magnetic fabrics of the miocene ignimbrites from west cameroon implications for pyroclastic flow source and sedimentation abstract the miocene ignimbrites of mounts bambouto and bamenda located in the central part of cameroon volcanic line are generally made of welded and non welded massive lapilli tuff and lithic breccias these discontinuous deposits cover a total area of 180 km 2 with thickness ranging from 25 to 200 m the different facies contain several lithic fragments of mainly trachytic nature the devitrified matrix of the welded ignimbrites is constituted by sanidine anorthoclase quartz plagioclase clinopyroxene biotite fe ti oxides and devitrified fiammes anisotropy of magnetic susceptibility ams is used to characterize magnetic fabrics and to provide an estimate of flow direction of each ignimbrite sheet magnetic mineralogy results from different flow units show that titanomagnetite titanohematite maghemite and goethite with grain size ranging from coarse md to very fine sp are the main magnetic carriers of these ignimbrites inferred transport directions based on the ams data and field indicators show that bambouto caldera is the source of main pyroclastic deposits of mount bambouto in southwestern mount bamenda santa mbu caldera or bambouto caldera constituted the probable emission center of mbengwi bamenda and mbu ignimbrite sheets whereas magnetic fabrics of bambili sabga and big babanki ignimbrites demonstrate that these deposits were emitted from a northeastern source namely oku vent in mount oku a small number of subvertical ams fabrics correspond to rocks possibly modified by an elutriation process

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

rapid high temperature formation of large scale rheomorphic structures in the 2 06 ma huckleberry ridge tuff idaho usa in the teton river canyon eastern idaho the ca 2 06 ma 130 m thick huckleberry ridge tuff exhibits large scale rheomorphic fold geometries defined by eutaxitic fabrics parallel to both the primary internal zonation and the basal contact with older strata paleomagnetic data from a large amplitude 150 m northwest trending overturned fold near the failed teton dam indicate folding above maximum magnetization unblocking temperatures 580 c the in situ characteristic remanent magnetization chrm direction is indistinguishable from previous studies of undeformed huckleberry ridge tuff and a fold test is negative k minimized at 100 unfolding anisotropy of magnetic susceptibility data reveal magnetic foliation planes that dip northeast roughly parallel to the axial surface of the fold because deformed and undeformed huckleberry ridge tuff exposures preserve the same anomalous chrm direction large scale rheomorphic structures in the tuff must have formed rapidly at high temperatures shortly after development of compaction fabrics post welding high temperature deformation is consistent with field evidence indicating rapid plastic secondary deformation of much of the tuff prior to devitrification

paleomagnetism magnetic fabric and 40ar 39ar dating of pliocene and quaternary ignimbrites in the arequipa area southern peru 40ar 39ar ages and paleomagnetic correlations using characteristic remanent magnetizations chrm show that two main ignimbrite sheets were deposited at 4 86 0 07 ma la joya ignimbrite lji and at 1 63 0 07 ma arequipa airport ignimbrite aai in the arequipa area southern peru the aai is a 20 100 m thick ignimbrite that fills in the arequipa depression to the west of the city of arequipa the aai is made up of two cooling units an underlying white unit and an overlying weakly consolidated pink unit radiometric data provide the same age for the two units as both units record exactly the same well defined paleomagnetic direction 16 sites in the white unit of aai dec 173 7 inc 31 2 95 0 7 k 2749 and 10 sites in the pink unit of aai dec 173 6 inc 30 3 95 1 2 k 1634 showing no evidence of secular variation the time gap between emplacement of the two units is unlikely to exceed a few years the 50 m thick well consolidated white underlying unit of the arequipa airport ignimbrite provides a very specific magnetic zonation with low magnetic susceptibilities high coercivities and unblocking temperatures of nrm above 580 c indicating a ti poor titanohematite signature the anisotropy of magnetic susceptibility ams is strongly enhanced in this layer with anisotropy values up to 1 25 the fabric delineated by ams was not recognized neither in the field nor in thin sections because most of the aai consists in a massive and isotrope deposit with no visible textural fabric pumices deformation due to welding is only observed at the base of the thickest sections ams within the aai ignimbrite show a very well defined pattern of apparent imbrications correlated to the paleotopography with planes of foliation and lineation dipping often at more than 20 toward the expected vent buried beneath the nevado chachani volcanic complex in contrast with the relatively small extent of the thick aai the la joya ignimbrite covers large areas from the altipano down the piedmont ti poor titanomagnetites are the dominant magnetic carriers and ams values are generally lower than 1 05 magnetic foliations are sub horizontal and lineations directions are scattered in the lji the ams fabrics are probably controlled by post depositional compaction and welding of the deposit rather than transport dynamics

emplacement dynamics of alkaline volcanic and subvolcanic rocks in trindade island brazil abstract anisotropy of magnetic susceptibility ams and anhysteretic remanent magnetization aarm are tools to characterize flow structures and emplacement conditions of volcanic and subvolcanic bodies directly related to magnetic fabrics an ams and aarm study was performed to contribute to the understanding of the processes involved in magma transportation and emplacement dynamics of the volcanic and subvolcanic rocks of trindade island and their implications on the evolution of the volcanic field trindade island is located in the south atlantic ocean at 1260 km to the east of the espirito santo state coast in brazil the island is composed of lava flows intrusions and pyroclastic rocks of alkaline sio2 undersaturated nature forming five geological units aged 3 9 0 25 ma during fieldwork 17 sampling sites were established in the phonolitic necks and melanephelinitic dyke of the oldest unit trindade complex and in the melanephelinitic a a flows of the youngest units morro vermelho valado and paredao volcano formations totaling 160 oriented mini cores and 504 rock specimens the analysis of petrofabric magnetic mineralogy ams and aarm diagrams suggests the dominance of a low coercivity magnetic mineral phase represented by low ti titanomagnetite titanomagnetite in the phonolitic necks and melanephelinitic dyke these bodies display normal fabric multi domain and vortex grains and sub vertical high angle magnetic foliation and lineation the melanephelinitic a a lavas present at least two magnetic mineral phases of high and low coercivity titanomagnetite maghemite and hematite with normal intermediate or inverse fabrics vortex or single domain grains mean magnetic foliation and lineation suggest that lavas flowed to ne data interpretation indicates that the subvolcanic bodies were emplaced vertically during the initial stages of island formation followed by lavas flowing to the northeast the orientation of the dyke and lava feeders along nnw sse leads to the conclusion that the tensional field during the evolution of volcanism remained the same with nw direction for during approximately 4 my this study is an example for the understanding of physical processes in the evolution of world oceanic islands

