eruptive processes and caldera formation in a nested downsagcollapse caldera cerro panizos central andes mountains abstract the cerro panizos ignimbrite center in the central andes mountains produced two ignimbrite sheets and many lava flows the ignimbrite of quebrada cienago was erupted at 7 9 ma and effusive eruptions continued until the two cooling units of the crystal rich dacitic cerro panizos ignimbrite were emplaced at 6 7 ma the lower unit has no laterally continuous flow breaks and was erupted from a single vent or small cluster of vents with limited fluctuation in discharge lithic fragments reach significant concentrations 5 only in the uppermost two meters of this cooling unit where they document vent wall collapse or the opening of a new vent the upper cooling unit contains many flow units with variations in welding thicknesses and lithic fragment concentrations implying an unsteady eruption column the opening of many vents and probable caldera collapse triangulation of anisotropy of magnetic susceptibility flow direction measurements locate a single vent for the lower cooling unit whereas the upper cooling unit had many vents within the present dome cluster a 15 km diameter topographic depression marked by inward dips of 4 8 at the cooling unit contact is centered on the vent area of the lower cooling unit the depression is interpreted as a downsag caldera formed during emplacement of the lower cooling unit collapse began late in the eruption of the lower cooling unit and continued through the emplacement of the upper cooling unit resurgent magmatism occurred as lava flows that mundated the caldera area a ring of dacite domes erupted until at least 6 1 ma in the northern half of the downsag caldera traces the margin of a collapse caldera associated with the upper cooling unit maximum caldera subsidence 353 km 3 is not enough to account for the erupted volume 652 km 3 dre minimum the ash poor crystal rich nature up to 50 crystals in the pumice 75 in the matrix of the cerro panizos pyroclastic flows resulted in poor retention of gases and there is little evidence of fluidization the high crystal content however favored modified grain flow processes in which several particle support mechanisms combine with grain dispersive forces it is postulated that the flows were initially partially fluidized but rapidly lost their gases and grain flow processes grew in relative importance grain flow processes destroyed evidence of fluidization before deposition

sedimentary response to intra arc extension controls on oligo miocene deposition sarcidano sub basin sardinia the oligo miocene sardinian rift is a rare example of a well exposed intra arc basin comprising several independent depocentres synthesis of field observations from one such depocentre the sarcidano sub basin in central sardinia not only allows a new and integrated sedimentological and tectonostratigraphic history for the area but also provides insights into controls on syn and post rift sedimentation in intra arc settings initial rifting during the late rupelian early chattian oligocene occurred on nw se trending normal faults and produced a half graben topography which controlled the dispersal of locally supplied coarse continental clastic sediments latest chattian aquitanian miocene ne sw trending normal faulting defined a more complex block topography that remained underfilled in all areas except those which received voluminous ignimbritic volcanic material subsequent marginal and shallow marine sedimentation progressively infilled the fault topography as sea level rose deposition of coarse clastic sediments eroded from fault scarps was focused in the hanging wall depocentres of the intersecting fault trends whilst platform carbonates became established in topographically higher areas protected from clastic sedimentary supply finally as the relative rise in sea level drowned sediment source areas in the burdigalian deeper marine marlstones dominated sedimentation the results demonstrate that the sarcidano sub basin was dominated by a mixed volcanic siliciclastic carbonate basin fill with abrupt facies changes developed in response to complex fault defined topography relative rise in sea level and an episodic volcanic supply whilst the rift structure largely controlled the provenance supply and accumulation of sediments relative sea level primarily controlled the types of facies that developed

