geology geochronology and tectonic setting of the jorullo volcano region michoacan mexico abstract the jorullo monogenetic volcano erupted 250 years ago at the southern border of the trans mexican volcanic belt tmvb an area that records a long history of magmatic and tectonic activity the oldest rocks that crop out in the area are early oligocene 32 7 0 2 ma dioritic to granitic 60 72 wt sio2 plutons intruded by younger to contemporaneous 30 3 0 1 and 33 3 0 7 ma aplitic 76 78 wt sio2 dikes and mineralized veins volcanic rocks related to these intrusives form a thick sequence exposed in cliffs from the base of which an altered andesite 58 wt sio2 was collected the nearby epithermal cu mineralization of the inguaran mining area to the southeast is also related to this tertiary volcanic episode the oldest expression of the tmvb in this area is represented by pliocene 3 2 2 4 ma basaltic andesite to andesite 53 63 wt sio2 lavas with pyroxene olivine forming extensive mesas eroded mounds and thick elongate flows that were unconformably deposited on top of the oligocene rocks their emplacement was followed by erosion approximating a rate of 100 m ma over the past 3 ma this led to the formation of an inverted topography reflecting the higher resistance to erosion of these lavas that were originally deposited in valleys and today constitute prominent mesas during the last 1 ma at least twenty six monogenetic scoria cones formed in the study area erupting about 10 2 km3 of lava and pyroclastics dominantly composed of olivine and pyroxene bearing andesites 61 vol dre lesser basaltic andesites 30 vol dre and a minority of olivine rich basalts 10 vol dre these eruptive products include the k rich basaltic trachyandesites of cerro la pilita a scoria and lava cone located close to jorullo the 27 ka san hilario and esmeralda lavas which are located 13 km southeast of jorullo are the southernmost eruptive products of the tmvb and also the youngest volcanoes after jorullo in this area mapping of main faults and scoria cone alignments on regional and local scales reveals that the ascent of dikes feeding quaternary eruptions in the jorullo area was controlled by west northwest and west southwest basement structures northeast oriented subduction related horizontal compressional stress probably associated with crustal fractures and a major northwest trending seismically active fault zone that passes near paricutin and jorullo

linking basin scale and pore scale gas hydrate distribution patterns in diffusion dominated marine hydrate systems the goal of this study is to computationally determine the potential distribution patterns of diffusion driven methane hydrate accumulations in coarse grained marine sediments diffusion of dissolved methane in marine gas hydrate systems has been proposed as a potential transport mechanism through which large concentrations of hydrate can preferentially accumulate in coarse grained sediments over geologic time using one dimensional compositional reservoir simulations we examine hydrate distribution patterns at the scale of individual sand layers 1 20 m thick that are deposited between microbially active fine grained material buried through the gas hydrate stability zone ghsz we then extrapolate to two dimensional and basin scale three dimensional simulations where we model dipping sands and multilayered systems we find that properties of a sand layer including pore size distribution layer thickness dip and proximity to other layers in multilayered systems all exert control on diffusive methane fluxes toward and within a sand which in turn impact the distribution of hydrate throughout a sand unit in all of these simulations we incorporate data on physical properties and sand layer geometries from the terrebonne basin gas hydrate system in the gulf of mexico we demonstrate that diffusion can generate high hydrate saturations upward of 90 at the edges of thin sands at shallow depths within the ghsz but that it is ineffective at producing high hydrate saturations throughout thick greater than 10 m sands buried deep within the ghsz furthermore we find that hydrate in fine grained material can preserve high hydrate saturations in nearby thin sands with burial

short migration of methane into a gas hydrate bearing sand layer at walker ridge gulf of mexico 1 the mechanisms governing distribution of natural gas hydrates in marine sediment systems are not well understood we focus on a hole in the gulf of mexico walker ridge block 313 hole h where a 2 5 m thick gas hydrate bearing sand occurs within a 152 m thick fine grained mud interval containing gas hydrate in fractures the gas hydrate bearing sand is surrounded by two distinct hydrate free zones 10 m thick above and 3 m thick below we hypothesize that microbial methane generated within the hydrate free zones diffused into the sand and formed gas hydrate we show that the amount of methane produced in the hydrate free zones is likely enough to explain the gas hydrate content of the sand layer additionally we show that there is enough time for dissolved methane to diffuse from the hydrate free zones into the sand we conclude that methane transport over significant distances via fluid flow is not required but that microbial methane could migrate short distances to form gas hydrate in the sand layer

gas origin and migration in the ulleung basin east sea results from the second ulleung basin gas hydrate drilling expedition ubgh2 abstract we have investigated the gas chemistry of marine sediments up to 250 m below seafloor m bsf recovered during the second ulleung basin gas hydrate drilling expedition ubgh2 to identify and compare the gas origin and its migration at seismically non chimney truncated chimney and chimney sites in this basin the molecular and isotopic data c1 c2 ratios 300 13 c ch 4 d ch 4 kim et al 2012 and thought to reflect two phase methane transport processes in chimney sites deep sourced methane migrates independently of the carbon dioxide which remains dissolved in the aqueous phase leading to the extremely low c values observed in the upper 30 m bsf upward methane diffusion at non chimney sites may also add a transport induced fractionation as evidenced in mixing diagrams but this process does not significantly affect the c values 13 c co 2 shows usually minimum values at the sulfate methane transition zone smtz at all sites however in chimney sites these values have a much more depleted signature indicating faster rates of methane consumption by anaerobic oxidation of methane aom supported by the higher methane flux moreover here the massive hydrate that forms in shallow sediments incorporates gases with a lower c1 c2 ratio higher c2 c1 than the surrounding sediment as documented in hydrate bound gas bg and void gas vg samples gas hydrate dissociation during the core recovery is released enriched ethane c2h6 trapped in gas cage which leads to have high c2h6 in vg samples

the relationship between quaternary volcanism in central mexico and the seismicity and structure of subducted ocean lithosphere late quaternary volcanism in central mexico is related to the subduction of young ocean lithosphere at the middle america trench along arc variations in seismicity volcano structure and composition of volcanic products bear a remarkable correlation with the age and structural framework of the downgoing slab morphological and petrographic characteristics of major volcanoes within the trans mexican volcanic belt tmvb serve to distinguish two calc alkaline subprovinces 1 a western arc averaging 60 km in width associated with aseismic subduction of the rivera plate the main cones of this region are dominated by two pyroxene andesites comprise volumes 70 km 3 and stand less than 3 000 m above sea level 2 a broad central and eastern arc related to subduction of a gently inclined segment of the cocos plate bounded by the rivera transform and the tehuantepec ridge major volcanic edifices possess summit elevations in the range 4 000 to 6 000 m have appropriately larger volumes typically 200 km 3 and are constructed with a high proportion of amphibole bearing lavas the boundary between these subprovinces is marked by a north south oriented structural depression the colima graben and it coincides with a 100 km offset in the volcanic front extensional tectonism in the colima graben accompanied by mixed calc alkaline and alkaline volcanism of potassic affinity is likely related to a hinge type transform fault which marks the cocos rivera plate juncture in the downgoing slab a third segment of ocean floor is presently interacting with continental lithosphere south of the gulf of tehuantepec where quaternary volcanism is weakly developed within a tectonically complex region that marks the diffuse cocos noam caribbean triple junction the northern limit of this triple junction is defined by the seismically active isthmus fault which may be related to alkaline volcanic activity at san andres tuxtla a tectonic reconstruction based on the evolution of oceanic crust reveals that the distribution of intermediate depth earthquakes along the arc is directly dependent upon the age of the subducted slab ocean lithosphere younger than approximately 20 m y is subducted aseismically at convergence rates approaching 9 cm yr the length of the inclined seismic zone indicates that the time constant for thermal relaxation in the slab is approximately 4 m y the tmvb overlies the aseismic extension of this young ocean lithosphere several aspects of this study have a bearing on the segmented nature of converging margins in general 1 the tectonic evolution of the ocean floor may determine the nature of segmentation at the site of subduction 2 the complete record of volcanism in the tmvb over the past million years can be related to the present plate configuration 3 alkaline and calc alkaline volcanism have developed contemporaneously at a converging plate margin 4 lineaments in volcanic arcs may reflect the structural complexity of the crust rather than segment boundaries in the subducted slab

