic units on port au port peninsula and is supported by the restored cross sections

structure and eruptive mechanisms at surtsey volcano iceland glassy basalt tuff was the primary material cored in 1979 from a 181 m deep drill hole on the east tuff ring of surtsey volcano despite the fact that the hole extends 122 m below sea level all the core is similar to the exposed tephra composing the two tuff rings of the island the tuff includes abundant accretionary lapilli and tuff vesicles indicating that it was all deposited subaerially during the growth of the tuff rings repeated hydromagmatic explosion cycles began with a series of intermittent tephra finger explosions leading up to continuous uprush explosions which lasted for several minutes to several hours this nozzle like continuous activity produced eruption columns 100 250 m in diameter and 500 2000 m in height which probably quarried several hundred metres below the ground surface the continuous uprush explosion type provides a reasonable mechanism to excavate a diatreme from the top down during construction of the tuff rings concentric faults repeatedly downdropped a funnel like structure 400 800 m in diameter several hundred metres thus accounting for the presence of subaerially deposited tephra in the drill core far beneath sea level ring dykes later intruded upward along these faults and fed small lava flows heat in the surface tephra probably originated primarily from these shallow intrusions

tropical marine climate during the late paleozoic ice age using trace element analyses of brachiopods abstract the late paleozoic ice age can be considered an important analogue to the modern ice age but comparisons between the two time intervals have been hampered by the difficulty of resolving climatic changes that occur over short e g seasonal time scales in the paleozoic record as a first step toward overcoming these limitations this study employed mg ca trace element ratios sampled across the growth bands of six specimens of the brachiopod composita to assess differences in tropical marine mean temperature and seasonality in visean and moscovian time these time intervals bracket the onset of the late paleozoic ice age which occurred in mid serpukhovian time and thus provide a snapshot of climate changes from a greenhouse to an icehouse world shells were screened for diagenetic alteration by cathode luminescence and by comparing the values of mn sr na and b to those reported for modern brachiopods in addition a 87sr 86sr value measured for one specimen agreed with a published strontium isotope curve for the late paleozoic era both the mean and range of mg ca values were larger in visean than moscovian time suggesting colder and less seasonal temperatures during the late paleozoic ice age the decline in seasonality was due to a greater proportional reduction of maximum values which presumably represent summer temperatures than minimum values these relative differences between the pre ice age and ice age tropical climate provide a first step toward understanding the broad patterns of seasonal scale changes in temperature during a critical time in earth s history