formation of high grade ignimbrites part ii a pyroclastic suspension current model with implications also for low grade ignimbrites analogue experiments in part i led to the conclusion that pyroclastic flows depositing very high grade ignimbrite move as dilute suspension currents in the thermo fluid dynamical model developed the degree of cooling of expanded turbulent pyroclastic flows dynamically evolves in response to entrainment of air and mass loss to sedimentation initial conditions of the currents are derived from column collapse modeling for magmas with an initial h2o content of 1 3 wt erupting through circular vents and caldera ring fissures the flows spread either longitudinally or radially from source up to a runout distance that increases with higher mass flux but decreases with higher gas content temperature bottom slope and coarser initial grain size progressive dilution by entrainment and sedimentation causes pyroclastic currents to transform into buoyant ash plumes at the runout distance the ash plumes reach stratospheric heights and distribute 30 80 of the erupted material as widespread co ignimbrite ash pyroclastic suspension currents with initial mass fluxes of 107 1012 kg s can spread for tens of kilometers with only limited cooling although they move as supercritical strongly entraining currents for the eruption conditions considered here with increasing eruption mass flux cooling during passage through the fountain diminishes while cooling during flow transport increases the net effect is that eruption temperature exerts the prime control on emplacement temperature pyroclastic suspension currents can form welded ignimbrite across their entire extent if eruption temperature is to 1 3 tmw the minimum welding temperature high eruption rates a large fraction of fine ash and a ring fissure vent favor the formation of extensive high grade ignimbrite for very hot eruptions producing sticky partially molten pyroclasts analysis of particle aggregation systematics shows that factors favoring longer runout also favor more efficient aggregation which reduces runout as a result very high grade ignimbrites cannot spread more than a few tens of kilometers from their source in cooler pyroclastic currents particles do not aggregate and the sedimentation process may involve re entrainment of particles which potentially leads to more extensive cooling and longer runout such effects however are only significant when net erosion of substrate occurs model results can be employed to estimate mass flux and duration of ignimbrite eruptions from measured ignimbrite masses and aspect ratios the model also provides an alternative explanation of the observed decrease in h lratios with ignimbrite mass

the bad step tuff a lava like rheomorphic ignimbrite in a calc alkaline piecemeal caldera english lake district lava like tuffs are lithologically indistinguishable from lavas and form part of a temperature and composition controlled continuum from low grade tuffs which are non welded or slightly welded through high grade densely welded tuffs extremely high grade tuffs which may be agglutinated right to their upper and lower contacts to spatter fed lava flows in some high grade tuffs a component of nonparticulate flow may postdate emplacement and deposition but in extremely high grade tuffs non particulate deformation normally occurs during emplacement and deposition in such cases syn depositional non particulate deformation previously called primary welding and non particulate slumping previously called secondary flowage processes overlap and are continuous one into the other so that distinction between them and their resultant structures is unrealistic and inapplicable therefore the term rheomorphism should be used to embrace all types of non particulate flow the bad step tuff is the most lava like of a sequence of rheomorphic calc alkaline rhyolitic ignimbrites emplaced during a climactic caldera forming eruption episode in the english lake district it is a ponded sheet 40 to 400 m thick which comprises a basal crudely stratified heterolithic breccia a thick flow laminated and locally vesicular central part which beomes increasingly flow folded upwards and an upper autobreccia despite an absence of vitroclastic textures within the main laminated part field relations show it to be a tuff diagnostic criteria are 1 a gradation within a lithophysal zone from unambiguous vitroclastic matrix of the basal lithic breccia upwards into the central flow laminated tuff 2 only rate autobreccia at the base of the sheet but ubiquitous autobreccia at the top of the sheet and 3 close textural similarity with localized intensely rheomorphic parts of associated ignimbrites that widely display unequivocal vitroclastic textures where their rheomorphism is less marked the extremely high grade character of the bad step tuff may reflect its proximal setting in a piecemeal type caldera high emplacement temperatures resulted from high rate but low velocity vent emission from fissures along numerous cross cutting calderafloor faults producing very low boil over eruption columns and proximal ponding

sediment slump structures a review of diagnostic criteria and application to an example from newfoundland abstract in the study of the history of deformed sedimentary rocks it is important that the cause of the deformation and the timing of deformation relative to metamorphic and tectonic events be assessed there is a continuous gradation between the deformation of freshly deposited sediments by gravitational forces and the deformation of well lithified sediments by tectonic forces so determining the degree to which gravity tectonism and lithification influenced deformation can be very difficult the characteristics that may be considered in determining the origin of deformational structures in sedimentary rocks can be divided into five categories ductile deformation structures 1 can provide definitive criteria for recognizing post lithification deformation but not pre lithification deformation this also holds for 2 brittle deformation structures and decollements overprinting of sediment reworking or remobilization structures 3 are the best criteria for recognizing pre lithification structures fabrics 4 can be very useful but are as yet poorly understood and there are misconceptions in the literature about what sort of fabrics are or are not found in unlithified sediments spatial relationships 5 may also tell us a great deal but are most useful when taken in conjunction with other types of evidence complex fold patterns in sandstones on farmer head north central newfoundland provide a case study for criteria that may be used to determine the degree of lithification during deformation although these folds have been interpreted as slump folds by earlier workers fabrics and spatial relationships point to a tectonic post lithification origin for the structures a conclusion which has significant implications for the interpretation of the regional geology