origin of andesite in the deep crust and eruption rates in the tancitaro nueva italia region of the central mexican arc the tancitaro nueva italia region 4400 km 2 is located within the west central mexican volcanic belt and spans 80 km of arc length along the volcanic front on the basis of 65 40 ar 39 ar ages it is shown that a total volume of 326 57 km 3 of magma has erupted from 200 vents since 1 2 ma nearly a third of this volume 103 5 km 3 was erupted as crystal rich andesite from a large stratovolcano volcan tancitaro whereas the remaining two thirds 223 52 km 3 span the complete range from 51 to 63 wt sio 2 are relatively crystal poor and were erupted from peripheral vents the total proportions of erupted lava types are 6 7 basalt 18 basaltic andesite and 74 76 andesite there is no rhyolite and only a trace of dacite there is no evidence that any significant magma differentiation occurred in the upper crust along this arc segment based on phenocryst abundances and modes major and trace element data as well as phase equilibrium experiments from the literature the following mechanisms for formation of andesite in this region can be ruled out 1 crystal fractionation of basalt basaltic andesite in the upper crust 2 magma mixing of basalt basaltic andesite with dacite rhyolite and 3 assimilation of granitoid by basalt basaltic andesite instead the same body of evidence indicates that the andesites were formed primarily in the deep crust through partial melting 20 of hornblende rich 40 gabbronorite the basaltic andesites are required owing to their elevated ni concentrations to be hybrids of the andesitic partial melts with basaltic residual liquids that have undergone variable two pyroxene fractionation in the deep crust

deep tow and sea beam studies of dueling propagating ridges on the east pacific rise near 20 40 s nontransform ridge axis discontinuities such as propagating rifts overlapping spreading centers oscs and saddle points occur frequently along the mid ocean ridge system we used sea beam and deep tow to study an example of a relatively large nontransform offset near 20 40 s on the east pacific rise epr the two spreading center tips are 15 km apart and separated by a diffuse shear zone a broad v shaped wake of disturbed topography and magnetics occurs on the flanks of the epr near the offset on a fine scale the offset is tectonically much more complex than either the typical osc or propagating rift structures for two reasons first it is a dueling propagator in which structures produced by successive short term episodes of northward and southward propagation have overprinted each other with migration toward the south prevailing over the long term second its offset distance is such that its tectonic behavior may be transitional from that of a propagating rift to that of an osc system the offset s rate of migration to the south has been 20 mm yr over the last 2 m y far less than total opening rate of 162 mm yr distinct propagation events may have been much more rapid however surging north and south at rates of 160 230 mm yr in the process several fossil spreading ridges 30 50 km long have been abandoned and rafted to the side we propose a fine scale tectonic evolution in which a propagating spreading center either curves inside or runs outside of a soon to be abandoned propagating rift tip we call this process self decapitation dueling propagation at 20 40 s may be caused by competing magmatic sources at shallow points along the epr one to the north near 20 s and one to the south near 21 30 s the 20 40 s dueling propagator occurs at a long wavelength low in the axial depth profile between these competing sources in general the division of spreading centers into segments separated by ridge axis discontinuities occurs at low points along the axis and may be caused by the pattern of upper mantle convection and the factors governing flow of magma along the strike of the ridge

monogenetic vulcanism in sierra chichinautzin mexico the variation in the activity patterns of the chichinautzin volcanic rocks is discussed this sequence of lavas and pyroclastic deposits is located in the central part of the mexican volcanic belt directly south of mexico city and is typical of its quaternary monogenetic vulcanism

magnetic hysteresis in young mid ocean ridge basalts dominant cubic anisotropy magnetic hysteresis data from young mid ocean ridge basalts include samples with saturation remanence to saturation magnetization mrs ms ratios greater than 0 5 the theoretical limit for an assemblage of single domain grains with uniaxial anisotropy under the usual assumption of dominant uniaxial anisotropy the narrow single domain grain size distribution implied by these high mrs ms values is difficult to reconcile with petrographic and remanence data that suggest the presence of larger multidomain grains dominant cubic anisotropy provides a plausible explanation for the high mrs ms ratios and if generally valid requires reinterpretation of granulometric and domain state inferences made from hysteresis data

dike emplacement near paricutin volcano mexico in 2006 a major seismic swarm occurred near paricutin volcano between the end of may and early july 2006 more than 700 earthquakes with magnitude ml exceeding 2 4 were located paricutin located in the michoacan guanajuato volcanic field in western mexico is well known as the site of the 1943 eruption in which a new 400 m cinder cone was constructed in what had been farmland the 2006 swarm exhibits all of the characteristics typically associated with swarms of volcanic origins the earthquake rate showed the typical ramp up and ramp down over the course of several days magnitudes were evenly distributed in time with a notably high b value of 2 45 the earthquake locations cluster around a northeast striking trend extending approximately 6 km over the first two weeks hypocenters migrated steadily a few hundred meters per day rising from 9 to 5 km depth and moving northeast about 5 km on approximately june 7 the ascent of hypocenters stalled for the next three weeks hypocenters held their depth while migrating laterally back to the southwest focal mechanisms during the first part of the swarm reflected the increased stress caused by dike inflation following june 7 the stress orientation changed and became more consistent with the inflation of horizontal sill like structures though only limited information is available from the seismic swarm preceding the 1943 eruption several features including the swarm duration and magnitude relationships were comparable to those of the 2006 episode the strong indicators of a magmatic origin to the 2006 swarm suggest that at this location there are few if any traditional seismic discriminants that could be used to distinguish which seismic swarms and dike emplacement events might culminate in eruption

tectonic and magmatic controls on the location of post subduction monogenetic volcanoes in baja california mexico revealed through spatial analysis of eruptive vents post subduction 12 5 ma to less than 1 ma monogenetic volcanism on the baja california peninsula mexico formed one of the densest intra continental areas of eruptive vents on earth it includes about 900 vents within an area 700 km long n s and 70 to 150 km wide w e this study shows that post subduction volcanic activity was distributed along this arc and that modes exist in the volcano distribution indicating that productivity of the magma source region was not uniform along the length of the arc vent clustering vent alignments and cone elongations were measured within eight monogenetic volcanic fields located along the peninsula results indicate that on a regional scale vent clustering varies from north to south with denser spatial clustering in the north on the order of 1 9 10 1 vents km2 to less dense clustering in the south on the order of 7 8 10 2 vents km2 san quintin san carlos jaraguay and santa clara are spatially distinct volcanic fields with higher eruptive vent densities suggesting the existence of individual melt columns that may have persisted over time in contrast the san borja vizcaino san ignacio and la purisima vent fields show lower degrees of vent clustering and no obvious spatial gaps between fields thus indicating an area of more distributed volcanism insight into the lithospheric stress field can be gained from vent alignments and vent elongation measurements within the fields located along the extinct subduction related volcanic arc elongation patterns of cinder cones and fissure fed spatter cones vent clusters and vent alignments trend nw se and n s within the santa clara field located more to the west within the forearc elongation patterns of the same volcanic features trend ne sw these patterns suggest that magmatism was more focused in the forearc and in the northern part of baja california than in its southern region within the extinct arc magma ascent created volcano alignments and elongate cones parallel to nw se to n s oriented tectonic structures in the forearc the existence of n s and ne sw oriented volcanic features indicates a rotation in the stress field orientation compared to the arc