magnetic structural and geochronologic evidence bearing on volcanic sources and oligocene deformation of ash flow tuffs northeast nevada magnetic properties of mid tertiary volcanic rocks west of jiggs in northeast nevada were investigated for the purposes of interpreting igneous structural and tectonic processes in this part of the basin and range province anisotropy of magnetic susceptibility ams patterns reflect flow fabrics and suggest previously unknown sources for these ash flow tuffs paleomagnetic and structural evidence suggest counterclockwise relative rotation of the southern part of the area with respect to the north new stratigraphic chemical and k ar isotopic data support these interpretations dacite to rhyolite ash flow tuffs of the indian well formation were subdivided into two main units the lower and predominant tuff of jiggs 35 8 37 0 ma and the unconformably overlying but areally restricted tuff of hackwood 30 8 ma the jiggs unit has two polarities whereas the hackwood has only a reversed polarity remanence together these units with tilt correction define a pole 92 e 87 n dp 6 dm 8 for approximately 30 37 ma this pole is concordant with coeval north american reference poles indicating that this zone of approximately 30 km ns extent has not undergone significant vertical axis rotation relative to the north american reference andesite lavas of normal magnetic polarity and of 32 5 ma age characterize the diamond hills immediately to the south we interpret this region from both structural evidence and discordant paleomagnetic direction to have rotated approximately 25 counterclockwise relative to the indian well volcanic units to the north the apparent rotation of the diamond hills is possibly the result of drag on the left lateral garcia fault which limits the diamond hills on the southwest analysis of ams data suggests by patterns of the k1 axes two distinct sources for the jiggs unit a northern buried source and a central partially buried source lithologic evidence consistent with proximal vent facies is found near the latter source an unusual pattern in the ams k3 axes suggested as resulting from the tilt correction applied to initial dip is consistent with the interpreted flow directions tight age constraints can be placed on the deformation of the main ash flows of the jiggs unit in the 35 8 to 30 8 ma interval because the diamond hills unit is slightly younger and appears to have been affected by the same deformative episode as the jiggs the regional deformation affecting both units can be more narrowly confined to the 32 5 30 8 ma interval this deformation is inferred to coincide with the major early tertiary episode of crustal extension in this region of the basin and range

correction to the 40ar 39ar ages and tectonic setting of the middle eocene northeast nevada volcanic field by william e brooks charles h thorman and lawrence w snee

magnetic properties and emplacement of the bishop tuff california anisotropy of magnetic susceptibility ams and characteristic remanence were measured for 45 sites in the 0 76 ma bishop tuff eastern california thirty three sites were sampled in three stratigraphic sections two in owens gorge south of long valley caldera and the third in the adobe lobe north of long valley the remaining 12 sites are widely distributed but of limited stratigraphic extent weakly indurated highly porous to dense welded ash flow tuffs were sampled saturation magnetization vs temperature experiments indicate two principal iron oxide phases low ti magnetites with 525 570 c curie temperatures and maghemite with 610 640 c curie temperatures af demagnetization spectra of isothermal remanent magnetizations are indicative of magnetite maghemite predominantly in the multidomain to pseudo single domain size ranges remeasurement of ams after application of saturating direct fields indicates that randomly oriented single domain grains are also present the degree of anisotropy is only a few percent typical of tuffs the ams ellipsoids are oblate with kmin axes normal to subhorizontal foliation and kmax axes regionally aligned with published source vents for 12 of 16 locality means kmax axes plunge sourceward confirming previous observations regarding flow sense topographic control on flow emplacement is indicated by the distribution of tuff deposits and by flow directions inferred from kmax axes deposition east of the benton range occurred by flow around the south end of the range and through two gaps benton notch and chidago gap flow down mammoth pass of the sierra nevada is also evident at least some of the adobe lobe in the northeast flowed around the west end of glass mountain eastward flow directions in the upper owens gorge and southeast directions in the lower owens gorge are parallel to the present canyon suggesting that the present drainage has been established along the pre bishop paleodrainage characteristic remanence directions from 45 sites 267 samples yield an overall mean of d 348 i 53 for the bishop tuff a correlation is found in two of the three profiles between density and remanence inclination a mean remanence direction based on 13 localities together with data from uncompacted xenoliths and data from the ash fall tuff at lake tecopa is d 353 i 54 k 172 95 2 9 n 15

magnetic fabrics of the bloodgood canyon and shelley peak tuffs southwestern new mexico implications for emplacement and alteration processes anisotropy of magnetic susceptibility ams of the middle tertiary bloodgood canyon and shelley peak tuffs of the mogollon datil volcanic field has been used to 1 evaluate the ability of ams to constrain flow lineations in low susceptibility ash flow tuffs 2 establish a correlation between magnetic fabric magnetic mineralogy tuff facies and characteristics of the depositional setting and 3 constrain source locations of the tuffs the tuffs are associated with the overlapping bursum caldera and gila cliff dwellings basin the high silica bloodgood canyon tuff fills the gila cliff dwellings basin and occurs as thin outcrops outside of the basin the older shelley peak tuff occurs as thin outcrops both along the boundary between the two structures and outside of the complex ams data were collected from 16 sites of bloodgood canyon tuff basin fill 19 sites of bloodgood canyon tuff outflow and 11 sites of shelley peak tuff sites were classified on the basis of within site clustering of orientations of principal susceptibility axes based on the categories of knight et al 1986 most microscopically visible oxide minerals in the bloodgood canyon tuff outflow and basin fill and in the shelley peak tuff are members of the hematite ilmenite solid solution series however irm acquisition data indicate that bloodgood canyon tuff basin fill and shelley peak tuff have magnetic mineralogy dominated by single or pseudo single domain magnetite and that the magnetic mineralogy of the bloodgood canyon tuff outflow is dominated by hematite hematite in bloodgood canyon tuff outflow is likely to be the result of deuteric and or low temperature alteration of magnetite and iron silicate minerals bulk magnetic susceptibility is higher in magnetite dominated ash flow tuff bloodgood canyon tuff basin fill and shelley peak tuff than it is in hematite dominated ash flow tuff bloodgood canyon tuff outflow bloodgood canyon tuff outflow has the highest total anisotropy h of the three units followed by shelley peak tuff and bloodgood canyon tuff basin fill all three ash flow tuffs are genearlly characterized by oblate susceptibility ellipsoids with those of the bloodgood canyon tuff basin fill nearest to spherical at high values of total anisotropy shelley peak tuff susceptibility ellipsoids attain a prolate shape those of bloodgood canyon tuff outflow attain an increasingly oblate shape three factors may influence differences in total anisotropy and susceptibility ellipsoid shape 1 ash which travelled the greatest distance before deposition may show the best development of magnetic fabric particularly of magnetic lineation 2 deposition of ash in a closed basin may inhibit laminar flow throughout the sheet and the resulting development of flow textures and 3 replacement of magnetite and iron silicates preferentially oriented within the foliation plane by hematite with strong crystalline anisotropy may enhance the magnetic susceptibility within that plane scatter in ams axis orientation within sites may result from 1 greater orientation inaccuracy in block sampled than in fielddrilled samples 2 rheomorphism and 3 low accuracy of ams measurement in low susceptibility ashflow tuffs evaluation of flow lineation based on ams of sites with well clustered k1 axes indicates that 1 bloodgood canyon tuff basin fill flowed along a generally northwest southeast azimuth 2 shelley peak tuff located on the boundary of the bursum caldera and the gila cliff dwellings basin flowed along a nearly east west azimuth and 3 bloodgood canyon tuff outflow sites have k1 susceptibility axes generally radial to the bursum gila cliff dwellings complex but within site scatter of k1 orientations is generally too large to draw conclusions about flow lineation orientation limited petrographic work on pilot thin sections adds flow direction information to ams derived flow lineation information

rupture velocity inferred from near field shear strain analysis s u m m a r y we propose a new technique to determine the rupture velocity of large strike slip earthquakes by means of simple numerical ground motion simulations we show that when the rupture penetrates a shallow layer of sediment or fractured rock shock waves propagate along the surface fault trace in the forward rupture direction such shock waves which are insensitive to the complexity of slip over the fault plane propagate at a phase velocity equal to the rupture speed we show that those shock waves can be easily isolated in the frequency domain and that phase velocity can then be simply obtained from shear strain