cooling degassing and compaction of rhyolitic ash flow tuffs a computational model previous models of degassing cooling and compaction of rhyolitic ash flow deposits are combined in a single computational model that runs on a personal computer the model applies to a broader range of initial and boundary conditions than riehle s earlier model which did not integrate heat and mass flux with compaction and which for compound units was limited to two deposits model temperatures and gas pressures compare well with simple measured examples the results indicate that degassing of volatiles present at deposition occurs within days to a few weeks compaction occurs for weeks to two to three years unless halted by devitrification near emplacement temperatures can persist for tens of years in the interiors of thick deposits even modest rainfall significantly chills the upper parts of ash deposits but compaction in simple cooling units ends before chilling by rainwater influences cooling of the interior of the sheet rainfall does however affect compaction at the boundaries of deposits in compound cooling units because the influx of heat from the overlying unit is inadequate to overcome heat previously lost to vaporization of water three density profiles from the matahina ignimbrite a compound cooling unit are fairly well reproduced by the model despite complexities arising from numerous cooling breaks uncertainties in attempts to correlate in detail among the profiles may be the result of the non uniform distribution of individual deposits regardless it is inferred that model compaction is approximately valid thus the model should be of use in reconstructing the emplacement history of compound ash deposits for inferring the depositional environments of ancient deposits and for assessing how long deposits of modern ash flows are capable of generating phreatic eruptions or secondary ash flows

growth destruction and facies architecture of effusive and explosive volcanics in the miocene shama basin southwest of saudi arabia subaqueous subaerial volcanism in a lacustrine setting abstract this paper presents for the first time detailed stratigraphical and petrographical analyses of the harrat shama volcanic basin hsvb of miocene age in sw saudi arabia this basin is filled by abundant volcanics intercalated with fluvio lacustrine sedimentary successions these successions are characterized by a syneruptive intereruptive cycle which results from the intercalation periods of volcanics and volcaniclastic sedimentation along with periods of fluvial incision two depositional sequences showing an overall fining upward trend are preserved within the hsvb the first comprises hyaloclastites zeolite bearing bedded tuffs and bedded accretionary lapilli tuffs interpreted as deep lake whereas the second is made up of extensive pumiceous pyroclastic flows peperitic breccias and basaltic lava flows which are in turn capped by siliciclastic deposits of lacustrine deposits the passage between two sequences is defined by lateral variations of the lithofacies and textural features reflecting a change in paleogeography the fining upward volcaniclastic deposits of the hsvb sequence are interpreted to represent the sedimentation pattern induced by tectonic subsidence with the influence of volcaniclastic sediment input indicating well established proximal distal relationship hsvb was evolved through three stages including pre syn and post caldera stages the first two stages correspond to proximal aggradational syn eruptive event whereas the latter stage refers to distal degradational inter eruptive event these stages overlap in time as shown by the alternation of explosive effusive and their resedimented deposits reflecting diverse eruptive sequences formed by different fragmentation processes the difference in the latter coupled with the variability in the magma water interaction and the nature of substrate are responsible for such diversity resulting in progressive environmental changes from subaqueous to subaerial regime from bottom to top the development of such contrast in the depositional sequences reflects variation in the sediment flux and subsidence rate during the evolution of the hsvb

the emplacement of a large chemically zoned rheomorphic lava like ignimbrite the sgurr of eigg pitchstone nw scotland the sgurr of eigg pitchstone on the isle of eigg nw scotland is a crystal rich trachydacitic partially vitrophyric rock which has previously been interpreted both as a lava and as a sill we interpret this rock as a chemically zoned rheomorphic lava like ignimbrite that formed during a sustained pyroclastic eruption the sgurr of eigg pitchstone can be subdivided into discrete emplacement units distinguished by the following features 1 their present day weathering characteristics 2 the orientation spacing and morphology of the columnar joints 3 sharp undulating boundaries with marked topographic breaks the absence of weathered surfaces palaeosols pyroclastic fall deposits or sedimentary rocks at emplacement unit boundaries suggests deposition from a single eruption the emplacement units some of which display upper and basal vitrophyres represent distinct depositional packages that record several rapidly emplaced ignimbrites which welded cooled and devitrified as a simple essentially single cooling unit during eruption from a sustained low pyroclastic column the sgurr of eigg pitchstone displays a pervasive base parallel flow banding which is folded into intrafolial recumbent isoclinal folds the flow banding and folds indicate that rheomorphism occurred throughout deposition the sgurr of eigg pitchstone is interpreted as an erosional remnant of an extensive ignimbrite sheet the first such unit recorded within the north atlantic igneous province