el estribo volcanic complex evolution from a shield volcano to a cinder cone patzcuaro lake michoacan mexico abstract el estribo volcanic complex evc is located in the northern part of the michoacan guanajuato volcanic field within the trans mexican volcanic belt tmvb el estribo is located at the southern edge of the e w patzcuaro fault that belongs to the patzcuaro jaracuaro graben a western extension of the e w morelia acambay fault system stratigraphy geochronology chemistry and mineral assemblages suggest that the volcanic complex was constructed in two periods separated by a 100 ka volcanic hiatus a emission of lava flows that constructed a shield volcano between 126 ka and b mixed phreatomagmatic to strombolian activity that formed a cinder cone 28 ka the magmas that fed these monogenetic volcanoes were able to use the same feeding system the cinder cone itself was constructed by strombolian fallouts and remobilized scoria beds followed by an erosion period and by a mixed phreatomagmatic to magmatic phase strombolian fallouts ending with lava flows soft sedimentary deformation of beds and impact sags cross bedding as well as pitting and hydrothermal cracks found in particles support the phreatomagmatic phase the erupted magmas through time ejected basaltic andesitic lava flows 56 21 58 88 sio 2 that built the shield volcano and then basaltic andesitic scoria 57 65 59 05 sio 2 that constructed the cinder cone although they used the same feeding system the geochemical data and the mineral chemistry of the magmas indicate that the shield volcano and the cinder cone were fed by different magma batches erupted thousands of years apart therefore the location of el estribo volcanic complex along an e w fault that has generated two sector collapses of the shield volcano to the north may be directly linked to this complex redistribution of the magmatic paths to the surface our findings show that magmatic feeding systems within monogenetic volcanic fields could be long lived questioning the classic view of the monogenetic nature of their volcanoes and yielding information about the potential volcanic risk of these settings usually considered risk free

morphometry of scoria cones and their relation to geodynamic setting a dem based analysis abstract the morphometry of a great number of scoria cones belonging to volcanic fields of various geodynamic settings has been measured and analyzed addressing the question whether there is a relation between the prevalent cone shape in a given field and the geodynamic setting of the field itself morphometric analysis was carried out on freely downloadable digital elevation models dems the accuracy of the used dems and the associated error in scoria cone morphometry were determined by cross comparing high resolution lidar derived dems usgs ned tinitaly dem and aster gdem the 10 m tinitaly 01 and usgs ned dems are proven to be suitable for scoria cone morphometry whereas aster gdem can be used reliably for cones with volume greater than 30 10 6 m 3 according to a detailed morphometry of all scoria cones we propose that the cones related to subductional setting show relatively higher values of h c o w co and lower values of w cr w co than the cones related to extensional setting the detected differences can be imputable to peculiar eruption dynamics resulting in slight but systematic changes in shape and differences in lithological and sedimentological characteristics that govern post eruptive erosion to constrain the pathway of scoria cone erosion the detected morphometric changes were also interpreted using a simple linear degradation model utilizing the obtained simulation results the inferred initial cone base and the age of scoria cones we calculated a diffusion coefficient k for several dated cones which are related to the prevalent climate our results despite the high error associated allow to assess the median k for all volcanic fields due to the complexity of the factors behind it is not easy to understand if the prevalent shape characterizing a certain volcanic field is due mainly to sin eruptive or post eruptive mechanisms however our distinction between the two main geodynamic settings may be the first step to decipher these factors

self similar clustering of cinder cones and crust thickness in the michoacan guanajuato and sierra de chichinautzin volcanic fields trans mexican volcanic belt abstract the spatial clustering of basaltic vents in monogenetic volcanic fields has been used as a proxy for crustal thickness in extensional and back arc tectonic settings the basaltic vents have a fractal clustered distribution self similar clustering described by a power law the power law is defined over a range the size range of the distribution of values in this case the vents separation delimited by a lower and an upper cut offs here we apply the fractal clustering analysis to the two largest monogenetic volcanic fields of the trans mexican volcanic belt tmvb a continental arc built on different crustal terranes the michoacan guanajuato volcanic field mgvf located in the central western tmvb includes over 1000 vents of late pliocene to quaternary age built on attenuated crust of mesozoic to tertiary age the sierra de chichinautzin volcanic field scvf in the central eastern tmvb is composed of 220 late pleistocene to holocene vents laying above thicker crust of precambrian to tertiary age monogenetic vents in both volcanic fields show self similar clustering with fractal exponent d 1 67 in the range 1 3 38 km mgvf and d 1 56 in the range 1 5 32 km scvf the upper cut off uco for the power law distribution of the mgvf well fits the crustal thickness below the volcanic field as derived from independent geophysical data the uco value of scvf indicates a crust thickness of about 32 km this value is in agreement with new geophysical data that indicate magma underplating the crust beneath the volcanic field area

seismic structure of the southern east pacific rise seismic data from the ultrafast spreading 150 to 162 millimeters per year southern east pacific rise show that the rise axis is underlain by a thin less than 200 meters thick extrusive volcanic layer seismic layer 2a that thickens rapidly off axis also beneath the rise axis is a narrow less than 1 kilometer wide melt sill that is in some places less than 1000 meters below the sea floor the small dimensions of this molten body indicate that magma chamber size does not depend strongly on spreading rate as predicted by many ridge crest thermal models however the shallow depth of this body is consistent with an inverse correlation between magma chamber depth and spreading rate these observations indicate that the paradigm of ridge crest magma chambers as small sill like midcrustal bodies is applicable to a wide range of intermediate and fast spreading ridges

the ad 500 700 late classic el astillero and el pedregal volcanoes michoacan mexico a new monogenetic cluster in the making the recent identification of holocene volcanic clusters in small areas within the michoacan guanajuato volcanic field mgvf opens several questions regarding future volcanic hazard assessments in this region documenting vent alignments and eruption recurrence intervals within clusters will provide parameters necessary for making temporal and spatial hazard evaluations here we present a possible new case of a small cluster consisting of only two monogenetic volcanoes el astillero and el pedregal located in the 4400 km2 tancitaro nueva italia region in the southwestern part of the mgvf only 25 km to the south of paricutin volcano we determined from paleomagnetic and radiocarbon dating that el astillero and el pedregal most likely erupted one after the other between ad 500 and 700 within the late classic period of mesoamerican archeology while the eruptions were likely separated by a short period of time the exact length is difficult to ascertain after the 6 years of total estimated eruption duration of the two volcanoes both together occupied an area of 14 7 km2 and emitted a dense rock equivalent dre volume of magma of 0 5 km3 notable characteristics of the eruptions include a switch from the explosive activity exclusive of el astillero strombolian to effusive activity early after the initiation of the el astillero eruption a shift in the active vents and a progressive change in the bulk magma composition from basaltic andesite to andesite throughout the duration of the eruption this activity first formed the el astillero scoria cone and tephra deposits followed by its lava field and ended with the emplacement of the el pedregal viscous lavas the discovery of pre hispanic pottery sherds and obsidian artifacts underneath the el astillero tephra fallout unambiguously attests to human activities in the area before the eruption judging by their eruptive style the eruptions probably had a limited impact on the small area affected and the surrounding human activities but the hazard for this area remains since el astillero and el pedregal could represent the initial stages of a new cluster that is still in the making if so another eruption should be expected in this area again