geologic origin of magnetic volcanic glasses in the kbs tuff volcanic glass shards and banded pumice from the kbs tuff of northern kenya exhibit marked variations in magnetic susceptibility and color colorless to dark brown the darker glass shards exhibit superparamagnetism which they now know to be carried by a population of tiny microcrystals of fe rich cubic oxide approx 20 to approx 100 a in size thought to be magnetite a theory for their origin is one of nucleation and growth precipitation in quenched homogeneous glass colorless at temperatures of approx 700 1000 sup 0 c during and immediately subsequent to eruption results from high temperature heating experiments on kbs shards support this idea the precipitate appears in the kbs shards as a consequence of their cooling history and is the origin of their coloring the origin cannot lie in negligible compositional differences between the colorless and darkest shards on the other hand banded pumice from the kbs tuff has both dark and colorless glasses of differing compositions the pumice appears banded because precipitation occurred preferentially in the fe rich glass although magma mixing may have played a role in the eruption of these materials on the basis of their survey of distal eruptive products it would appear that the volumetric amount of the maficmore end member dark pumice was insubstantial less

orientation of logs in the taupo ignimbrite as an indicator of flow direction and vent position the taupo ignimbrite is an unwelded rhyolitic ignimbrite that originated about 1 820 yr ago from an eruptive vent within lake taupo in the taupo volcanic zone new zealand this ignimbrite is rich in carbonized plant matter including many logs the logs lie subhorizontally throughout all levels of the ignimbrite but are especially common in the lower half regionally the logs show a strong radial orientation indicating flow directions and position of the source vent log orientation is thus a reliable indicator of vent position

pleistocene ignimbrites of western armenia paleomagnetic and magnetic anisotropy constraints on flow direction and stratigraphy abstract voluminous early to middle pleistocene pyroclastic flow deposits were generated from mount aragats aragats volcanic province avp one of the largest volcanoes in the arabia eurasia continental collision zone here we report paleomagnetic results in combination with anisotropy of magnetic susceptibility ams analysis from 31 outcrops belonging to six ignimbrite units of the western part of the avp covering an area of more than 5000 km2 paleomagnetic directions analysis of samples from 26 outcrops of the three younger ignimbrite units yields a well defined normal polarity direction of d 339 2 i 55 7 k 278 4 and 95 1 7 the virtual coincidence of the paleomagnetic directions seems to point to an emplacement of the units within a restricted time of similar geomagnetic secular variation i e few hundred years however new isotope 40ar 39ar ages of the three units suggest two separated events at 0 65 and 0 75 ma we cannot fully exclude the possibility of similar paleomagnetic directions at 0 65 and 0 75 ma or the occurrence of a widespread remagnetization event at 0 65 ma but both scenarios are highly unlikely in contrast to that five sites from the three older units all show reversed magnetic polarity which implies an age of emplacement older than the last magnetic polarity reversal at 780 ka this agrees with recent 40ar 39ar ages of the three older units at 0 93 ma the paleomagnetic scatter of the older units is high compared to the younger ones even if paleomagnetic directions and radiometric age are similar available data cannot unequivocally indicate if the three older units were generated from the same or three different eruptions the high quality ams data result in three different types of fabric lineated foliated and undetermined that are all commonly observed in the studied pyroclastic deposits accordingly we use lineation tilt of foliation or both for the determination of paleoflow direction results reveal a radial flow pattern surrounding mt aragats which is however highly influenced by topographic features such as valleys even in proximity of the eruption center

anisotropy of magnetic susceptibility in relation to source vents of ignimbrites empirical observations the anisotropy of magnetic susceptibility ams of young undeformed ashflow deposits e g bishop tuff california bandlier tuff new mexico show alignment of the maximum axis of magnetic susceptibility kmax with inferred flow lines the sense of flow is deduced from the predominant plunge bias of kmax axes these principles are discussed in relation to four deformed settings of ashflow deposits in the great basin of the western united states the magnetic susceptibility data provide valuable but not always unambiguous insight into probable source areas various complications arise in interpreting the ams data in terms of flow directions but these can be reduced by integration with geologic observations

the 40ar 39ar ages and tectonic setting of the middle eocene northeast nevada volcanic field widespread middle to late eocene calc alkalic volcanism which formed the northeast nevada volcanic field marks the earliest tertiary volcanism in the northern basin and range the central part of this major field in northeast nevada and adjacent utah is herein defined by 23 40ar 39ar ages that range from 42 6 to 39 0 ma rock chemistry from 12 localities stratigraphic position of the volcanic rocks above a regional middle eocene unconformity volcanic setting and lithology the type area is at nanny creek in the northern pequop mountains nevada where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs the intermediate composition rocks are locally derived throughout the volcanic field whereas the sources for rhyolite ash flow tuffs are unknown the uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region in the central part of the field the middle eocene volcanic rocks rest with depositional angular discordance on deformed middle paleozoic to triassic strata ostracode bearing limestone probably of early eocene age is locally present below the volcanic rocks in the western and southeastern parts of the field these middle eocene volcanic rocks rest with depositional angular discordance on lower eocene lacustrine strata of the elko and white sage formations respectively this angular discordance documents a middle eocene deformational event previously unrecognized in the region

magnetic fabric and implications for pyroclastic flow and lahar emplacement albano maar italy 1 the albano polygenetic maar is the youngest eruptive center of the quiescent colli albani volcano located near the city of rome the most recent activity of the albano maar extends from 23 ka into the holocene and produced the small volume basic phreatomagmatic peperino albano pa ignimbrite and more recently phreatomagmatic surge and lahar deposits related to the overspill of the albano maar lake we have performed an anisotropy of magnetic susceptibility ams study in order to define the relationships between the magnetic fabric and the flow mechanisms of the pa and of the phreatomagmatic deposits and lahars ams results indicate different transport and or depositional systems for the veneer and valley pond facies in the pa ignimbrite and for the lahar deposits ams also demonstrates that flow directions are mainly controlled by the paleotopography the paleotopographic control has been interpreted in terms of talweg sedimentation even at proximal locations where deposition occurs from dilute pyroclastic flows furthermore ams results clearly evidence a southward provenance for holocene post pa ignimbrite units cropping out in the ciampino plain and confirm their origin from albano maar lake overspill we demonstrate that ams is a reliable marker to determine paleoflow directions also in small volume phreatomagmatic ignimbrites and in syneruptive lahar deposits and can be successfully used to define their depositional systems

the bishop tuff new insights from eruptive stratigraphy the 0 76 ma bishop tuff from long valley caldera in eastern california consists of a widespread fall deposit and voluminous partly welded ignimbrite the fall deposit f exposed over an easterly sector below and adjacent to the ignimbrite is divided into nine units f1 f9 with no significant time breaks except possibly between f8 and f9 maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for f1 f8 as ca 90 hrs the ignimbrite ig is divided into chronologically and or geographically distinct packages of material earlier packages ig1 were emplaced mostly eastward are wholly intraplinian coeval with fall units f2 f8 lack phenocrystic pyroxenes and contain few or no glass mountain derived rhyolite lithic fragments earlier packages ig2 were erupted mostly to the north and east are at least partly intraplinian interbedded with fall unit f9 to the east contain pyroxenes and have lithic fractions rich in g