welded tuffs deformed into megarheomorphic folds during collapse of the mcdermitt caldera nevada oregon the montana mountains contain part of a resurgent dome complex within the southwestern sector of the mcdermitt caldera exposed intracaldera units consist of welded tuffs lavas domes and epiclastic deposits the majority of volcanic units have a comenditic composition but a few units are nonperalkaline and biotite bearing a deformed series of comenditic welded tuffs occur in a strongly folded rheomorphic zone more than 500 m thick the geometry of vitrophyres flow bands foliations and lineations causes the rheomorphic zone to superficially resemble a series of intrusive domes however the contacts between units are conformable the vitrophyres are continuous from one fold to the next and the units display pumice shards and other features common in welded tuffs the deformed tuff sequence lies on top of biotite tuffs and epiclastic deposits that are tilted and faulted but not folded and beneath relatively undeformed tuffs and lake deposits this relationship suggests that their large scale folds in the deformed sequence developed shortly after caldera collapse

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

eruption and emplacement of a basaltic welded ignimbrite during caldera formation on gran canaria the 14 1 ma old composite ignimbrite cooling unit p1 45 km3 on gran canaria comprises a lower mixed rhyolite trachyte tuff a central rhyolite basalt mixed tuff and a slightly rhyolite contaminated basaltic tuff at the top the basaltic tuff is compositionally zoned with a an upward change in basalt composition to higher mgo content 4 3 5 2 wt b variably admixed rhyolite or trachyte commonly 5 wt and c an upward increasing abundance of basaltic and plutonic lithic fragments and cognate cumulate fragments the basaltic tuff is divided into three structural units i the welded basaltic ignimbrite which forms the thickest part c 95 vol and is the main subject of the present paper ii poorly consolidated massive bomb and block rich beds interpreted as phreatomagmatic pyroclastic flow deposits and iii various facies of reworked basaltic tuff tuff unit i is a basaltic ignimbrite rather than a lava flow because of the absence of top and bottom breccias radial sheet like distribution around the central tejeda caldera thickening in valleys but also covering higher ground and local erosion of the underlying p1 ash a gradual transition from dense rock in the interior to ash at the top of the basaltic ignimbrite reflects a decrease in welding the shape of the welding profile is typical for emplacement temperatures well above the minimum welding temperature a similar transition occurs at the base where the ignimbrite was emplaced on cold ground in distal sections in proximal sections the base is dense where it was emplaced on hot felsic p1 tuff the intensity of welding especially at the base and the presence of spherical particles and of mantled and composite particles formed by accretion and coalescence in a viscous state imply that the flow was a suspension of hot magma droplets the flow most likely had to be density stratified and highly turbulent to prevent massive coalescence and collapse model calculations suggest eruption through low pyroclastic fountains 1000 m high with limited cooling during eruption and turbulent flow from an initial temperature of 1160 c the large volume of 26 km3 of erupted basalt compared with only 16 km3 of the evolved p1 magmas and the extremely high discharge rates inferred from model calculations are unusual for a basaltic eruption it is suggested that the basaltic magma was erupted and emplaced in a fashion commonly only attributed to felsic magmas because it utilized the felsic p1 magma chamber and its ring fissure conduits evolution of the entire p1 eruption was controlled by withdrawal dynamics involving magmas differing in viscosity by more than four orders of magnitude the basaltic eruption phase was initially driven by buoyancy of the basaltic magma at chamber depth and continued degassing of felsic magma but most of the large volume of basalt magma was driven out of the reservoir by subsidence of a c 10 km diameter roof block which followed a decrease in magma chamber pressure during low viscosity basaltic outflow

