calibration and testing of upland hillslope evolution models in a dated landscape banco bonito new mexico 1 in this study we tested upland hillslope evolution models and constrained the rates of regolith production colluvial transport and eolian deposition over geologic time scales in a dated volcanic landscape in northern new mexico using field measurements of regolith thickness geochemical analyses of regolith bedrock and regional dust numerical modeling of regolith production and transport and quantitative analyses of airborne light detection and ranging lidar digital elevation models dems within this volcanic landscape many topographically closed basins exist as a result of compressional folding and explosion pitting during eruption the landscape has evolved from an initial state of no regolith cover at 40 5 ka to its modern state which has highly weathered regolith ranging from 0 to 3 m with local thickness values controlled primarily by topographic position our models constrain the maximum rate of regolith production in the study area to be in the range of 0 02 to 0 12 m kyr 1 and the rate of colluvial transport per unit slope gradient to be in the range of 0 2 to 2 7 m 2 kyr 1 with higher values in areas with more aboveground biomass we conclude that a depth dependent colluvial transport model better predicts the observed spatial distribution of regolith thickness compared to a model that has no depth dependence this study adds to the database of estimates for rates of regolith production and transport in the western united states and shows how dated landscapes can be used to improve our understanding of the coevolution of landscapes and regolith cover

partitioning of sr ba rb y and lree between plagioclase and peraluminous silicic magma trace element partition coefficients between plagioclase and coexisting glass matrix have been determined for twenty nine rhyodacite rhyolite samples from nine volcanic centers strontium partition coefficients form two clear positive trends when plotted against the an content of plagioclase one trend has a steep slope with d sr between 5 6 and 15 8 t1 whereas the other trend has a gentle slope with d sr between 1 2 and 7 6 t2 barium partition coefficients show similar patterns a steeply sloped trend is formed by samples with d ba between 1 6 and 8 8 and a gently sloped trend by samples with d ba 1 d sr and d ba in plagioclase correlate positively and both d sr and d ba correlate positively with temperature samples with large d sr and d ba are all from rocks with 1 wt cao these rocks are peraluminous and also have low total sr and ba partition coefficients for sr and ba are influenced by whole rock cao in high al rhyolite systems where these systems have low cao the opportunity increases for other divalent cations to enter the plagioclase structure strontium and ba will then enter the m position of plagioclase to balance the charge deficiency caused by the substitution of al for si because of the potent influence of whole rock composition on sr and ba partitioning for low cao rocks values of partition coefficients in petrogenetic modeling needs to be selected carefully

structure and emplacement of a rhyolitic obsidian flow little glass mountain medicine lake highland northern california many rhyolitic obsidian flows show consistent stratigraphic relations among textural units exposed in the flow fronts of undissected flows and in cross sections of older flows the stratigraphy of the holocene little glass mountain rhyolitic obsidian flow consists of from bottom to top air fall tephra deposits basal breccia coarsely vesicular pumice obsidian finely vesicular pumice and surface breccia slightly crystalline rhyolite occurs near the vent areas a model of obsidian flow emplacement based on these textural relations is presented this stratigraphy may reflect the distribution of volatiles within the magma source region with the interlayered contact between coarsely vesicular pumice and overlying obsidian indicating stratification of volatiles in the magma body the distribution of pumiceous and glassy zones along with the orientations of flow banding can be used to map the surface structure of rhyolitic obsidian flows this complex structure as mapped on the little glass mountain flow reflects both the initial flow stratigraphy and subsequent deformational processes during emplacement of the lava three processes disrupt the initial flow configuration the rise of coarse pumice diapirs from the base of the flow the inward propagation of fractures in areas of extension and surface folding in sites of flow parallel compression subvertical flow banding in vent areas indicates that fracturing accompanies the emergence of lava most of the observed upper surface of a dome originates as a fracture plane the structure of domes that form over vent areas may reflect the orientation of dike like conduits as well as the local state of stress during extrusion