stratigraphy paleomagnetism and magnetic fabric of the toba tuffs constraints on the sources and eruptive styles the toba depression in north central sumatra is a complex of several overlapping calderas resulting from three major ignimbrite forming eruptions within the depression the upland masses of samosir and northern uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment hitherto interpreted to be two parts of a single resurgent dome this study has demonstrated that the welded tuffs of samosir and uluan have different magnetic polarities and therefore at least two different ignimbrites are present the samosir uluan massif may consist of parts of two resurgent domes the first ignimbrite eruption occurred at 0 84 ma and produced a very thick 400 m densely welded unit having a reversed polarity anisotropy of magnetic susceptibility ams flow direction and lithic size data indicate that the source lies in the southern part of the toba depression and the thick deposit of uluan is thought to have ponded in a 40 km wide caldera the second ignimbrite is normally magnetized ams flow direction data indicate two separate source vents one to the north in the haranggaol caldera and another to the south the thick deposit at samosir is thought to have ponded in the southern caldera coarse sediments then accumulated over samosir and northern uluan and were capped by lacustrine deposits a renewed episode of resurgence then uplifted samosir island and possibly the northern part of uluan at approximately 0 075 ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the toba depression this ignimbrite is mostly nonwelded and normally magnetized part of the uluan dome was destroyed by collapse of the sibandung caldera and latung graben and concomitant with renewed subsidence of the haranggaol and porsea calderas

high resolution magnetic susceptibility imaging of geological thin sections pilot study of a pyroclastic sample from the bishop tuff california u s a high resolution magnetic susceptibility imaging is a new technique for studying the magnetic properties of geological thin sections the two dimensional distribution of both remanent and induced magnetization can be determined with a spatial resolution 1 0 mm that is similar to the size of phenocrysts in the sample amongst many problems in rock magnetism to which it could be applied the technique holds great potential for understanding the origin of the anisotropy of magnetic susceptibility in pyroclastic flows preliminary tests on a single sample of ignimbrite indicate that secondary iron titanium oxide particles deposited within vesicle walls prior to their collapse are responsible for the bulk susceptibility

grain orientation and depositional ramps as flow direction indicators of a large scale pyroclastic flow deposit in japan flow lineation was measured using the orientation of the elongated crystals and lithic fragments in the large scale welded ata pyroclastic flow deposit in southwestern japan in order to analyze the effect of preflow topographic relief three different types of lineations were obtained flow lineations radially away from the source were obtained only for samples collected from the surface of the pyroclastic flow plateau samples collected along the floor of narrow valleys have lineations parallel to the axis of valley samples collected from valley walls tend to be parallel to the slope of the valley this evidence suggests the following process earlier flow material moves along the valley axis in the narrow valley bottom when the flow is trapped within a valley it moves toward the bottom of the valley just prior to the final settlement when the basement relief is filled up with the earlier flow units the flow maintains its original radial movement until final settlement asymmetrical depositional ramps on the original surface of the pyroclastic flow provide an independent indicator of the flow direction

anisotropy of magnetic susceptibility data bearing on the transport direction of mid tertiary regional ignimbrites candelaria hills area west central nevada in west central nevada the oligocene candelaria pyroclastic sequence reaches a local thickness of up to 1 3 km in what has been referred to as the candelaria trough but more generally the accumulation of ash flow tuffs and related volcanic rocks is less than 300 m thick complete to near complete outcrops are scattered over about 3200 km2 in the candelaria hills and surrounding ranges of the southern walker lane structural zone three regionally extensive compound cooling units within the overall sequence 25 8 ma metallic city 24 1 ma belleville and 23 7 ma candelaria junction tuffs have distinguishing characteristics and are the focus of study at 106 sites anisotropy of magnetic susceptibility ams data provide an estimate of transport direction of each tuff inferred transport directions based on the ams data are corrected for a modest clockwise yet variable magnitude vertical axis rotation that affected these rocks in late miocene to pliocene time as revealed by paleomagnetic studies the ams data show a somewhat orderly pattern of magnetic fabrics that we interpret to define unique transport directions for the metallic city and candelaria junction tuffs the low susceptibility and degree of anisotropy of the belleville tuff limits our interpretation from this pyroclastic deposit the metallic city and candelaria junction tuffs typically show gentle south southeast and southeast dipping magnetic fabric imbrication respectively and very gently plunging magnetic lineations these ams fabric elements indicate the tuffs were transported to the north northwest and northwest respectively the ams fabric data from the metallic city and candelaria junction tuffs suggest relatively unrestricted flow during emplacement evidence across the 3 200 km2 area to support more regionally controlled channelized flow into and or flow along the east northeast west southwest axis of the candelaria trough is lacking the ignimbrites clearly filled a topographic depression inferred to have formed concurrent with early localized basin and range extension during pyroclastic emplacement but based on the uniformity of ams fabric data we infer that the depression quickly filled and did not hinder flow across the region unrecognized eruptive centers for the three ignimbrites may lie buried beneath neogene basin fill sediments south southeast of the candelaria hills or concealed below younger deposits farther southeast into the palmetto mountains

magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy magnetic fabric investigations of pyroclastic deposits from phlegrean fields southern italy

distribution anisotropy the cause of ams in igneous rocks understanding of ams in pristine basic igneous rocks in terms of flow orientation of ellipsoidal magnetite grains is thwarted by the fact that these grains 1 typically are irregularly equant or skeletal rather than distinctly ellipsoidal and 2 usually crystallize interstitially and late after magma flow has ceased however the distribution of such grains will be relatively anisotropic if they grew in residual liquid volumes within a preferredly oriented by flow silicate template this constraint may impose a slight preferred shape orientation to the irregular magnetite grains during their growth but in addition we suggest it is the anisotropic magnetic interaction between the grains which is the basic cause of ams experiments involving the casting of magnetite epoxy mixtures in various foliated or lineated glass templates have yielded samples with the expected ams symmetry i e k3 perpendicular to glass plates or k1 parallel to glass rods slicing of near isotropic synthetic blank and natural rock cores and interleaving with glass slides showed a progressive increase in anisotropy of the blank from 0 4 to 2 and the natural rock from 0 9 to 2 9 in both the k3 axis migrates to be perpendicular to the glass slides

magnetic fabric flow directions and source area of the lower miocene peach springs tuff in arizona california and nevada we have used anisotropy of magnetic susceptibility ams to define the flow fabric and possible source area of the peach springs tuff a widespread rhyolitic ash flow tuff in the mojave desert and great basin of california arizona and nevada the tuff is an important stratigraphic marker from the colorado plateau to barstow california a distance of 350 km however the location of its source caldera is unknown dated at 18 5 ma by 40ar 39 ar the tuff erupted during the early stages of miocene extension along the lower colorado river the thicker accumulations 100 m occur at kingman arizona and in the piute mountains california on opposite sides of the colorado river extensional corridor our ams studies produced well defined magnetic lineations in 30 of 42 sites distributed throughout the tuff typical ratios of the principal ams axes are 1 01 for the magnetic lineation kmax kint and 1 02 for the foliation kint kmin the bulk magnetic susceptibility of the peach springs tuff averages 2 0 10 3 in the si unit system the subhorizontal lineations which presumably parallel the flow directions form a pattern radiating outward from the approximate center of the outcrop area magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff the lineation and imbrication indicate a source region near the southern tip of nevada defining the best intersection of the ams lineations required restoration of major extension strike slip faulting and associated tectonic rotation in the disrupted tuff the optimum intersection of magnetic lineations lies in the southern black mountains of arizona on the eastern side of the colorado river extensional corridor no caldera structures are known from that area but the area contains thick sections of the peach springs tuff above a silicic volcanic center the caldera may be buried under younger deposits in the mohave valley of arizona tertiary granite in the newberry mountains may represent a deeper level of the peach springs tuff vent that has been exhumed by detachment faulting