distribution and significance of crystalline perlitic and vesicular textures in the ordovician garth tuff wales diverse spherulitic and granular crystalline fabrics perlitic textures and fabrics related to the growth and migration of vesicles occur in the garth tuff a largely welded ordovician ignimbrite defining the distribution of such textures helps to constrain the ignimbrite s cooling and degassing history suites of spherulitic and perlitic textures closely reflect variation in cooling rates seven facies are defined based on the style and intensity of crystallisation 1 a medium to coarsely crystalline equigranular facies 2 an intensely spherulitic facies 3 a sparsely spherulitic facies 4 a pectinate facies 5 a microcrystalline to cryptocrystalline equigranular facies 6 a lithophysal facies and 7 a transitional perlite pectinate facies textural changes from facies 1 to 5 reflect progressively higher cooling rates facies 1 occurs in proximal settings in the ignimbrite s core facies 2 to 5 successively envelop facies 1 with facies 2 becoming the dominant fabric in the ignimbrite s core in medial settings facies 5 is typically developed in the originally glassy perlitic zones at the ignimbrite s welded margins crystallisation under hydrous conditions is reflected by second boiling textures in the sporadically developed lithophysal facies the seventh facies reflects a subtle interplay between cooling hydration and crystallisation which locally prevented perlitic fracturing the distribution of amygdales reflects patterns of volatile migration and entrapment in the lower levels of the ignimbrite amygdales occur in irregular concentrations or rare subvertical pipe like structures pipe like structures attest to fumarolic activity while the ignimbrite was in a rheomorphic state amygdales are widespread and evenly distributed in the upper levels of the ignimbrite however the top of the welding profile is characterised by a thin poorly vesiculated originally vitrophyric horizon that abruptly caps an intense concentration of amygdales ductile and brittle fabrics developed during the upward migration of gas microscopic drag folds occur around some amygdales jigsaw fit to clast rotated breccias originated through both late stage pneumatic fracturing and autobrecciation vaporisation of water at the flow base provided a significant source of volatiles in addition to gas released during cooling and crystallisation secondary alteration has enhanced or modified some fabrics perlitic zones were susceptible to patchy chlorite sericite carbonate diagenetic alteration diagenesis and metamorphism have contributed to the infilling of vesicles

authigenic chromium enrichments in proterozoic ironstones abstract chromium isotopes have recently emerged as a proxy for atmospheric oxygen levels in the precambrian chromium cr enrichments along with a suite of other redox sensitive metals have also been used as marine redox proxies for decades despite significant interest and growing use of this system which has been used to argue for low oxygen levels through the mid proterozoic leading up to the rise of animals there is a poor understanding of the host phases of cr in the sedimentary record without this context interpretation of measured isotopic signatures and concentrations of cr in various sedimentary archives can be ambiguous in order to shed light on this problem we have used electron microprobe elemental mapping to identify authigenic and detrital cr phases we have examined a range of ironstone samples that have been previously analyzed for cr isotopes and find that cr is predominantly associated with iron rich allochems instead of detrital cr mineral phases we did not find evidence for detrital chromite or other cr rich detrital minerals additionally we find evidence for authigenic ti enrichments and decoupling of al and ti elements traditionally used as detrital tracers similar to what has been reported in other slowly accumulating metal rich sediments e g iron manganese nodules lastly we have explored a range of possible sedimentary cr isotope signals that could result from changes in the percentage of detrital versus authigenic cr incorporated into a sample providing clear evidence for the ambiguity in interpretations of this isotope system without petrographic context

sedimentation and welding processes of dilute pyroclastic density currents and fallout during a large scale silicic eruption kikai caldera japan abstract sedimentation and welding processes of the high temperature dilute pyroclastic density currents and fallout erupted at 7 3 ka from the kikai caldera are discussed based on the stratigraphy texture lithofacies characteristics and components of the resulting deposits the welded eruptive deposits unit b were produced during the column collapse phase following a large plinian eruption and preceding an ignimbrite eruption and can be divided into two subunits units b l and b u unit b l is primarily deposited in topographic depressions on proximal islands and consists of multiple thin u shows densely welded stratified facies composed of alternating lithic rich and pumice rich layers the layers mantle lower units and are sometimes viscously deformed by ballistics the sedimentary characteristics of unit b l such as welded stratified or cross stratified facies indicate that high temperature dilute pyroclastic density currents were repeatedly generated from limited magma water interactions it is thought that dense brittle particles were segregated in a turbulent current and were immediately buried by deposition of hot lighter pumice rich particles and that this process repeated many times it is also suggested that the depositional temperature of eruptive materials was high and the eruptive style changed from a normal plinian eruption through surge generating explosions unit b l into an agglutinate dominated fallout eruption unit b u on the basis of field data welded pyroclastic surge deposits could be produced only under specific conditions such as 1 rapid accumulation of pyroclastic particles sufficiently hot to weld instantaneously upon deposition and 2 elastic particles interactions with substrate deformation these physical conditions may be achieved within high temperature and highly energetic pyroclastic density currents produced by large scale explosive eruptions