characterization of gas hydrate reservoirs by integration of core and log data in the ulleung basin east sea abstract examinations of core and well log data from the second ulleung basin gas hydrate drilling expedition ubgh2 drill sites suggest that sites ubgh2 2 2 and ubgh2 6 have relatively good gas hydrate reservoir quality in terms of individual and total cumulative thicknesses of gas hydrate bearing sand hybs beds in both of the sites core sediments are generally dominated by hemipelagic muds which are intercalated with turbidite sands the turbidite sands are usually thin to medium bedded and mainly consist of well sorted coarse silt to fine sand anomalies in infrared core temperatures and porewater chlorinity data and pressure core measurements indicate that gas hydrate occurrence zones ghoz are present about 68 155 mbsf at site ubgh2 2 2 and 110 155 mbsf at site ubgh2 6 in both the ghoz gas hydrates are preferentially associated with many of the turbidite sands as pore filling type hydrates the hybs identified in the cores from site ubgh2 6 are medium to thick bedded particularly in the lower part of the ghoz and well coincident with significant high excursions in all of the resistivity density and velocity logs gas hydrate saturations in the hybs range from 12 to 79 with an average of 52 based on pore water chlorinity in contrast the hybs from site ubgh2 2 2 are usually thin bedded and show poor correlations with both of the resistivity and velocity logs owing to volume averaging effects of the logging tools on the thin hybs beds gas hydrate saturations in the hybs range from 15 to 65 with an average of 37 based on pore water chlorinity in both of the sites large fluctuations in biogenic opal contents have significant effects on the sediment physical properties resulting in limited usage of gamma ray and density logs in discriminating sand reservoirs

stratigraphy geomorphology geochemistry and hazard implications of the nejapa volcanic field western managua nicaragua abstract the nejapa volcanic field nvf is located on the western outskirts of managua nicaragua it consists of at least 30 volcanic structures emplaced along the n s nejapa fault which represents the western active edge of the managua graben the study area covers the central and southern parts of the volcanic field we document the basic geomorphology stratigraphy chemistry and evolution of 17 monogenetic volcanic structures ticomo a b c d and e altos de ticomo nejapa san patricio nejapa norte motastepe el hormigon la embajada asososca satelite refineria and cuesta el plomo a and b stratigraphy aided by radiocarbon dating suggests that 23 eruptions have occurred in the area during the past 34 000 years fifteen of these eruptions originated in the volcanic field between 28 500 and 2 130 yr bp with recurrence intervals varying from 400 to 7 000 yr most of these eruptions were phreatomagmatic with minor strombolian and fissural lava flow events a future eruption along the fault might be of a phreatomagmatic type posing a serious threat to the more than 500 000 inhabitants in western managua

variations in oceanic crustal magnetization systematic changes in the last 160 million years the amplitudes of marine magnetic anomalies show a clear worldwide pattern of systematic variation when viewed as a function of age globally the amplitudes decrease with age over the first 20 to 30 million years a phenomenon that has been attributed to the low temperature oxidation of the magnetic mineral titanomagnetite in the upper extrusive rocks in oceanic crust older than 40 million years however the amplitudes of the magnetic anomalies increase with increasing age and remain at elevated levels for the entire period between 80 and 160 million years in order to examine the processes responsible for this elevated crustal magnetization in older oceanic crust we compiled all of the existing rock magnetic data from relevant deep sea drilling project and ocean drilling program sites that sampled extrusive rocks from normal ocean crust this compilation shows that the laboratory measurements of the magnetization of the upper basement rocks closely reflect that of the anomaly amplitudes both in the decrease due to oxidation in the first 20 to 30 million years and in the increase in magnetization in older 80 million years crust this positive correlation between anomaly amplitudes and upper crustal magnetization supports the argument that a major source of the marine magnetic anomalies is in the upper extrusive volcanic rocks further the curie temperature data show that oxidation of the magnetic minerals in oceanic basalts occurs largely within the first 30 million years and does not increase significantly beyond that point finally the strong positive correlation between the intensity of the magnetization of the drill core samples and the saturation magnetization argues that the higher magnetic anomaly amplitudes over older crust are related to a change in an intrinsic property of the upper crust specifically we propose that the observed elevated magnetization of older ocean crust is due primarily to an increased abundance of magnetic feti oxides in the older crustal rocks this large increase in abundance of the feti oxides may be related to a systematic increase in bulk feti content in older tholeiitic basalts or more likely is due to a difference in the partitioning of the iron and titanium content between the silicate and oxide phases

structure of the michoacan guanajuato volcanic field mexico abstract the distribution and the alignment of volcanoes and fault scarps provide new insights into the structural framework of the michoacan guanajuato volcanic field a total of 1894 volcanoes occur over an area of 34 500 km 2 contour maps of the density of volcanoes and average volcano height indicate a shift from polygenetic to monogenetic volcanism from north to south in the field volcano edifice height has a bimodal density distribution which can be described by two populations with mean heights of 95 m and 510 m plotting the distribution of these two populations on separate contour maps reveals a n35e trend in monogenetic volcanoes and a n60w trend in polygenetic volcanoes fault scarps cut tertiary volcanic rocks and older quaternary volcanoes but are overprinted by volcanoes and lava flows in the central part of the field fault scarps define two fault zones oriented n60e in the northeastern part of the area and n60w in the northwest part of the area e w trending faults are common and alignments of volcanoes often parallel these faults regional trends in volcano distribution appear to be related to plate convergence the n60w trend of the polygenetic volcanoes is colinear to the western trans mexican volcanic arc parallel to the middle america trench and parallel to a major fault zone and represents an extinct volcanic arc the n35e band of monogenetic volcanoes may reflect the segmentation of the subducted plate

spatial distribution and structural analysis of vents in the lunar crater volcanic field nevada usa volcanoes within monogenetic volcanic fields often are arranged in alignments and clusters which are related to effects of magma source geometry in the upper mantle principal stress orientations and crustal structures on their magma feeding systems we use cluster analysis with dendrogram vent morphometric analysis and field structural data to explore the relationships between volcanoes and tectonic features in the plio pleistocene part of the lunar crater volcanic field lcvf pancake range nevada usa which includes 96 monogenetic volcanic edifices totaling 119 vents structural analysis identified three main sets of faults with dip slip kinematics mostly normal with a few examples of thrust faults striking n s e w and ne sw the ne sw set comprises dip slip faults with a dominant normal component of movement which are consistent with the modern state of stress based upon the world stress map database spatial distribution pattern analysis suggests a clustered distribution of vents in the lcvf and gis based spatial density analysis shows that these clusters trend mostly ne sw morphometric study of the monogenetic cones which provides information on feeder dike orientation where dikes are not directly exposed suggests dominant nne ssw to ne sw orientations of near surface inferred dikes an amount of 27 out of 31 inferred feeder dikes within the lcvf is parallel to the present orientation of the greatest principal horizontal stress hmax as suggested by world stress map data derived from hydrofracturing and earthquake focal mechanisms in some cases dike strike is parallel with that of pre existing quaternary dip slip faults we suggest that the spatial distribution of vents is related to domains of different scales of partial melting and compositional heterogeneity in the upper mantle source which is substantiated by geochemical data the relationship of feeder dikes with respect to shallow tectonic structures although somewhat ambiguous at lcvf is consistent with behavior that is intermediate between volcanic fields with high and low long term magma fluxes