flow directions in ash flow tuffs a comparison of geological and magnetic susceptibility measurements tshirege member upper bandelier tuff valles caldera new mexico usa this study concludes that the elongation axis k 1 of the ellipsoid of anisotropic magnetic susceptibility ams is a suitable proxy for flow axis in ashflow tuffs 153 oriented samples 176 specimens were studied from 18 sites in the 1 1 ma tshirege member of the bandelier tuff these sites are distributed around the valles caldera at distances of 5 25 km outside of the rim k 1 axes correlate well with postulated radial flow axes at 13 sites k 1 also agrees with measured geological flow indicators mainly imbricated larger clasts at 7 sites at 2 of the 5 sites where significant disagreement is seen between theoretical radial flow directions and measuredk 1 axes thek 1 axes correspond well with geological flow indicators indicating that the divergence of flow from the predicted radial flow pattern is real two major topographic buttresses are suggested as the cause of flow divergence for the tshirege ash flows the san pedro buttress northwest of the caldera and the san miguel buttress in the southeast in situk 1 axes plunge about 7 toward the source at two thirds of the sites therefore the plunge ofk 1 is a plausible in situ indicator for thedirection of flow multiple flow zones in sections of several meters thickness indicate changes of flow direction that are both rapid and large during ash flow emplacement these observations raisre the question of how best to represent mean flow directions in ash flow sheets by eigenvector methods by vector sum methods or by modes a method for measuring imbrication of larger clasts using apparent dips in vertical joints is outlined imbrication determined in this way at one third of the sites dips toward the source i e up flow the minimum k 3 axis of the ams ellipsoid correlates with the flow foliation rather than with the larger clast imbrication the flow axes of ash flows correspond with thek 1 axes not with the declination ofk 3 axes as suggested by some authors initial dip of the sampled ash flows is not large and does not affect the paleomagnetic remanence direction which is reversed with a mean ofd 173 5 i 38 4 95 3 4 n 18 this mean is not different at the 95 confidence level from that of earlier workers the mean pole at 098 0 e 74 8 n a 95 3 3 n 18 is about 15 far sided relative to the expected time averaged geomagnetic pole suggesting a history of emplacement too short to adequately average secular variation

paleocurrent direction measurements in a volcanic setting by means of anisotropy of magnetic susceptibility a case study from the lower miocene tepoztlan formation transmexican volcanic belt central mexico sources of ancient volcanic rocks are often unknown if they are either eroded and or covered by younger deposits this problem as well as the provenance of reworked volcaniclastic fluvial and mass flow deposits can be partially solved by the application of anisotropy of the magnetic susceptibility ams for massive and poorly sorted volcaniclastic rocks in particular this may be the only way of finding reliable transport directions and therefore allowing for paleogeographic reconstructions here we present a data set of 428 ams measurements and 249 measurements of sedimentary paleocurrent indicators from the miocene tepoztlan formation at the southern edge of the transmexican volcanic belt central mexico the highest degree of reliability of ams measurements is gained for data from lava samples and the lowest from mass flows sedimentary structures in sandstones and conglomerates such as trough cross stratification asymmetric ripple marks and the shape of scours and channels could be used to calibrate the results from ams data and to prove their reliability ams data on fluvial deposits point to a drainage system with a w e flow direction indicating an outflow of the river system into the ancient gulf of mexico

identifying ash flow sources with directional data an application to the kizilkaya ignimbrite central anatolia clast shape preferred orientation or anisotropy of magnetic susceptibility ams data are increasingly used to constrain the source location of ash flow tuffs little attention has been paid to processing of these directional data for the accurate and objective identification of the sources this topic is the focus of this paper as applied to locating the source of the 5 ma kizilkaya ignimbrite central turkey first geological criteria allowing a rough identification of source vent location are discussed second ams data are processed to more accurately locate the source of this widespread deposit hillhouse and wells 1991 present a method which consists of determining the areal overlapping density function of the ams error fans defined by the kmax confidence ellipse applications to the kizilkaya ignimbrite show various possible source positions however a unique anomaly occurs in every model suggesting a source in the derinkuyu basin another method is proposed here and is named the average radial angular difference arad function it consists of calculating the average angular difference of lines that join a given source point to the sampling sites with the lineations corresponding to the ams data interpreted in terms of kinematic axes application with the data obtained at the 44 ams sampling sites shows that the function decreases to 35 in the derinkuyu basin if the analysis is narrowed to the presumed 12 most proximal sites the function provides values of 20 in the central part of the derinkuyu basin the identified source area occurs near the source zone of older miocene ignimbrites and coincides with a gravity anomaly and with a basaltic shadow zone

meandering flow of a pyroclastic density current documented by the anisotropy of magnetic susceptibility ams in the quartz latite ignimbrite of the pleistocene monte cimino volcanic centre central italy abstract a pattern of shifting current direction is revealed by the anisotropy of magnetic susceptibility ams in the quartz latite ignimbrite associated with the pleistocene cimini dome complex in central italy high field ams documents that both paramagnetic and ferrimagnetic minerals contribute to the magnetic fabric of the ignimbrite the results of low field ams studies suggest that the pyroclastic density current changed both direction and character during deposition of each flow unit that makes up the ignimbrite the meandering behaviour of the currents can be observed on the scale of 15 cm increments in the fine grained basal layer 2a of one flow unit in an area lacking significant palaeotopography ams data from 1 2 and 4 metre increments through the flow units also show complex changes in flow direction throughout the deposition of each unit studied the most closely aligned ams fabrics occur near the bases of flow units and dispersion increases upward this trend may represent decreased shear between the flow and the substrate or increased turbulence in the upper depositional zone of the pyroclastic density current or both suggesting a progressively waning current in terms of capacity or particle concentration the phenomena observed in this study have broad implications for the general methodology of ams studies detailed vertical profiles made up of several individual sample sites within 1 metre thickness intervals can reveal small scale current flow variations that may not be evident if sampling is done more sparsely