large scale rheomorphic shear deformation in miocene peralkaline ignimbrite e gran canaria the single ignimbrite cooling unit e average thickness 28 m volume ca 30 km3 forms the uppermost member of the miocene upper mogan formation on gran canaria it is strongly chemically zoned from basal first erupted comendite peralkaline rhyolite to late erupted trachyte and apart from an upper trachytic zone it is densely welded e was emplaced onto a surface inclined ca 2 5 from the source caldera detailed mapping of key sections up to 300 m long exposed in barranco walls ca 10 km from the caldera margin reveals structures that are interpreted to have been produced by rheomorphic deformation of the ignimbrite along shear zones the shear zones formed within the lower viscosity comenditic tuff extensional structures include mega boudinage and decapitated sequences and compression resulted in sequence repitition by overthrusting mechanisms traditionally thought to be important during rheomorphic deformation of welded tuffs compaction lateral creep folding vertical density driven diapirism cannot account for these features which reflect lateral post compactional rheomorphic movement locally in excess of 800 m we suggest the following sequence of events emplacement of the several flow units compaction with little lateral movement rheomorphic deformation during and after compaction layers of secondary porosity developed within the comenditic tuff possibly where upward escape of gas was prevented by overlying relatively impermeable layers of densely compacted ignimbrite these structurally weak layers of high porosity subsequently acted as shear zones

when did sardinia stop rotating new palaeomagnetic results previous palaeomagnetic results from tertiary volcanic rocks of sardinia suggest that the island underwent a quick counterclockwise rotation between 20 5 and 19 ma in order to test this hypothesis a new palaeomagnetic study was carried out on sediments and volcanic rocks with well controlled biostratigraphic or radiometric ages younger than the suggested age for the end of the rotation unrotated palaeomagnetic directions obtained from the upper ignimbrites radiometric age 18 5 19 7 myr and the aquitanian tuffs collected in the anglona region n sardinia would suggest that the rotation ended earlier than 20 5 myr the directions found in the volcanics are perhaps the result of secular variation rather than differential rotation of the anglona region with respect to surrounding regions two sites belonging to the upper ignimbritic unit si2 in the monte traessu monte rispisu area logudoro region exhibited a large amount of rotation d 320 4 i 60 which may be age related radiometric age 19 9 myr four dikes radiometric age 18 3 16 7 myr in the arcuentu volcanic complex arburese region yield results d 348 4 i 33 8 that indicate a further amount of rotation after their emplacement stable end point directions were successfully isolated from three upper burdigalian langhian sedimentary sites biostratigraphic age 18 15ma from the marmilla and logudoro regions consistent paleomagnetic directions obtained from these sites d 352 i 49 indicate that a small amount of rotation was still to be completed in the langhian time this result is consistent with the history of sardinian volcanism which did not end until 13 ma

ropy flow structures a neglected indicator of magma flow direction in sills and dikes ropy flow structures that form in dikes and sills are very similar to ropy lava structures pahoehoe but are of intrusive origin they are unusual among magma flow indicators in that they provide a frozen record of the magma flow our observations suggest that they occur in large vesicles which form as a result of local repeated pressure drops during propagation and emplacement of the magma adiabatic gas expansion causes a ductile rim to form around a vesicle this rim is then sheared by the underlying flow anisotropy of magnetic susceptibility data from the contact zones confirm the flow data from the ropy flow structures but the results from deeper within the body both locally and regionally yield different yet consistent flow orientations this discrepancy suggests that a distinction can be made between the magma flow close to the contact and the deeper regional flow patterns in sills and dikes