initial results from vc 1 first continental scientific drilling program core hole in valles caldera new mexico valles caldera 1 vc 1 is the first continental scientific drilling program csdp core hole drilled in the valles caldera and the first continuously cored well in the caldera region the objectives of vc 1 were to penetrate a hydrothermal outflow plume near its source to obtain structural and stratigraphie information near the intersection of the ring fracture zone and the precaldera jemez fault zone arid to core the youngest volcanic unit inside the caldera banco bonito obsidian coring of the 856 m well took only 35 days to finish during which all objectives were attained and core recovery exceeded 95 vc 1 penetrates 298 m of moat volcanics and caldera fill ignimbrites 35 m of precaldera volcaniclastic breccia and 523 m of paleozoic carbonates sandstones and shales a previously unknown obsidian flow was encountered at 160 m depth underlying the battleship rock tuff in the caldera moat zone hydrothermal alteration is concentrated in sheared brecciated and fractured zones from the volcaniclastic breccia to total depth with both the intensity and rank of alterations increasing with depth alteration assemblages consist primarily of clays calcite pyrite quartz and chlorite but chalcopyrite and sphalerite have been identified as high as 450 m and molybdenite has been identified in a fractured zone at 847 m carbon 13 and oxygen 18 analyses of core show that the most intense zones of hydrothermal alteration occur in the madera limestone above 550 m and in the madera and sandia formations below 700 m this corresponds with zones of most intense calcite and quartz veining thermal aquifers were penetrated at the 480 540 and 845 m intervals although these intervals are associated with alteration brecciation and veining they are also intervals where clastic layers occur in the paleozoic sedimentary rocks

stratigraphic relations and lithologic variations in the jemez volcanic field new mexico over 100 radiometric dates and recent detailed geologic mapping allow some refinements of the stratigraphic relations of major units and generalization of temporal lithologic variations in the jemez volcanic field volcanism had begun in the area by about 16 5 ma with episodic eruptions of alkaline basalts by 13 ma alkaline volcanism had been replaced with eruptions of more voluminous olivine tholeiite high silica rhyolite derived from melts of lower crust also was erupting by about 13 ma basalt and high silica rhyolite continued to be erupted until about 7 and 6 ma respectively but effusions of dominantly andesitic differentiates of basalt that began as early as about 12 ma volumetrically overshadowed all other eruptive products between 10 and 7 ma from 7 to 3 ma the dominant erupted lithology was dacite which appears to have been generated by mixing of magmas whose compositions are approximated by earlier andesites and high silica rhyolites less than 4 3 ma volcanism was dominated by eruption of rhyolitic tuffs field relations geochemistry and dates specifically indicate the following with regards to stratigraphie relations 1 distinctions among basalt of chamisa mesa paliza canyon formation basalts and lobato basalt for other than geographic reasons are artificial basaltic volcanism was continuous in volcanic field from 13 to 7 ma 2 canovas canyon and bearhead rhyolites form a continuum of high silica rhyolite volcanism from 13 to 6 ma 3 hypabyssal and volcanic rocks of the cochiti mining district probably represent the exhumed interior of a keres group volcano s 4 temporal overlaps exist among the major stratigraphie groups which may imply some genetic relations and 5 the tewa group formation cerro rubio quartz latite may more appropriately be considered part of the tschicoma formation of the polvadera group preliminary analysis of hydrothermal alteration in the context of the volcanic stratigraphy suggests at least three distinct hydrothermal events have occurred in the volcanic field s history

the rio caliente ignimbrite analysis of a compound intraplinian ignimbrite from a major late quaternary mexican eruption the rio caliente ignimbrite is a multi flow unit orcompound ignimbrite formed during a major late quaternary explosive rhyolitic eruption of la primavera volcano mexico the eruption sequence of the ignimbrite is complex and it occurs between lower and upper plinian air fall deposits it is therefore anintraplinian ignimbrite air fall layers pyroclastic surge mudflow and fluviatile reworked pumice deposits also occur interbedded between ignimbrite flow units a chaotic near vent facies of the ignimbrite includes co ignimbrite lag breccias segregated from proximal pumice flows the facies locates a central vent but one which could not have been associated with a well defined edifice many of the lithics in the exposed lag breccias and near vent facies of the ignimbrite appear to be fragments of welded rio caliente ignimbrite and indicate considerable vent widening or migration during the eruption nearer vent the ignimbrite is thickest and composed of the largest number of flow units here it is welded and is a simple cooling unit evidence suggests that it was only the larger thicker pumice flows that escaped to the outer parts of the sheet detailed analysis of four flow units indicates that the pumice flows were generally poorly expanded less mobile flows which would be produced by collapse of low eruption columns the analogy of a compound ignimbrite with a compound lava flow is therefore good a compound lava flow forms instead of a simple one when the volumetric discharge rate or intensity is low and in explosive eruptions this predicts lower eruption column heights a corollary is that the ignimbrite has a high aspect ratio