an ams study of different silicic units from the southern parana etendeka magmatic province in brazil implications for the identification of flow directions and local sources abstract in the southern portion of the parana etendeka magmatic province in brazil extensive silicic dacite rhyolite deposits occur at the top of a sequence of low ti pahoehoe to rubbly basalts the internal architecture of the silicic deposits and their eruptive style as well as the location of their sources are still unsatisfactorily known in an attempt to provide independent evidence for flow directions in deposits previously characterized as effusive and test the hypothesis of local sources we carried out anisotropy of magnetic susceptibility ams studies on the two main silicic units caxias do sul dacites and santa maria rhyolites with the best exposures in an area previously mapped in detail magnetic anisotropies were determined on oriented cylindrical specimens from a total of 28 sites rock magnetism properties indicate that pseudo single domain magnetite carries the fabrics and the remanence magnetic fabrics were determined by applying anisotropy of low field magnetic susceptibility ams and anisotropy of anhysteretic remanent magnetization aarm both ams and aarm tensors are coaxial indicating that the ams fabric is not affected by the effect of magnetite single domain grains magnetic data from several dacitic coulees caxias do sul unit indicate flows from se to nw the location and spatial distribution of these lavas support the hypothesis of local sources aligned along a ne sw trend these data are in agreement with the alignments of structures dome shaped hills observed in field work and dem images on the other hand magnetic data obtained in santa maria rhyolites indicate that flow directions in two different areas are distinct towards nw ne and w suggesting that they derived from different emission centers so regarding the silicic volcanism in the studied region our data do not support the model which classifies the entire silicic volcanism of the province as extensive rheomorphic pyroclastic deposits released from a central conduit in contrast we propose the occurrence of local volcanic events implying in the existence of different sources possibly characterized by local emission centers

the reliability of anisotropy of magnetic susceptibility ams data as flow direction indicators in friable base surge and ignimbrite deposits italian examples abstract the base surge deposits in linosa and procida islands italy show minimum axes k3 of the anisotropy of magnetic susceptibility ams that are mostly parallel to the flow directions inferred from field evidence but sometimes perpendicular to them samples from different heights above the base of an ignimbrite bed vulsini area italy showing a coarse tail reverse grading and a fine grained basal layer have different magnetic fabrics at 10 and 40 cm from the base the declinations of maximum axes k1 of the ams are approximately 90 from the theoretical flow direction whereas at 60 and 75 cm from the base the k1 axes are approximately parallel to the flow direction this bed also shows a decrease upward of the magnetic foliation lineation and degree of anisotropy suggesting that the ams data could be used to obtain information about flow mechanisms the airm data in the case of the ignimbrite bed show that the long shape axes of the ferromagnetic grains are probably aligned with the maximum axes of the ams for this reason the change in the magnetic fabric at different distances from the base is probably due to differences in the flow behaviour rather than the domain state of magnetite grains this study shows that the ams lineations can indicate the flow direction in base surge and ignimbrite deposits but other geological and magnetic information is necessary to assess if these lineations are flow perpendicular or flow parallel

structural rotation and volcanic source implications of magnetic data from eocene volcanic rocks sw idaho eocene crystal rich ash flow tuffs in the southwest part of the challis volcanic field in idaho yield a biotite kar isochron data near 45 38 0 19 ma this age determination is in good agreement with a previous result 1 from this locality near poison creek which corrected for modern decay constants is near 44 7 0 8 ma af and thermal demagnetization experiments on 113 oriented cores from sixteen sites in these challis ash flow tuffs provide a tilt corrected mean paleomagnetic direction with i 63 6 d 019 4 95 6 2 n 15 k 39 4 the corresponding paleomagnetic pole 74 9 n 313 3 e a95 8 9 lies clockwise 34 0 9 8 relative to a synthetic 40 ma reference pole for north america 2 the observed inclination 63 6 is close to that expected 61 8 for this locality and age therefore a clockwise tectonic rotation of about 34 is inferred to have affected these rocks in the interval 40 to 15 ma approximately accompanying basin and range regional extension ams analyses of these ash flow tuffs yield a predominant e w trend for the maximum susceptibility axis assuming that this axis reflects the primary flow fabric of the rock and that this direction was also rotated by the amount indicated in the paleomagnetic results a flow source along a trend n55 e s55 w is calculated a source region northeast of the study area in the main region of development of the challis volcanic field is consistent with these results

the northeast nevada volcanic field magnetic properties and source implications 1 magnetic susceptibility and remanence studies on late eocene ash flow deposits of the northeast nevada volcanic field nenvf were undertaken at 39 sites in four sampling localities dolly varden mountains 20 sites nanny creek area of the pequop mountains 11 sites eastern windermere hills 7 sites and southern snake mountains 1 site these deposits span a narrow age range from 41 08 to 39 08 ma based on 40ar 39ar data brooks et al 1995a 1995b and on new k ar ages presented here characteristic remanences reveal that all ignimbrites sampled are reversely magnetized the characteristic directions of remanent magnetization do not significantly deviate from the expected direction computed from the 42 ma mean pole for north america indicating that no major vertical axis rotations have affected the sampling regions since eocene time anisotropy of magnetic susceptibility studies undertaken to decipher flow trends and possible source areas for these ash flow tuff units of the nenvf are consistent with a source region near the toano range this places the ash flow source area east of nanny creek the type area for the nenvf the suspected source region lies close to the wells fault system however tertiary detachment faulting basin and range normal faulting erosion and sedimentation have obscured the suspected source region a more clearly defined zone of eocene igneous activity in the central pilot range to the east cannot be ruled out as a potential source area

interpreting anomalous magnetic fabrics in ophiolite dikes abstract anisotropy of magnetic susceptibility ams may reveal mineral orientation distributions defining magmatic flow axes in igneous dikes the mafic silicates are the best indication of magmatic flow but fe ti accessories may contribute more to the bulk susceptibility if the orientation distributions of the two subfabrics are incongruent anomalous fabrics will occur that do not reflect magma flow axes for ophiolite dikes ocean floor metamorphism changes the mineralogy producing new fe oxides by retrogression and exsolution from mafic silicates and by the oxidation of primary oxides incompatibly oriented ferro magnetic subfabrics may be isolated by anisotropy of anhysteretic remanence aarm anomalous ams fabrics in ophiolites elsewhere have been attributed to inverse fabric contributions from single domain magnetite in varying combinations however in ophiolite dikes from the troodos ophiolite of cyprus anomalous fabrics arise from ocean floor metamorphism extensively or completely replacing the original magnetite and titanomagnetite accessory phases with titanomagnetite fe 2 4 ti 0 6 o 4 tm60 and its oxidised versions titanomaghemite to varying degrees according to depth beneath the ocean floor distance from spreading axis and proximity to transform faults at best the oxide orientation distribution defined by aarm could only be indirectly related to magma flow if its nucleation orientation controlled by a host lattice however more commonly the topotactic lattice reorganization produces weaker arm fabric anisotropies although recrystallized oxidised tm60 dominates the bulk low field susceptibility its anisotropy is generally too feeble to compete with the flow fabric defined by the ams contribution from paramagnetic mafic silicates

the afragola settlement near vesuvius italy the destruction and abandonment of a bronze age village revealed by archaeology volcanology and rock magnetism abstract public works in progress in the campanian plain north of somma vesuvius recently encountered the remains of a prehistoric settlement close to the town of afragola rescue excavations brought to light a bronze age village partially destroyed and buried by pyroclastic density currents pdcs of the vesuvian pomici di avellino eruption 3 8 14c ka bp and subsequently sealed by alluvial deposits volcanological and rock magnetic investigations supplemented the excavations careful comparison between volcanological and archaeological stratigraphies led to an understanding of the timing of the damage the buildings suffered when they were struck by a series of pdcs the first engulfed the village located some 14 km to the north of the inferred vent and penetrated into the dwellings without causing major damage the buildings were able to withstand the weak dynamic pressure of the currents and deviate their path as shown by the magnetic fabric analyses some later collapsed under the load of the deposits piled up by successive currents stepwise demagnetization of the thermal remanent magnetization trm carried by potsherds embedded in the deposits yields deposition temperatures in the order of 260 320 c fully consistent with those derived from pottery and lithic fragments from other distal and proximal sites the fairly uniform temperature of the deposits is here ascribed to the lack of pervasive air entrainment into the currents this in turn resulted from the lack of major topographical obstacles along the flat plain the coupling of structural damage and sedimentological analyses indicates that the currents were not destructive in the afragola area but trm data indicate they were still hot enough to cause death or severe injury to humans and animals the successful escape of the entire population is apparent from the lack of human remains and from thousands of human footprints on the surface of the deposits left by the first pdcs people were thus able to walk barefoot across the already emplaced deposits and escape the subsequent pdcs the rapid cooling of the deposits was probably due to both their thinness and heat dissipation due to condensation of water vapour released in the mixture by magma water interaction