culture based investigations of key microbial functional groups in gas hydrate bearing sediments of the krishna godavari basin in offshore india abstract results of national gas hydrate program 02 nghp 02 expedition have confirmed the resource potential of sand hosted gas hydrate accumulations in the deep offshore of krishna godavari basin kg basin nghp 02 has identified two types of gas hydrate accumulations one is thick layer type units developed in sand rich facies at depths of a few 100 m below sea floor mbsf and the other fracture type unit of variable thickness at shallow depths fracture type and layer type gas hydrates were observed in areas b c and e in the kg basin as defined under nghp 02 the importance of each type of gas hydrate occurrence as discovered during nghp 02 has led to the need for microbial profiling different classes of microbes anaerobic fermentative microbes afm methanotrophs and sulphate reducing bacteria srb were studied to understand the microbial system for biogenic gas generation in sedimentary sequences encountered in eight core holes 4 in area b 3 in area c and 1 in area e as established during nghp 02 this study reveals the availability of highest concentration of methane generating microbes that are closely associated with high total organic carbon toc content in the identified gas hydrate bearing sedimentary units cored in each borehole the presence of microbial consortia for methane generation through both processes of reduction of co2 and fermentation of formate acetate has been confirmed stratigraphic correlation between drill sites have also been generated to better understand the spatial relationship of various microbial processes an observation from the study is that the entire late miocene age sediments has potential to generate biogenic gas in holes nghp 02 05b 17b 22b and 23b late to early pleistocene age sediments are showing methane generating activity in holes nghp 02 01c 08b and 09b while early pleistocene to late pliocene age sediments contains methane generating microbes in holes nghp 02 05b 17b 19b 22b and 23b the existence of methane generating biogenic microbial system in areas b c and e of kg basin has been confirmed

a model for the diffusive growth of hydrate saturation anomalies in layered sediments 1 sediment particles affect the phase behavior of gas hydrates both by increasing the surface energy where pore geometry forces hydrate crystals to attain high curvatures and through wetting interactions that cause aqueous films to coat particle surfaces these effects produce only slight changes to the gas solubility through most of the hydrate stability zone so the particle size has only a modest influence on the rate of hydrate accumulation when the sediments are homogeneous in hydrate reservoirs however discontinuous changes in sediment properties are common and such stratigraphic boundaries often coincide with hydrate anomalies these anomalies are a natural consequence of variations in subsurface sediment properties by accounting for sediment hydrate interactions i show how compositional diffusion supplies the growth of hydrate spikes in coarse grained sediments immediately adjacent to hydrate free regions hfrs in more fine grained sediments where the solubility is slightly elevated over timescales comparable with milankovitch cycles hydrate spikes are typically less than a meter in width and contain essentially all of the hydrate that would have otherwise occupied the much larger adjacent hfr if sediment heterogeneities were absent hydrate can form in the more fine grained sediments only once the spike achieves a sufficiently high saturation level often 90 of pore volume that the solubility is continuous across the stratigraphic boundary the wetting interactions that stabilize much of the residual liquid when hydrate forms an interconnected skeleton spanning many pore diameters can also partially unload sediment particle contacts and lead to the growth of segregated hydrate nodules and lenses

cinder cone clustering in the transmexican volcanic belt implications for structural and petrologic models cinder cone distributions have most often been characterized using univariate statistics here a new technique to volcano distribution studies cluster analysis is applied to cinder cone distribution in the central transmexican volcanic belt tmvb a total of 1016 cinder cones are identified over an area of approximately 60 000 km2 application of cluster analysis reveals structure in cinder cone distribution using a search radius parameter of 16 km 75 of the cinder cones within the central tmvb are found within eight clusters of 45 159 cones each these clusters are each 2000 to 5000 km2 in area only 22 cones are found within clusters of three or fewer cinder cones indicating that clustering is a pervasive phenomenon some petrologic variation is evident among clusters low mg alkaline cinder cones are found within a single cluster 360 400 km from the trench application of alignment analysis techniques including the two point azimuth method the hough transform and two dimensional fourier analysis demonstrates that cinder cone alignments have common orientations on a regional scale within three clusters all located in the southernmost part of the michoacan guanajuato volcanic field in the western portion of the study area these alignments consist of tens of cinder cones are 20 50 km long and are all oriented with azimuths of 020 040 parallel to the direction of plate convergence high mg lavas which last fractionated at pressures in excess of 8 kbar are only found associated with these alignments indicating that magma transport is significantly enhanced in these areas although local alignments of three to six cinder cones occur within mapped fault zones that transect the area regional cinder cone alignment patterns and the distribution of clusters themselves do not appear to be affected by the presence of these fault zones the cluster analysis the alignment analysis and some petrologic data support the hypothesis that the tmvb is segmented near 101 w

estimation of the global amount of submarine gas hydrates formed via microbial methane formation based on numerical reaction transport modeling and a novel parameterization of holocene sedimentation this study provides new estimates for the global offshore methane hydrate inventory formed due to microbial ch4 production under quaternary and holocene boundary conditions a multi 1d model for particular organic carbon poc degradation gas hydrate formation and dissolution is presented the novel reaction transport model contains an open three phase system of two solid compounds organic carbon gas hydrates three dissolved species methane sulfates inorganic carbon and one gaseous phase free methane the model computes time resolved concentration profiles for all compounds by accounting for chemical reactions as well as diffusive and advective transport processes the reaction module builds upon a new kinetic model of poc degradation which considers a down core decrease in reactivity of organic matter various chemical reactions such as organic carbon decay anaerobic oxidation of methane methanogenesis and sulfate reduction are resolved using appropriate kinetic rate laws and constants gas hydrates and free gas form if the concentration of dissolved methane exceeds the pressure temperature and salinity dependent solubility limits of hydrates and or free gas with a rate given by kinetic parameters global input grids have been compiled from a variety of oceanographic geological and geophysical data sets including a new parameterization of sedimentation rates in terms of water depth we find prominent gas hydrate provinces offshore central america where sediments are rich in organic carbon and in the arctic ocean where low bottom water temperatures stabilize methane hydrates the world s total gas hydrate inventory is estimated at 0 82 x 10sup13 m3 2 10 x 10sup15 m3 ch4 at stp conditions or equivalently 4 18 995 gt of methane carbon the first value refers to present day conditions estimated using the relatively low holocene sedimentation rates the second value corresponds to a scenario of higher quaternary sedimentation rates along continental margins our results clearly show that in situ poc degradation is at present not an efficient hydrate forming process significant hydrate deposits in marine settings are more likely to have formed at times of higher sedimentation during the quaternary or as a consequence of upward fluid transport at continental margins