a gis based volcanic hazard and risk assessment of eruptions sourced within valles caldera new mexico abstract the objective of this study is to evaluate the spatial extent of a possible future eruption using a gis based volcanic hazard tool designed to simulate pyroclastic fallout and density currents pdcs as well as lava flows and to assess the social and economic vulnerabilities of the area at risk simulated pyroclastic fallout deposits originating from the el cajete crater within the valles caldera jemez mountains volcanic field new mexico are calibrated to isopach and lithic isopleth maps of the lower and upper el cajete as constructed by wolff et al 2011 the change in the axial orientation of fallout deposits between the lower and upper el cajete is best matched using seasonal variations in wind speed and direction based on modern atmospheric records the calibration of pdcs is based on the distribution and run out of the battleship rock ignimbrite once calibrated hazards are simulated at a second vent location determined from probability distributions of structural features the resulting hazard simulation maps show the potential distribution of pyroclastic fallout pdcs and lava flows indicating areas to the s se of valles caldera to be at greatest risk to assess hazard preparedness social vulnerability is evaluated for all census designated places cdps within the study site based on methods by the hazards and vulnerability research institute twenty four variables were selected as proxies of social vulnerability and a principal component analysis was used to generate eight components which account for 64 of the total variance the eight component scores are summed into a final score for each cdp and the standard deviations from the mean of the scores are mapped with the cdps allowing for an easy visualization of areas that are considered more socially vulnerable economic vulnerability is evaluated through a multi criteria evaluation of population infrastructure road types and land use each variable is categorized and assigned a value representing relative vulnerability based on cost and importance the variables are assigned weights relative to one another through a pairwise comparison and summed together into a final map showing the distribution of economic vulnerability which is useful when used with the hazard simulation maps for targeting areas for mitigation to reduce economic loss in order to evaluate the overall risk the hazard simulation maps and vulnerability assessments are aggregated through weighted linear combination and pairwise comparison matrices creating a total of three risk maps although the actual maps provide greater detail overall based on the criteria chosen the risk maps show that pyroclastic fallout has the greatest impact effecting areas up to 50 km s se of the valles caldera including highly vulnerable cities such as los alamos and white rock the pdc and lava flow hazards however impact significantly smaller areas primarily disturbing forested land the methodology presented in this paper allows for a robust analysis of the risks posed by eruptions sourced from the valles caldera and is especially useful in focusing mitigation strategies to reduce the loss from such hazardous events

the eruptive and magmatic history of the youngest pulse of volcanism at the valles caldera implications for successfully dating late quaternary eruptions abstract new 40ar 39ar and u th ages provide insight into the youngest eruptions at the valles caldera and also reveal previously unknown pulses of magmatism the youngest eruptive units collectively termed the east fork member of the valles rhyolite include the el cajete pyroclastic beds and co erupted battleship rock ignimbrite and the disconformably overlying banco bonito lava flow previous attempts to date these units using a variety of techniques yielded ages ranging from 1 ma new 40ar 39ar ages were generated using the high sensitivity multi collector argus vi mass spectrometer which provides more than an order of magnitude increase in precision compared to most single detector mass spectrometers 40ar 39ar dating of single crystals yields a range of ages of which the youngest populations are interpreted to represent the eruption age sanidine ages indicate that the el cajete pyroclastic beds and battleship rock ignimbrite erupted at 74 4 1 3 ka whereas the banco bonito lava erupted at 68 3 1 5 ka populations of older crystals represent variably degassed xenocrysts explaining why previous 40ar 39ar bulk step heating analyses yielded spuriously old and irreproducible results u th dating of unpolished zircon surfaces also yield multiple age populations which range from the eruption age to 350 ka indicating protracted magmatism during the 453 ka long eruptive hiatus prior to the eruption of the east fork member 40ar 39ar and u th ages indicate that the east fork member represents a short 6 1 2 8 ka eruptive cycle from a longer lived magmatic system beneath the southern caldera equally short repose periods similar to the interval between the el cajete battleship rock and banco bonito eruptions are possible during future volcanism at the valles caldera results demonstrate that detailed geochronology using single crystal and in situ techniques is necessary for understanding the eruptive history and magmatic evolution at some young volcanic systems