the lake natron footprint tuff northern tanzania volcanic source depositional processes and age constraints from field relations the engare sero footprint site situated on the southern shore of lake natron in northern tanzania has been reported to host one of the best preserved sets of fossilized hominid footprints in the

fabrics facies and flow through a large volume ignimbrite pampa de oxaya chile the miocene oxaya formation exposed along the western andean slope in northern chile represents one of the largest ignimbrite provinces on earth in this study magnetic fabric data were acquired from a 1 km long core drilled vertically through a single cooling unit of the oxaya formation the ca 22 ma cardones ignimbrite samples for magnetic analysis were obtained every 20 m from the fine grained matrix of the core detailed measurements of the variation in bulk magnetic properties including natural remanent magnetization nrm susceptibility and anisotropy of magnetic susceptibility ams were used to monitor changes in magnetic mineralogy as well as changes in the strength and orientation of the magnetic fabric throughout the flow ams ellipsoid orientation and shape reflect rapid deposition from a concentrated granular fluidized flow and constrain both the location of the source caldera for this catastrophic eruption and processes of transport and deposition in this large volume ignimbrite after utilizing the magnetic remanence to correct for rotation about the core axis well grouped and imbricated petrofabric orientations reveal a well defined sw 247 transport direction down a proto western andean slope indicating syn or post welding flow and confirming the deformed lauca caldera as the likely source of the eruption systematic variations in fabric shape t and intensity p with depth reveal a predominately oblate fabric toward the top and base of the flow and predominately prolate fabrics in the center these vertical changes in fabric reveal that this massive apparently homogeneous deposit has a systematic layering this layering reflects depth dependent temperature variations temporal changes in the flow boundary zone during deposition and post depositional processes

estimates of flow direction for calc alkaline welded tuffs and paleomagnetic data reliability from anisotropy of magnetic susceptibility measurements central san juan mountains southwest colorado abstract flow directions are estimated from the measurement of the magnetic fabric of 106 samples collected at 18 sites in four welded tuff units in the central san juan mountains of southern colorado the estimates assume that the tuffs generally flowed directly away from the extrusive vents and that the lineations of magnetic grains within the tuffs represent the flow direction at individual sites errors in the estimation may arise from topographic variation rheomorphism post emplacement mass flow within the tuff and other factors magnetic lineation is defined as the site mean anisotropy of magnetic susceptibility maximum azimuth a test on the flow directions for individual units is based on the projection of lineation azimuths and their intersection within or near the known source caldera for the tuff this test is positive for the four units examined paleomagnetic results for these tuffs are probably reliable indicators of the geomagnetic field direction in southwest colorado during the time 28 2 26 5 ma of emplacement

single domain particles in rocks and magnetic fabric analysis magnetic susceptibility anisotropy is commonly used as a guide for determining stress and flow axes in rocks and for assessing the suitability of anisotropic rocks for palaeomagnetic purposes we have recently demonstrated however in laboratory produced samples that as theoretically expected susceptibility anisotropy is dependent on particle size multidomain particles of magnetite or similar strongly magnetic particles with shape anisotropy exhibit a maximum susceptibility parallel to their easy remanent magnetization axes whereas uniaxial single domain particles exhibit a maximum susceptibility perpendicular to their easy magnetization axes hence susceptibility anisotropy does not yield an unambiguous determination of particle alignment in rocks independent of domain state by demonstrating uniaxial single domain anisotropy characteristics for the first time in a rock specimen we show here that susceptibility measurements by themselves may not correctly identify the degree or type of alignment of ferro or ferrimagnetic particles in rocks in particular apparently foliated rocks can actually be lineated and vice versa moreover rocks with low anisotropy of susceptibility can have a much higher anisotropy of remanence remanence anisotropy methods are shown to give unambiguous determinations of the degree and type of alignment

paleomagnetic and magnetic fabric studies of the san gaspar ignimbrite western mexico constraints on emplacement mode and source vents abstract paleomagnetic and magnetic fabric data for the 4 8 ma san gaspar ignimbrite one of the largest in western mexico are used to investigate the source vents emplacement mechanism and tectonics rock magnetic properties from distant parts of the ignimbrite are similar suggesting relatively homogeneous mineralogy of the unit isothermal remanence and continuous susceptibility temperature experiments point to low to medium ti titanomagnetites as the main magnetic carriers hysteresis ratio parameters of most samples fall in the pseudo single domain grain size region wasp waisted hysteresis loops were identified corresponding to high hcr hc values stepwise thermal and alternating field demagnetization shows that secondary components are completely removed below 20 mt or 400 c thereafter the characteristic component is isolated with small within site dispersion of mean direction 95

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

paleomagnetic and ams studies of the el castillo ignimbrite central east mexico source and rock magnetic nature abstract lithological petromagnetic paleomagnetic and magnetic fabric studies are employed to determine the flow direction and the location of the source of the 2 44 to 2 21 ma el castillo ignimbrite in the central east mexico based on the increasing matrix to pumice ratio and decreasing pumice size the ignimbrite field is divided into the northwestern central and south southeastern sectors lithological comparisons among the three sectors reveal that the ignimbrite had flowed from nw to se and the source is in the nw part of the study area thermomagnetic results concur with the increasing matrix proportions from the proximal to the distal sector the coercivity and magnetization ratios of the hysteresis parameters are lower in the se sector than in the nw and central sectors the dominant flow direction inferred through magnetic fabrics at most sites is nw to se which coincides with the direction inferred from lithological comparisons however at some sites magnetic fabrics demonstrate flow towards ene or other various directions the paleomagnetic analysis and field observations reveal that these anomalous directions are a consequence of anticlockwise block rotation and tilting due to normal and lateral faulting in the region

vent sites and flow directions of the otowi ash flows lower bandelier tuff new mexico the nonwelded ash flow tuffs of the 1 4 m y old lower otowi member of the bandelier tuff on the flanks of the jemez mountains erupted from as many as seven ring fracture vents bounding the large toledo caldera at the center of the volcanic complex field evidence for these multiple vents includes 1 broad radial distribution of large volumes of ash flow tuff 2 areal variation in the compositions of lithic inclusions with basalt andesite clasts more abundant in the western than in the eastern sectors 3 areal contrasts in welding and zonation with welding best developed in the western sections 4 paleotopography and 5 flow directions determined from field measurements of elongated pumice fragments and charred trees