magnetization of axial lavas from the southern east pacific rise 14 23 s geochemical controls on magnetic properties although the spatial association of iron rich lavas and high amplitude magnetic anomalies is well documented a causal link between enhanced iron content and high remanent magnetization has been difficult to establish here we report magnetic data from approximately 250 samples with 8 16 feo total iron as feo from the southern east pacific rise epr that provide strong support for the presumed geochemical dependence of remanent intensity the limited age range 0 6 ka of axial lavas from this ultrafast spreading ridge 150 mm yr full rate effectively minimizes variations resulting from time dependent changes in geomagnetic intensity or low temperature alteration systematic sampling relative to the chilled margin illustrates that substantial grain size related variations in magnetic properties occur on a centimeter scale both microprobe data and curie temperatures suggest that the average groundmass titanomagnetite composition in the southern epr samples is approximately constant modal modified ulv6spinel content 0 67 over a wide range of lava compositions saturation magnetization and saturation remanence are highly correlated with feo r 0 73 and 0 83 respectively indicating that more iron rich lavas have higher abundances of otherwise similar titanomagnetite we show that there is a good correlation between natural remanent magnetization nrm and feo provided that sufficient specimens are used to determine the average nrm of a sample r 0 63 because the range of iron contents in mid ocean ridge basalts is limited the best fit slope 4 44 a m per feo in an ambient field of 0 030 mt should provide reasonable bounds on the equatorial magnetization of submarine lavas 10 a m at 8 5 feo and 50 a m at 16 feo finally we demonstrate that along axis variations in nrm closely parallel geochemical changes along the southern epr where magnetization values deviate significantly from those predicted from the range of measured feo contents these discrepancies may reflect additional unrecognized geochemical variability

mount elbert gas hydrate stratigraphic test well alaska north slope overview of scientific and technical program abstract the mount elbert gas hydrate stratigraphic test well was drilled within the alaska north slope ans milne point unit mpu from february 3 to 19 2007 the well was conducted as part of a cooperative research agreement cra project co sponsored since 2001 by bp exploration alaska inc bpxa and the u s department of energy doe in collaboration with the u s geological survey usgs to help determine whether ans gas hydrate can become a technically and commercially viable gas resource early in the effort regional reservoir characterization and reservoir simulation modeling studies indicated that up to 0 34 trillion cubic meters tcm 12 trillion cubic feet tcf gas may be technically recoverable from 0 92 tcm 33 tcf gas in place within the eileen gas hydrate accumulation near industry infrastructure within ans mpu prudhoe bay unit pbu and kuparuk river unit kru areas to further constrain these estimates and to enable the selection of a test site for further data acquisition the usgs reprocessed and interpreted mpu 3d seismic data provided by bpxa to delineate 14 prospects containing significant highly saturated gas hydrate bearing sand reservoirs the mount elbert site was selected to drill a stratigraphic test well to acquire a full suite of wireline log core and formation pressure test data drilling results and data interpretation confirmed pre drill predictions and thus increased confidence in both the prospect interpretation methods and in the wider ans gas hydrate resource estimates the interpreted data from the mount elbert well provide insight into and reduce uncertainty of key gas hydrate bearing reservoir properties enable further refinement and validation of the numerical simulation of the production potential of both mpu and broader ans gas hydrate resources and help determine viability of potential field sites for future extended term production testing drilling and data acquisition operations demonstrated that gas hydrate scientific research programs can be safely effectively and efficiently conducted within ans infrastructure the program success resulted in a technical team recommendation to project management to drill and complete a long term production test within the area of existing ans infrastructure if approved by stakeholders this long term test would build on prior arctic research efforts to better constrain the potential gas rates and volumes that could be produced from gas hydrate bearing sand reservoirs

volcano tectonic segmentation of the northern andes data from landsat images published geologic and geophysical reports and field observations are used to document eight major transverse boundaries that divide the northern andes into segments about 125 km long most of the segment boundaries have no apparent relation to structure on the nazca plate being subducted and may instead represent fundamental and long lasting structures of the continent subduction of the carnegie ridge results in increased uplift of the andes widespread and chemically diverse volcanism and major faulting and seismicity subduction of a highly fractured zone in southern ecuador has led to the formation of a separate platelet with a distinct orientation and to a notable change in volcanic style from andesitic stratovolcanoes to rhyolitic ash flow sheets recognition of the segmented structure of the northern andes provides a framework for interpreting details of the regional geology and illustrates that not all segment boundaries are related to subduction

capillary effects on hydrate stability in marine sediments 1 we study the three phase liquid gas hydrate stability of the methane hydrate system in marine sediments by considering the capillary effects on both hydrate and free gas phases the capillary pressure a measure of the pressure difference across a curved phase interface exerts a key control on the methane solubility in liquid hydrate l h and liquid gas l g systems by calculating the l h and l g solubilities as a function of water depth pressure and pore size interface curvature we show how the solubility requirements for forming both gas hydrate and free gas can be met in a three phase zone the top of the three phase zone shifts upward in sediments as the water depth increases and the mean pore size decreases the thickness of the three phase zone increases as the distribution of pore sizes widens the top of the three phase zone can overlie or underlie the bulk three phase equilibrium depth at blake ridge we predict that the three phase zone is 27 7 m thick and that the top of the three phase zone lies 13 m above the predicted bulk equilibrium depth this reconciles the observation of the bottom simulating reflector bsr at blake ridge that is shallower than the predicted bulk equilibrium depth in contrast at hydrate ridge where water depth is shallower we predict that the three phase zone is 20 4 m thick and that the top of the three phase zone lies 0 7 m below the predicted bulk equilibrium depth our model which predicts an upward shift in the top of free gas occurrence with increasing water depth pressure is compatible with worldwide observations that the bsr is systematically shifted upward relative to the bulk equilibrium depth as water depth pressure is increased

the effect of lithologic heterogeneity on diffusive methane migration in hydrate systems abstract short range diffusive gas migration has been proposed as a means of supplying methane to coarse grained hydrate bearing sediments in this scenario methane is generated microbially in fine grained sediments and moves diffusively into adjacent coarse grained sediments due to a dissolved methane concentration gradient caused by pore size contrasts previous work has shown that short range diffusion is sufficient to supply thin isolated sands 1 m thickness with enough methane to achieve high hydrate saturations 80 in systems with a high ratio of clay to sand several notable hydrate reservoirs such as those encountered at daini atsumi knoll in the nankai trough and walker ridge block 313 and green canyon block 955 in the gulf of mexico exhibit tens of meters of alternating sand silt and clay layers that are decimeters in thickness the coarse grained layers in these settings are typically filled with hydrate to saturations exceeding 80 while short range diffusion is capable of filling thin isolated sands with hydrate thicker sands within more heterogeneous systems require mechanisms other than or in addition to short range diffusion using 1 d numerical simulations we demonstrate that short range diffusive migration of microbial methane is insufficient for reaching consistently high hydrate saturations in such systems this work shows that a different mechanism for gas transport such as advective migration is necessary to achieve these observed hydrate saturations in layered systems