the emplacement history of a remarkable heterogeneous chemically zoned rheomorphic and locally lava like ignimbrite tl on gran canaria abstract ignimbrite tl on gran canaria is a complex compositionally zoned rheomorphic tuff that locally exhibits features previously considered to be diagnostic of lavas it is made up of two locally overlapping lobes of ignimbrite that were emplaced during a single eruptive episode the eastern lobe is high grade with rheomorphic zones and localised patches that are lava like the western lobe is extremely high grade more extensively lava like and welded to its top surface both parts are zoned with a basal comendite rich zone grading up through a mixed zone into an upper trachyte rich zone lithic contents and the relative proportions of comendite and trachyte pyroclasts vary with height each comendite rich zone is vitroclastic whereas each trachyte rich zone is partly lava like with local gradations into vitroclastic ignimbrite mixed zones are intermediate in character and locally show compositional banding gradational zoning in massive ignimbrite best seen in lower strain zones and welding fabrics that are pervasively lineated and oblique to bedding suggest that deposition was sustained agglutination was rapid and rheomorphic deformation began during the sustained deposition the viscosity and porosity of the agglutinate varied with height because successively deposited pyroclast populations varied in grainsize composition and temperature the hot agglutinate continued to compact and shear downslope after the density currents had dissipated causing further rheomorphic folding thrusting attenuation and autobrecciation the western lobe locally overlies the partly welded top of the eastern lobe in part because it advanced rheomorphically across it for at least 300 m hot state loading and auto intrusion occurred due to unstable density layering in the chemically zoned agglutinate deformation behaviour changed during cooling and degassing and because of heat transfer between juxtaposed agglutinates and localised retention of dissolved volatiles where there was an overlying impermeable cap

magnetic fabric welding texture and strain fabric in the nuraxi tuff sardinia italy anisotropy of magnetic susceptibility ams has been used to interpret flow directions in ignimbrites but no study has demonstrated that the ams fabric corresponds to the flow fabric in this paper we show that the ams and strain fabric coincide in a high grade ignimbrite the nuraxi tuff a miocene rhyolitic ignimbrite displaying a wide variability of rheomorphic features and a well defined magnetic fabric natural remanent magnetization nrm data indicate that the magnetization of the tuff is homogeneous and was acquired at high temperatures by ti magnetite crystals comparison between the magnetic fabric and the deformation features along a representative section shows that ams and anisotropy of isothermal remanent magnetization airm fabric are coaxial with and reproduce the shape of the strain ellipsoid magnetic tests and scanning electron microscopy observations indicate that the fabric is due to trails of micrometer size pseudo single domain magnetically interacting magnetite crystals microlites formed along discontinuities such as shard rims and vesicle walls mimicking the petrofabric of the tuff the fabric was thus acquired after deposition before late rheomorphic processes and accurately mimics homogeneous deformation features of the shards during welding processes and mass flow

the formation of high grade ignimbrites i experiments on high and low concentration transport systems containing sticky particles high grade ignimbrites are thought to be deposited by pyroclastic flows at temperatures exceeding minimum welding temperature or even solidus temperature corresponding pyroclastic flow particles range from plastic to partially liquid and are able to aggregate or coalesce this contrasts with particles in pyroclastic flows producing unwelded ignimbrite which are capable of elastic grain interactions the low aspect ratio and great areal extent of high grade ignimbrites requires transport in a particulate state either by a high concentration mass flow facilitated by fluidizing gas reducing internal friction or by b expanded turbulent flow of low but downward increasing concentration this paper presents experiments designed to investigate the effects of plastic to liquid particles on these two contrasting transport mechanisms gas fluidization experiments using polyethyleneglycole peg powders heated above minimum sintering tms and melting tm temperatures cover a wide range of fluidization velocities umf ua 0 6 ut but are always in the bubbly fluidization regime similar to fluidized ignimbrite ash where particle volume concentration outside the bubbles is high 10 1 when the powders reach a critical temperature tm t tms defluidization by catastrophic particle aggregation immediately commences in both stationary and laterally moving fluidized beds as well as in experiments using mixtures of high and low tm 30 wt peg powders when t tms of the lower tm powder this indicates that extended particulate transport at t tms is not possible at such high particle concentrations in the turbulent flow experiments liquid sprays of molten peg or water vertically injected into a high re 104 horizontal air flow form a low concentration 10 5 to 10 4 turbulent suspension current proximal formation of partially coalesced aggregates which settle faster than individual particles causes the measured downstream decay of sedimentation rate to be steeper than predicted by theory of single solid particle sedimentation from turbulent suspensions as particles become finer downstream and coalescence efficiency decreases in response to cooling more distally formed aggregates become too small and rare to modify sedimentation rate decay from that of suspension flows containing solid particles the key difference between the two transport systems is particle concentration c since particle collision rate rcoll c2 collision rates in fluidized beds are so high that all particles immediately aggregate when coalescence efficiency 1 ecoal 0 is larger than 10 3 low concentration suspensions on the other hand require much higher values of ecoal for significant aggregation to occur dilute pyroclastic flows will have higher particle volume fractions 10 3 than the experimental currents but then viscous pyroclasts should have lower coalescence efficiencies than peg droplets experimental results thus support an expanded turbulent transport mechanism of pyroclastic flows generating extensive high grade ignimbrite sheets