light lithophile element boron lithium abundances in the valles caldera new mexico vc 1 core hole boron and lithium abundance distributions are very different in the sedimentary and volcanic sections of the vc 1 core hole valles caldera new mexico in the sedimentary section the wide variations in boron and lithium abundances probably reflect both diagenesis and introduction of these elements in early hydrothermal fluids late higher temperature fluids appear to have leached boron and lithium from some rocks adjacent to fault zones and aquifers the correlation of boron with lithium suggests that these elements behave in similar ways during diagenesis and hydrothermal alteration the small variation of boron and lithium abundances in most of the vitreous volcanic rocks from the upper part of the core implies that their source was relatively homogeneous following primary devitrification boron appears to have been stable but lithium was mobile boron may have been stabilized by the development of boron bearing submicroscopic phenocrysts in the lavas or boron may be less soluble than lithium in relatively cool groundwater

alteration in the madera limestone and sandia formation from core hole vc 1 valles caldera new mexico core hole vc 1 penetrated the southwestern ring fracture zone of the 1 1 ma valles caldera and at a depth of 333 m intersected the top of the paleozoic section including the abo formation madera limestone and sandia formation reaching a total depth of 856 m the paleozoic rocks which consist of thin bedded limestone siltstone mudstone sandstone and local conglomerate are overlain by volcanic rocks of the caldera moat that are less than 0 6 ma diagenetic and at least three hydrothermal alteration stages were identified in the madera limestone and sandia formation diagenetic clay alteration was pervasive throughout the sedimentary rocks volcanic activity at 16 5 ma and continuing through the formation of the valles caldera resulted in high thermal gradients which caused recrystallization of diagenetic clay minerals interstratified smectite illite is the most diagnostic clay mineral throughout the section structurally the illite component in the ordered interstratified illite smectite changes gradationally from 70 at the top of the madera limestone to 95 at the base of the section in the sandia formation pyrite that occurs as small clots and lenses as well as finely disseminated is interpreted as being of diagenetic origin especially in organic rich beds low permeability of much of the paleozoic section precluded the deposition of hydrothermal minerals except in fractures and intergranular space in some of the more permeable sandstone and brecciated horizons three stages of hydrothermal mineral deposition are defined stage i is widespread and includes mainly chlorite calcite pyrite and interstratified smectite illite that was formed prior to caldera development at temperatures approximating 200 c stage ii is characterized by quartz sericite and scarce sulfides deposited locally by fluids at approximately 275 c shortly before or at the time of early caldera related volcanism the stage iii hydrothermal event associated with formation of the valles caldera caused complex brecciation accompanied by deposition of phengite and molybdenite at temperatures locally approaching 300 c mostly in rocks underlying the madera limestone during the present drilling program thermal aquifers were encountered at 483 m 113 c 533 m 125 c and 845 m 184 c very close to the stage ii and iii mineralized zones however measured temperatures of the present hydrothermal system are much lower than during the main hydrothermal events good permeability has been maintained in these three levels for a long time while the rest of the madera limestone and sandia formation was sealed by hydrothermal mineral precipitation

the effect of explosive eruption processes on geochemical patterns within pyroclastic deposits abstract the effects of magma fragmentation and atmospheric transport of pyroclasts in modifying tephra chemistry are quantitatively examined in order to assist in devising geochemical sampling strategies for young pyroclastic deposits with particular regard to air fall tephra magma fragmentation during explosive eruption results in crystal fractionation the extent of which increases with decreasing tephra particle size among the products of a single sustained plinian eruption variable atmospheric flight times of pyroclasts may cause simultaneous deposition of earlier erupted and later erupted material both of these processes will affect the degree and nature of chemical variations found in individual pyroclastic deposits their effects may be largely overcome by sampling coarse tephra within a narrow grain size range

non riemannian and fractal geometries of fracturing in geomaterials the mechanism of earthquakes is presented by use of the elastic dislocation theory with consideration of the continuous dislocation field the general problem of medium deformation requires analysis based on non riemannian geometry with the concept of the continuum with a discontinuity no more continuum here we derive the equilibrium equation navier equation for the continuous dislocation field by introducing the relation between the concepts of the continuous dislocation theory and non riemannian geometry this equation is a generalization of the laplace equation which can describe fractal processes like diffusion limited aggregation dla and dielectric breakdown 1313 moreover the kinematic compatibility equations derived from navier equation are the laplace equations and the solution of navier equation can be put in terms of functions which satisfy the biharmonic equation suggesting a close formal connection with fractal processes therefore the relationship between the non riemannian geometry and the fractal geometry of fracturing damage in geomaterials as earthquakes can be understood by using the navier equation moreover the continuous dislocation theory can be applied to the problem of the earthquake formation with active folding related with faulting active flexural slip folding related to the continuous dislocation field