a quaternary monogenetic volcanic field in the xalapa region eastern trans mexican volcanic belt geology distribution and morphology of the volcanic vents abstract the most abundant volcanic manifestations along the east west trending trans mexican volcanic belt tmvb are the scoria cones these have been grouped by other authors in extended monogenetic volcanic fields such as michoacan guanajuato chichinautzin apan and los tuxtlas here we present geological and morphological data of a relatively unknown group of monogenetic volcanoes located on the east flank of the cofre de perote volcano cp around the city of xalapa in the state of veracruz mexico within an area of about 2400 km2 the xalapa monogenetic volcanic field xmvf contains over 50 late quaternary volcanoes most of them are scoria cones but small shield volcanoes and tuff rings also occur the lava flows produced by these volcanoes are constrained by an abrupt topography and cover a great percentage of the surface on the eastern and northeastern flanks of cp between 3000 and 500 m a s l the representative rocks of the different volcanic centers include olivine basalt basaltic andesite with phenocrysts of plagioclase pyroxene and minor olivine and andesite with phenocrysts of plagioclase and pyroxene sio2 and al2o3 contents of the rocks vary between 45 and 62 wt and 15 to 18 wt respectively most of the basaltic rocks have mgo contents between 4 2 and 9 wt ni and cr concentrations between 23 and 180 and 10 to 380 ppm respectively with a typical calc alkaline behavior trace elements suggest two types of magmas the most abundant are characterized by an enrichment of lile and lree with negative anomalies of nb and ti which denote a calc alkaline affinity others are lile depleted and show high concentrations of mgo cr and ni which is typical of primary calc alkaline magmas the mean scoria cone morphological values are cone height hco 90 8 m cone diameter wco 686 38 m crater diameter wcr 208 49 m and 0 12 km3 for the cone volume we dated twelve different scoria cones using the 40ar 39ar method for the other cones we used morphologic indicators such as the ratio of the scoria cone height to basal diameter hco wco and maximum slope angle additionally vegetation type and cone and lava flow morphologies were also considered in order to determine rough relative ages in general the ages fall into three groups those older than 2 0 ma those between 0 25 0 40 ma and those less than approximately 0 1 ma

magnetic alteration of zero age oceanic basalt the youngest sampled submarine lava flow which erupted june 1993 on the juan de fuca ridge provides the basis for a tight constraint on the initial or zero age magnetization state of morb detailed profiles of magnetic hysteresis parameters curie temperatures and unblocking temperatures of nrm with respect to the chilled margin of a pillow fragment show evidence of significant oxidation which preferentially affected the finest grain size fraction and principal remanence carrier of the titanomagnetite magnetic mineralogy the oxidation must have occurred during or immediately after initial cooling implying that morb is already appreciably magnetically altered before aging nevertheless successful results of thellier paleointensity experiments on the basalt sample lend support to the idea that crustal magnetization represented by morb preserves a record of geomagnetic intensity variations that may be reflected in small scale magnetic anomalies

3 d basin scale reconstruction of natural gas hydrate system of the green canyon gulf of mexico our study presents a basin scale 3d modeling solution quantifying and exploring gas hydrate accumulations in the marine environment around the green canyon gc955 area gulf of mexico it is the first modeling study that considers the full complexity of gas hydrate formation in a natural geological system overall it comprises a comprehensive basin re construction accounting for depositional and transient thermal history of the basin source rock maturation petroleum components generation expulsion and migration salt tectonics and associated multi stage fault development the resulting 3d gas hydrate distribution in the green canyon area is consistent with independent borehole observations an important mechanism identified in this study and leading to high gas hydrate saturation 80 vol at the base of the gas hydrate stability zone ghsz is the recycling of gas hydrate and free gas enhanced by high neogene sedimentation rates in the region our model predicts the rapid development of secondary intra salt mini basins situated on top of the allochthonous salt deposits which leads to significant sediment subsidence and an ensuing dislocation of the lower ghsz boundary consequently large amounts of gas hydrates located in the deepest parts of the basin dissociate and the released free methane gas migrates upwards to recharge the ghsz in total we have predicted the gas hydrate budget for the green canyon area that amounts to 3 256 mt of gas hydrate which is equivalent to 340 mt of carbon 7 x 1011 m3 of ch4 at stp conditions and consists mostly of biogenic hydrates

the geomorphological evolution of the paricutin cone and lava flows mexico 1943 1990 abstract erosion and sedimentation processes in the paricutin cone and adjacent lava fields have been evaluated 47 years after the eruption through field study of the morphological characteristics of the area previous studies indicate an initial stage of accelerated erosion decreasing quickly after the stripping of the ash layers and cessation of the volcanic activities in the cone and crater area revegetation is the main process after the first stage of stripping of fine ash the lava fields are the least affected areas by erosional processes with vegetation development in minor depressions crevices and along the edges of the lava flows the llanos or lava dammed plains are the most active physiographic units with continuous deposition of sediments they are completely covered by vegetation or under agricultural use incision of channels along the lava field edge promotes the integration of the drainage system but still most of the affected area drains into the lave field present erosion rates are higher than normal rates and a complete rehabilitation of the pyroclastic area may extend over decades filling of sediments and revegetation of the lava fields will develop in a longer period of several millennia

semi automatic delimitation of volcanic edifice boundaries validation and application to the cinder cones of the tancitaro nueva italia region michoacan guanajuato volcanic field mexico abstract the shape and size of monogenetic volcanoes are the result of complex evolutions involving the interaction of eruptive activity structural setting and degradational processes morphological studies of cinder cones aim to evaluate volcanic hazard on the earth and to decipher the origins of various structures on extraterrestrial planets efforts have been dedicated so far to the characterization of the cinder cone morphology in a systematic and comparable manner however manual delimitation is time consuming and influenced by the user subjectivity but on the other hand automatic boundary delimitation of volcanic terrains can be affected by irregular topography in this work the semi automatic delimitation of volcanic edifice boundaries proposed by grosse et al 2009 for stratovolcanoes was tested for the first time over monogenetic cinder cones the method based on the integration of the dem derived slope and curvature maps is applied here to the tancitaro nueva italia region of the michoacan guanajuato volcanic field mexico where 309 plio quaternary cinder cones are located the semiautomatic extraction allowed identification of 137 of the 309 cinder cones of the tancitaro nueva italia region recognized by means of the manual extraction this value corresponds to the 44 3 of the total number of cinder cones analysis on vent alignments allowed us to identify ne sw vent alignments and cone elongations consistent with a ne sw max and a nw se min constructing a vent intensity map based on computing the number of vents within a radius r centred on each vent of the data set and choosing r 5 km four vent intensity maxima were derived one is positioned in the nw with respect to the volcano tancitaro one in the ne one to the s and another vent cluster located at the se boundary of the studied area the spacing of centroid of each cluster 24 km can be related to the thickness of the crust 9 10 km overlying the magma reservoir

burial driven methane recycling in marine gas hydrate systems abstract natural gas hydrate may be buried with sediments until it is no longer stable at a given pressure and temperature resulting in conversion of hydrate into free gas this gas may migrate upward and recycle back into the hydrate stability zone to form hydrate as of yet however no quantitative description of the methane recycling process has been developed using multiphase flow simulations to model burial driven gas hydrate recycling in this study we present a series of 1d multiphase transport simulations to investigate the methane recycling process in detail by invoking the effects of capillary phenomena on hydrate and gas formation in pores of varying size we find that a free gas phase can migrate a significant distance above the bulk base of hydrate stability since the top of the free gas occurrence is often identified as the base of the hydrate stability zone from seismic data our results demonstrate that not only could this assumption mischaracterize a hydrate system but that under recycling conditions the highest hydrate saturations can occur beneath the top of the free gas occurrence we show that the presence of pore size distributions requires a replacement zone through which hydrate saturations progressively decrease with depth and are replaced with free gas this replacement zone works to buffer against significant gas buildup that could lead to fracturing of overlying sediments this work provides a framework for simulating flow and transport of methane within the 3 phase stability zone from a mass conservation perspective