development of fluid veins during deformation of fluid rich rocks close to the brittle ductile transition comparison between experimental and physical models to understand the factors affecting the orientation and spacing of fluid veins in deforming silicate matrices experimental results on the deformation of sub micron flint have been examined new results and data from the literature show that deformation of flint at high temperature fluid rich conditions 6 vol fluid and strains less than 0 1 0 2 leads to the development of fluid banding orthogonal to consistent with theoretical expectations on the other hand above these strains and with fluid poor conditions 1 vol conjugate r1 and r2 riedel bands are observed related to brittle fracture these latter bands cross the first generation of 45 bands formed at lower it is of note that for strains up to 0 5 the strain rate remains uniform throughout the entire sample but that above this value deformation is concentrated in a single prominent riedel band to understand and rationalize these observations one dimensional numerical modelling of fluid rock separation during shear has been performed the model assumes a constant strain rate and uses the interstitial fluid dependence of the pressure solution viscosity of quartz at the beginning of the numerical simulation fluid and solid pressures are equal pf ps but shearing leads to the development of zones of compaction from which fluid is expelled pf drops and the solid viscosity rises sharply because strain rate is uniform across the bulk sample the model implies that local stress rises sharply in the compaction bands but remains low in zones where fluid accumulates indeed the model shows that in the zones of fluid extraction both the deviatoric stress and the excess pressure pf ps have the same amplitude their value exceeds the bulk shear stress by a factor of about five in light of these numerical results we tentatively suggest that in certain experimental studies low angle fluid rich veins may be initiated by transient embrittlement of the rock in zones of fluid loss subsequently a drop in permeability orthogonal to cracking may lead to compaction in that direction filling the cracks with fluid consideration of the strain rate conditions of the mantle calls into question whether low angle melt veins will occur or not in natural olivine basalt systems although such veins may be relevant for fluid percolation in quartz fault gouges

rheomorphic structures in a high grade ignimbrite the nuraxi tuff sulcis volcanic district sw sardinia italy deposits of the 15 8 ma nuraxi explosive eruption crop out in the sulcis volcanic district southwestern sardinia they consist of a decimeter thick pumice fallout layer overlain with no apparent temporal break by several tens of meters thick high grade ignimbrite the eruption was from a vent probably located 20 30 km north of s pietro island and it tapped a compositionally uniform rhyolite magma reservoir sio2 70 72 wt bearing about 20 vol crystals the ignimbrite consists of a a lower ignimbrite li 2 m thick fine grained and glassy grading upward into a partly devitrified lapilli tuff and b an upper ignimbrite ui that is an intensely welded to lava like lapilli tuff the deposit has an inferred maximum runout of about 80 km and overall dispersal thickness characteristic of a low aspect ratio ignimbrite over most of its outcrop the tuff rests on a flat horizontal paleosurface and forms an ignimbrite plateau with uniform thickness and structural characteristics however in some areas it shows extensive folding and over thickening as a result of rheomorphic flow into topographic depressions structural analysis of the ignimbrite reveals the presence of a pervasive foliation and lineation as well as macroscopic and microscopic flow structures occurrence of abundant widely distributed stretched cavities in the tuff indicates that a substantial amount of gas was trapped in the tuff during aggradation the observed pattern of structures of the ignimbrite plateau accords well with a conceptual model in which syn depositional structures are formed during non particulate basal laminar flow due to agglutination of juvenile particles within the lower part of the pyroclastic density current the flow was controlled by the substantial transfer of momentum from the high speed particulate upper part of the current to the top of the non particulate part of the current and the onset of a complex relationship between welding flow and sedimentation processes it is proposed in particular that elongated gas pockets played a major role in guiding the laminar shear within the non particulate part of the current in flat horizontal areas eventually promoting brittle failure of the deforming tuff mass along shear planes