chronology and pyroclastic stratigraphy of the may 18 1980 eruption of mount st helens washington many timed observations make it possible to subdivide the 9 hour plinian eruption of mount st helens on may 18 1980 into six phases defined by eruption style the phases are correlated with stratigraphic subunits of ashfall tephra and pyroclastic flow deposits the suite of pyroclastic deposits indicates that the eruption became more pumice rich and compositionally diverse with time perhaps owing to concurrent eruption of less evolved gas poor parts of the magma body with the more evolved gas rich parts the paroxysmal phase i 0832 0900 consisted of landslides lithic pyroclastic flows of a lateral blast and other explosions and a weak pre plinian column phase i pyroclastic deposits include lithic ash flow deposits intercalated with and overlying the voluminous debris avalanche deposit and basal pumice lapilli tephra that underlies a pisolitic ash layer the early plinian phase ii 0900 1215 consisted of vertical ejection of tephra with an early pulse of small pyroclastic flows on the upper flanks 1010 1035 a brief period of lithic ash ejection 1035 1100 and a pumice rich pulse that accompanied growth in height of the eruption column 1100 1215 deposits include minor pyroclastic flows on the crater rim and a reversely graded sequence of proximal tephra that include the lower pumice lapilli layer the lower lithic ash layer and the middle pumice lapilli layer all of which consist of evolved white dacitic pumice 63 64 sio2 during the early ash flow phase iii 1215 1500 the height of the eruption column decreased vertical ejection of tephra ceased and pyroclastic flows were fed from intermittent fountains phase iii deposits consist of a poorly exposed sequence 12 m of ash flow tuff that consist of many thin flow units 2 m each containing pumiceous white dacite 63 64 sio2 and denser gray silicic andesite 61 62 sio2 and fine grained ash cloud deposits interbedded with a nongraded middle pumice ash layer the climactic phase iv 1500 1715 developed in two stages fountain fed pyroclastic flows followed by a short pulse 1625 1715 of vigorous vertical ejection of tephra these stages were accompanied by the peak seismic energy release and peak eruption column height respectively climactic deposits consist of a thick 35 m sequence of thick lapilli rich ash flow sheets 4 12 m each with white and gray pumice and streaky scoria bands 60 sio2 in pumice breccia clasts and the reversely graded upper pumice lapilli layer that is interbedded with fine grained ash cloud deposits during the late ash flow phase v 1715 1815 eruption intensity waned but included a brief episode of small pyroclastic flows 1745 1815 phase v deposits consist of small distributary lobes of ash flow tuff containing white and gray pumice and minor fine ash deposits phase vi activity 1815 to may 19 1980 consisted of a low energy ash plume with transient increases in intensity while seismicity continued at depth sustained vertical discharge of phase ii prodeced evolved dacitewith high s cl ratios ash flow activity of phase iii is attributed to decreases in gas content indicated by reduced s cl ratios and increased clast density of the less evolved gray pumice climactic events are attributed to vent clearing and exhaustion of the evolved dacite

internal textures of rhyolite flows as revealed by research drilling recent drilling of 550 and 140 000 yr old rhyolite lava flows in the inyo dome chain california and valles caldera new mexico and field inspection of 8 ma flows in western arizona reveal a more detailed vertical zonation of lava textures in glassy rhyolite flows than has previously been recognized the upper vitrophyre of a flow can usually be subdivided into a surficial finely vesicular pumice underlain by obsidian coarsely vesicular pumice and a second obsidian layer beneath these layers is the crystalline center of the flow which in turn is underlain by a basal obsidian layer lower vitrophyre differences in lava vesicularity and crystallinity between zones in a flow result from primary effervescence devitrification lava rheology and migration of water vapor