numerical recognition of alignments in monogenetic volcanic areas examples from the michoacan guanajuato volcanic field in mexico and calatrava in spain abstract identification of geological lineaments using numerical methods is a useful tool to reveal structures that may not be evident to the naked eye in this sense monogenetic volcanic fields represent an especially suitable case for the application of such techniques since eruptive vents can be considered as point like features application of a two point azimuth method to the michoacan guanajuato volcanic field mexico and the calatrava volcanic province spain demonstrates that the main lineaments controlling the distributions of volcanic vents 322 in calatrava and 30 in michoacan approach the respective main compressional axes that dominate in the area i e the cocos north america plates convergence and the main betics compressional direction respectively considering the stress fields that are present in each volcanic area and their respective geodynamic history it seems that although volcanism may be a consequence of contemporaneous extensional regimes the distribution of the volcanic vents in these kinds of monogenetic fields is actually controlled by reactivation of older fractures which then become more favourable for producing space for magma ascent at near surface levels

spatial distribution and alignments of volcanic centers clues to the formation of monogenetic volcanic fields monogenetic basaltic volcanic fields occur worldwide in tectonic environments ranging from extensional to convergent understanding similarities and differences between these fields may help to characterize key controls on their generation such volcanic fields consist of numerous volcanic centers each of which represents a pathway of magma from its source to the surface we analyzed the spatial distribution of volcanic centers in 37 monogenetic volcanic fields and assuming that the distribution of volcanic centers relative to each other is matched by a similar source pattern within the mantle applied the following methods for each 1 the poisson nearest neighbor pnn analysis representing the degree to which the distribution of the volcanic centers departs from a predicted poisson distribution and 2 a volcanic alignment analysis to ascertain the preferential pathways if any used by the magma to reach the surface this is the first comprehensive global comparison of such analyses magma pathways within the brittle upper crust are influenced to various degrees by two end member situations 1 formation of new extension fractures perpendicular to the least compressive stress 3 and 2 re activation of pre existing fractures that are near parallel to the maximum principal stress 1 the results of the pnn analysis show that independently of the tectonic environment most volcanic fields display a clustered distribution of their volcanic centers alignment analysis shows that either the ambient tectonic environment exerts a strong influence on the preferential orientations of the volcanic alignments or that it is in competition with other factors e g pre existing structures local stress changes due to older intrusions overall these results indicate that the propagation of the magma and therefore the spatial distribution of the volcanic centers within volcanic fields is the product of an interplay between deep level influences i e nature of the magma source and shallow level influences i e stress field pre existing crustal structures of the lithosphere

volcan tancitaro michoacan mexico 40ar 39ar constraints on its history of sector collapse abstract volcan tancitaro is a 97 3 km3 stratovolcano located in the michoacan guanajuato volcanic field mgvf part of the trans mexican volcanic belt prior to this study there was only one k ar date 530 60 ka ban m hasenaka t delgado granados h takaoka n 1992 k ar ages of lavas from shield volcanoes in the michoacan guanajuato volcanic field mexico geofisica internacional 31 4 467 473 and one sector collapse event reported for this volcano in the literature garduno monroy v h corona chavez p israde alcantara i mennella l arreygue e bigioggero b chiesa s 1999 carta geologica de michoacan scale 1 250 000 universidad michoacana de san nicolas de hidalgo capra l macias j l scott k m abrams m garduno monroy v h 2002 debris avalanches and debris flows transformed from collapses in the trans mexican volcanic belt mexico behavior and implications for hazard assessment journal of volcanology and geothermal research 113 81 110 twenty six new 40ar 39ar ages indicate that volcan tancitaro became active 793 22 ka and that the most recent effusive activity occurred at 237 34 ka two catastrophic sector collapse events are identified and dated the first one occurred on the west side between 694 and 571 ka whereas the second one occurred on the east side between 261 and 238 ka the older collapse produced a 2 3 3 4 km3 debris avalanche and laharic deposit spread over 567 km2 whereas the more recent collapse left a 3 km wide horseshoe shaped scar on the eastern flank and produced a 3 6 7 0 km3 debris avalanche and laharic deposit that covers 654 km2 reconstruction of the main edifice of volcan tancitaro using arcgis software and digital elevation models indicates that the volume removed during the eastern sector collapse was 4 7 km3

high magnetization zones near spreading center discontinuities abstract three dimensional inversions of the magnetic field reveal the presence of zones of high magnetization over the propagating limb of large spreading center discontinuities the presence of high magnetization zones is confirmed by rock magnetic measurements where available magnetization highs are associated with basalts which tend to have high fe contents and low mg numbers these data suggest that high magnetization zones observed over large ridge axis discontinuities are associated with highly differentiated basalts enriched in iron following christie and sinton 1 such highly evolved basalts may be the result of shallow level crystal fractionation in small magma bodies with a low supply rate these small magma bodies are postulated to correspond to the first stages in the development of a sub axial magmatic system as a result of the propagation of one of the limbs of the offset into older lithosphere because high magnetization zones at large ridge axis discontinuities often correspond to gaps in the along axis extent of a seismically detectable magma chamber these magma bodies may be smaller than a few hundred meters rock magnetic measurements suggest that the enrichment in iron associated with increased differentiation may be accompanied in a few cases by an increase in the concentration of titanomagnetite within the basalts and in the magnetization of the rocks however the exact relationship between high magnetization intensities and iron enrichment is complex and unclear and may be significantly affected by factors such as magnetic mineralogy and crystallization history

magma intrusion near volcan tancitaro evidence from seismic analysis abstract between may and june 2006 an earthquake swarm occurred near volcan tancitaro in mexico which was recorded by a temporary seismic deployment known as the mars network we located 1000 events from this seismic swarm previous earthquake swarms in the area were reported in the years 1997 1999 and 2000 we relocate and analyze the evolution and properties of the 2006 earthquake swarm employing a waveform cross correlation based phase repicking technique hypocenters from 911 events were located and divided into eighteen families having a correlation coefficient at or above 0 75 90 of the earthquakes provide at least sixteen phase picks we used the single event location code hypo71 and the p wave velocity model used by the jalisco seismic and accelerometer network to improve hypocenters based on the correlation adjusted phase arrival times we relocated 121 earthquakes which show clearly two clusters between 9 10 km and 3 4 km depth respectively the average location error estimates are

the eruptive history of the patzcuaro lake area in the michoacan guanajuato volcanic field central mexico field mapping c 14 and 40ar 39ar geochronology abstract monogenetic volcanic fields occur in many tectonic settings representing an unpredictable and uncertain volcanic hazard the patzcuaro lake is located in the central part of the michoacan guanajuato volcanic field mgvf field mapping of an area of 560 km2 stratigraphy and new radiometric ages 40ar 39ar and 14c aided in the identification of 45 individual volcanic landforms formed during the past 3 9 ma these landforms comprise five medium size shield volcanoes 25 scoria cones 10 lava domes two lavas associated to scoria cones one fissural lava flow and two volcanic complexes these volcanic edifices erupted a minimum volume of 20 km3 since pliocene with an output rate of 0 02 km3 ka chemical composition of magmas ranges from 47 6 to 65 3 wt silica being dominantly andesites 76 vol followed by basaltic andesites 12 vol dacites 9 vol and basalts 3 vol volcanic products vary from mainly andesitic during the pliocene basaltic to dacitic during the pleistocene and basaltic andesitic to andesitic during the holocene all these rocks are the result of low dipping slab subduction related magmatism alignment of cones and structures follow ne sw n s and e w directions the youngest faults are associated with the e w morelia acambay fault system with historic earthquake activity the volcanic complexes el estribo and la muela have cone collapses to the north that correspond with the direction of maximum regional extension associated with the e w fault system the 3 9 ma history of monogenetic volcanism around the patzcuaro lake demonstrates that monogenic fields can be long lived volcanic features