intracaldera volcanic activity toledo caldera and embayment jemez mountains new mexico the toledo caldera was formed at 1 47 0 06 ma during the catastrophic eruption of the lower member bandelier tuff the caldera was obscured at 1 12 0 03 ma during eruption of the equally voluminous upper member of the bandelier tuff that led to formation of the valles caldera earlier workers interpreted a 9 km diameter embayment located ne of the valles caldera toledo embayment to be a remnant of the toledo caldera drill hole data and new k ar dates of toledo intracaldera domes redefine the position of toledo caldera nearly coincident with and of the same dimensions as the younger valles caldera the toledo embayment may be of tectonic origin or a small tschicoma volcanic center caldera this interpretation is consistent with distribution of the lower member of the bandelier tuff and with several other field and drilling related observations explosive activity associated with cerro toledo rhyolite domes is recorded in tuff deposits located between the lower and upper members of the bandelier tuff on the northeast flank of the jemez mountains recorded in the tuff deposits are seven cycles of explosive activity most cycles consist of phreatomagmatic tuffs that grade upward into plinian pumice beds a separate deposit of the same age and consisting of pyroclastic surges and flows is associated with rabbit mountain located on the southeast rim of the valles toledo caldera complex these are the surface expression of what may be a thicker more voluminous intracaldera tuff sequence the combined deposits of the lower and upper members of the bandelier tuff toledo and valles intracaldera sediments tuffs and dome lavas form what we interpret to be a wedge shaped caldera fill this sequence is confirmed by deep drill holes and gravity surveys this fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse we interpret the toledo valles caldera complex to be a pair of nearly coincident trapdoor calderas with the hinge on the west side and thick caldera fill in the east

extended cooling and viscous flow of large hot rhyolite lavas implications of numerical modeling results abstract a common misconception about rhyolite lava flows is that they cannot advance far from their vents and are constrained to be small due to their high viscosities although a high viscosity lava will indeed flow quite slowly it may advance a great distance if the lava efficiently retains heat and thus mobility and if the available magma supply is large to evaluate this possibility a finite difference numerical model was used to determine the rate of heat loss from units interpreted to be large rhyolite lavas cooling of such 100 300 m thick units is very slow and is further retarded by a vesicular carapace and by evolution of latent heat thermal history alone suggests that large thick lava flows could remain active for several decades to constrain the duration of flow advance of these units their inferred apparent viscosities are compared with those of active lava flows mean flow velocity of some large rhyolites in southwestern idaho usa was estimated based on the observation that the units must still have been advancing by viscous flow at the start of spherulite crystallization indicating perhaps a 10 to 18 year flow duration based on studies of obsidian dome california mean flow velocities would have been roughly 0 59 to 2 5 km yr and calculated average apparent viscosities of the flows ranged from 1 8 to 3 6 orders of magnitude greater than the viscosity of the lava itself this relationship also holds for active andesite and basaltic andesite block lava flows which have apparent viscosities 2 7 to 4 orders of magnitude greater than lava viscosities this implies that large rhyolites may indeed have been emplaced in the same manner as less silicic block lavas and that long distance viscous flow is not unrealistic for rhyolite lavas a good example of a large volume lava is the 15 km 3 rhyolite of the badlands sw idaho where vent relations show that lava effused from a fissure and advanced up to 9 km more extensive rhyolites could also be emplaced solely by effusion extended flow of these lavas may lead to the remelting and removal of basal crumble breccias and to the formation of welded shard textures in their margins both processes tending to obscure the lava flow origin of the units the great dimensions of lava like rhyolitic units with volumes of 10 to at least 200 km 3 are not a sufficient reason to assume they erupted as ignimbrites

the behavior of light lithophile and halogen elements in felsic magma geochemistry of the post caldera valles rhyolites jemez mountains volcanic field new mexico abstract post collapse rhyolite lava domes lava flows and pyroclastic rocks from valles caldera 1140 ka erupted from 1133 ka to approximately 520 60 ka have been sampled to study variations of light lithophile li be b and halogen f cl elements our principal objectives were 1 to examine the mobility of these elements during post eruptive devitrification and hydration and 2 to study their behavior during magma differentiation compared to fresh glassy samples devitrified rocks from the same dome are depleted in b li f and cl but not in be during devitrification be was immobile while the other elements were progressively more mobile in the order b

anisotropy of magnetic susceptibility in welded tuffs application to a welded tuff dyke in the tertiary trans pecos texas volcanic province usa consideration of published anisotropy of magnetic susceptibility ams studies on welded ignimbrites suggests that ams fabrics are controlled by groundmass microlites distributed within the existing tuff fabric the sum result of directional fabrics imposed by primary flow lineation welding and if relevant rheomorphism ams is a more sensitive indicator of fabric elements within welded tuffs than conventional methods and usually yields primary flow azimuth estimates detailed study of a single densely welded tuff sample demonstrates that the overall ams fabric is insensitive to the relative abundances of fiamme matrix and lithics within individual drilled cores ams determinations on a welded tuff dyke occurring in a choked vent in the trans pecos texas volcanic field reveals a consistent fabric with a prolate element imbricated with respect to one wall of the dyke while total magnetic susceptibility and density exhibit axially symmetric variations across the dyke width the dyke is interpreted to have formed as a result of agglutination of the erupting mixture on a portion of the conduit wall as it failed and slid into the conduit followed by residual squeezing between the failed block and in situ wallrock irrespective of the precise mechanism widespread occurrence of both welded tuff dykes and point welded aggregate pumices in pyroclastic deposits may imply that lining of conduit walls by agglutionation during explosive volcanic eruptions is a common process

a shallow attenuating anomaly inside the ring fracture of the valles caldera new mexico abstract spectral ratios of teleseismic direct and scattered p waves observed in the valles caldera new mexico show a systematic pattern of low amplitudes at sites inside the caldera relative to sites on or outside the ring fracture waveforms recorded at caldera stations are considerably more complex than those recorded outside the caldera the data used in this study were collected during a passive seismic monitoring experiment conducted in 1987 twenty four teleseismic events were recorded on two linear arrays spanning the caldera to first order the pattern of low amplitudes did not vary with source incidence angle or azimuth of approach and could not be explained by anomalous amplification at the ring fracture this observation suggests the presence of a shallow attenuating zone associated with the caldera fill material inside of the ring fracture we estimated the general features of the caldera s near surface structure for the two dimensional vertical cross section beneath the array using a modification of the aki larner discrete wavenumber method to forward model the observed amplitude variations our results indicate that the caldera fill material must be subdivided into at least two distinct zones a strongly attenuating lower zone extending to depths in excess of 4 km and a mildly attenuating surface layer to fit the data we had to assign an unrealistically low value to seismic q in the deeper attenuating anomaly we attribute this to the inability of the aki larner method to account for strong re direction of energy away from the caldera due to local heterogeneity that we could not include within the low q anomaly this interpretation is consistent with the pervasive fractured hydrothermal system that is known to exist in the caldera fill material

the el cajete series valles caldera new mexico three rhyolite eruptive units totaling 3 km3 dense rock equivalent volume are the youngest products from the valles caldera new mexico these pyroclastic and effusive units herein called the el cajete series were erupted over an appreciable time period with plinian and ignimbrite forming activity preceding an effusive phase by a lengthy but indeterminable interval new u th disequilibrium and published fission track dates support an age for the event between 170 and 130 ka the explosive phases formed the el cajete plinian deposit about 1 3 km3 minor dry pyroclastic surges and the battleship rock tuff about 1 0 km3 a valley confined welded ignimbrite the plinian eruption column is estimated to have been 28 km high during deposition of the most widely dispersed pumice fall unit slow effusion of the banco bonito obsidian lava flow 1 0 km3 onto a dissected surface cut into the el cajete and battleship rock pyroclastic deposits accompanied by minor explosive activity terminated the event twenty four bulk rock samples from all three eruptive stages exhibit little significant compositional variation and the three units are petrographically identical apart from differences arising from contrasting eruption styles very few phenocrysts appear to have been in equilibrium with their enclosing high silica rhyolitic liquid plagioclase grains are typically strongly resorbed while biotite and hornblende frequently occur in aggregate grains in which textures characteristic of plutonic rocks can be discerned these features result from partial melting of preexisting crustal igneous rock probably of dioritic to granodioritic composition and subsequent eruption of the resulting liquid plus restite crystals rapid generation and eruption of rhyolitic magma during this most recent phase of activity in the jemez mountains may imply that the valles magma system is presently in a state where small magma bodies are transient phenomena

partial homogenization of cryptoperthites in an ignimbrite cooling unit a single crystal x ray study of alkali feldspars of bulk composition or39 43 and or62 64 from a single cooling unit of battleship rock tuff northern new mexico reveals a trend of decreasing degree of exsolution from the non welded zone toward the densely welded center of the cooling unit crystals of bulk composition or62 64 range from cryptoperthite with both phases monoclinic in the nonwelded zone to virtually unexsolved crystals in the welded center of the cooling unit crystals of bulk composition or39 43 include crytpoperthites with both phases monoclinic and cryptoperthites with pericline twinned sodic lamellae with of the sodic phase increasing systematically from 87 3 in the nonwelded zone to 90 in the densely welded zone composition estimates based on unitcell parameters show decreasing compositional differences between coexisting lamellae toward the welded zone the feldspar crystals studied are interpreted to be xenocrysts which had undergone exsolution prior to incorporation in the erupting magma and which were then partially homogenized during emplacement and post emplacement cooling the data indicate a maximum re equilibration temperature of the feldspars of about 500 c and a more rapid cooling of the tuff than calculated for simple conduction in a uniform slab

calcrust 1985 seismic reflection survey whipple mountains detachment terrane california an overview calcrust is a new consortium of six academic institutions from california engaged in seismic reflection studies of the continental crust calcrust activities are concentrated in the southwestern u s where a diversity of geotectonic styles occur which are fundamental to the evolution of continental margins the good surface geologic control provided by excellent outcrop exposure and more than two decades of intensive mapping adds to the attractiveness of the region for deep crustal investigation current calcrust efforts are directed in large part toward development of a crustal transect from the colorado plateau stable north american craton to the salton trougwgulf of california continental riftinghansform the transect crosses a variety of tectonic elements central to the mesozoic and cenozoic evolution of the continental margin of western north america e g coney 1981 dickinson 1981 the line of transect is shown schematically in fig 1a also shown are the various major tectonic elements to be crossed both compressional and extensional structures are involved calcrust is not only addressing the seismic character and geometry of the individual elements or structures but also relationships between them the first work on the transect began during calcrust s pilot program from 1984 to 1986 it focussed on the detachment fault terrane of the whipple mountains in southeastern california fig 1b the whipple mountains are part of the metamorphic core complex of the north american cordillera davis et al 1979 1980 crittenden et al 1980 well exposed in the whipple mountains and common to the core complexes of arizona and southeastern california is a series of low angle normal or detachment faults of mid tertiary age indicating appreciable crustal extension also exposed in the range are zones of pronounced mylonitic foliation which are seen on a regional basis throughout large parts of the southwestern u s crittenden et al 1980 finally immediately south of the whipple mountains mesozoic thrusts and nappes hamilton 1982 are exposed in the riverside and big maria mountains fig ib two important questions are posed by the presence of these three structural elements detachments mylonites and thrusts in the vicinity of the whipple mountains

the taupo eruption new zealand i general aspects the ca a d 186 taupo eruption was the latest eruption at the taupo volcanic centre occurring from a vent at horomatangi reefs now submerged beneath lake taupo in the central north island of new zealand minor initial phreatomagmatic activity was followed by the dry vent 6 km 3 hatepe plinian outburst large amounts of water then entered the vent during the 2 5 km 3 hatepe phreatoplinian ash phase eventually stopping the eruption though large amounts of water continued to be ejected from the vent area causing gullying of the ash deposits after a break of several hours to weeks phreatoplinian activity resumed generating the 1 3 km 3 rotongaio ash notable for its fine grainsize and for containing significant quantities of non or poorly vesicular juvenile material the vent area then became dry again and eruption rates and power markedly increased into the 23 km 3 taupo ultraplinian phase which is the most powerful plinian outburst yet documented synchronous with this ultraplinian activity lesser volumes of non to partially welded ignimbrite were generated by diversion of ejecta from or partial collapse of the eruption column the rapid rate of magma withdraw al during this phase removed support from the vent area to trigger local vent collapse and initiate the catastrophic eruption of the ca 30 km 3 taupo ignimbrite finally after some years lava was extruded on to the floor of the reformed lake taupo and floating fragments derived from the lava carapace were driven ashore the known eruption volume is more than 65 km 3 while additional volumes are represented by primary material now beneath lake taupo and layer 3 to the ignim brite phases a total volume of more than 105 km 3 is likely equivalent to more than 35 km 3 of magma plus more than 3 km 3 of lithic debris airfall deposits more than 10 cm thick blanketed 30000 km 2 of land east of the vent while ignimbrite covers a near circular area of 20000 km 2 widespread and locally severe ground shaking occurred during but mostly after the eruption associated with subsidence in the lake taupo basin secondary deposits are abundant above and extending beyond the taupo ignimbrite consisting of the products of surface water interacting with the still hot ignimbrite and subsequent water reworking of the light pumiceous materials the complexity and size of this eruption preclude accurate forecasting of the size nature and return period of the inevitable next eruption from the taupo volcanic centre