fe oxide microcrystals in welded tuff from southern nevada origin of remanence carriers by precipitation in volcanic glass although it is widely recognized that remanent magnetism in ash flow tuffs is carried by fine grained fe oxides the origin mineralogy and significance of such magnetic carriers are not well understood the authors have obtained transmission electron microscope images of distinctive fe oxide microcrystals in rhyolitic samples located 3 8 7 6 and 18 5 m above the base of a 110 m thick section of the miocene tiva canyon member of the paintbrush tuff the fe oxide microcrystals are lath shaped and increase in size from 20 x 140 nm in the lowermost sample near base of the member to 120 x 800 nm in the uppermost sample within the flow interior microcrystals in this size range are within or close to the range of single domain grain size for magnetite electron diffraction and analytical x ray data indicate that the microcrystals in the lower two samples are cubic fe oxides magnetite maghemite with less than 10 mol ti end member and that those in the uppermost sample are manganiferous hematite systematic variations in magnetic properties are consistent with the observed variations in size and mineralogy of the microcrystals these microcrystals are morphologically distinct from grains that the authors interpret to be fragments of phenocrysts the morphology and spatial distribution ofmore the microcrystals as well as their increase in grain size from the rapidly cooled base of the ash flow sheet into the flow interior are consistent with an origin by nucleation and growth from volcanic glass at elevated temperature subsequent to emplacement less

palaeomagnetism and magnetic properties of the cappadocian ignimbrite succession central turkey and neogene tectonics of the anatolian collage abstract the cappadocian ignimbrite succession of central southern anatolia comprises at least nine major and two minor calc alkaline rhyolitic sheets emplaced at 1 2 ma intervals between 11 2 and 1 1 ma it records the last phase of neotethyan subduction during final emplacement of the tauride orogen in southern turkey this study reports magnetostratigraphy and describes associated rock magnetic properties remanence resides in ti poor titanomagnetites haematisation is locally produced by post emplacement oxidation but does not contribute significantly to the palaeomagnetic signature although secondary processes within the ignimbrite sheets have produced composite isothermal remanent magnetisation spectra and variable intensities of magnetisation weak anisotropy of magnetic susceptibility describes tensors with maximum axes close to bedding and minimum axes perpendicular to this plane directions of kmax with weak imbrication mostly suggest flow away from centres of eruption located by gravity and remote sensing older ignimbrites upper and lower goreme akdag zelve from the cardak centre are all of normal polarity later ignimbrites partly erupted from the derinkuyu centre comprise the sarimaden r cemilkoy r tahar r kizilkaya r incesu n and valibaba sofular r ignimbrites the overall reversed group mean is d i 174 51 n 10 units r 9 84 95 6 6 k 55 and all magnetisation directions from the upper goreme 9 0 ma onwards are rotated anticlockwise with respect to eurasian and african palaeofields this sense of rotation characterises most of central anatolia and averages 9 5 in this sector the rotation rate from 8 to 1 ma bp was 1 25 ma but it appears to have accelerated during the latter part of the quaternary to about an order higher than rates determined from gps rotation has resulted from extrusion of fault blocks during tectonic escape of the anatolian collage to the southwest and followed crustal thickening as the afro arabia plate has continued to impinge differentially into eurasia cenozoic magnetic inclinations are systematically shallower across anatolia than inclinations expected from the geocentric dipole model and the apparent polar wander of bordering major plates only part of this difference 400 km can be accommodated by northward movement and crustal thickening in central anatolia since mid miocene times shallow inclinations observed in the aegean region extend into turkey they are observed in young volcanics and appear to reflect a regional geomagnetic anomaly the pattern of neotectonic declinations across anatolia shows rotations that are strongly anticlockwise rotated in the east near the arabian pincer but diminish towards the west to become zero or slightly clockwise at the western extremity of the collage rotations also seem to become generally younger towards the south crustal deformation has therefore been distributed and the net effect of terrane extrusion to the west and south has been to expand the curvature of the tauride arc and by inference the cyprus arc

magma flow in the east greenland dyke swarm inferred from study of anisotropy of magnetic susceptibility magmatic growth of a volcanic margin summary volcanic passive margins vpms are characterized by large volumes of melt emplaced within the lithosphere during break up processes several data and a recently proposed conceptual model of volcanic margin development suggest that vpms are fed from localized crustal zones of magma storage underlying large polygenetic volcanoes localized above diapir like instabilities of the asthenosphere we investigated the magma flow pattern within the coastparallel dyke swarm of the east greenland vpm which is the only outcropping vpm over a distance of 125 km the 44 sampled dykes are representative of the successive families of intrusions igneous petrofabrics are constrained by the measurements of the anisotropy of magnetic susceptibility the magnetic fabrics are of medium to low anisotropy p 0 flow related fabrics are recorded in 75 per cent of the sampled dykes we infer the flow directions from the imbrication geometry of the magnetic foliation planes at the dyke margins and check the results by measuring the preferred orientation of plagioclase in thin sections cut in the magnetic principal planes due to probable fabric superposition the magnetic lineation represents the zone axis for the distribution of magnetic foliation plane we obtained 23 reliable flow directions that are predominantly horizontal and directed away from identified crustal reservoirs this flow pattern supports the proposed model of vpm growth and emphasizes the localized nature of the magma sources in the mantle the entire flood basalt sequence appears to have been fed by a restricted number of crustal reservoirs and associated dyke swarms

complex remanent magnetization in the kizilkaya ignimbrite central anatolia implication for paleomagnetic directions abstract pyroclastic flow deposits known as ash flow tuffs or ignimbrites are invaluable materials for paleomagnetic studies with many applications for geological and tectonic purposes however little attention has been paid to evaluating the consistency and reliability of the paleomagnetic data when results are obtained on a single volcanic unit with uneven magnetic mineralogy in this work we investigate this issue by concentrating on the kizilkaya ignimbrite the youngest large volume unit of the neogene ignimbrite sequence of the central anatolian volcanic province in turkey bringing evidence of significant magnetic heterogeneities in ignimbrite deposits magnetic mineralogy susceptibility magnetic remanence coercivity etc and emphasizing the importance of a stratigraphic sampling strategy for this type of volcanic rocks in order to obtain reliable paleomagnetic data six sections were sampled at different stratigraphic heights within the devitrified portion of the ignimbrite isothermal remanence measurements point to low ti titanomagnetite as the main magnetic carrier at all sites at some sites the occurrence of oxidized ti magnetite and hematite is disclosed the bulk susceptibility k m decreases vertically at two out of six sections its value for the topmost samples is commonly one order of magnitude lower than that of the samples at the base in most cases low k m values relate to high coercivity of remanence b cr values which range from 25 to 400 mt and to low s ratios measured at 0 3 t between 0 28 and 0 99 these data point to the occurrence of oxidized magnetic phases we therefore consider the k m parameter as a reliable proxy to check the ignimbrite oxidation stage and to detect the presence of oxidized ti magnetite and hematite within the deposit the characteristic remanent magnetization is determined after stepwise thermal and af demagnetization and clearly isolated by principal component analysis at most sites for these sites the site mean paleomagnetic direction is consistent with data from the literature at a few other sites the remanence is more complex the direction moves along a great circle during demagnetization and no stable end point is reached the occurrence of oxidized ti magnetite or hematite as well as two remanence components with overlapping coercivity and blocking temperature spectra suggest that the kizilkaya ignimbrite acquired first a thermal remanent magnetization and then during the final cooling or a short time later a secondary remanent magnetization component which is interpreted as a crm acquired during post emplacement devitrification processes notwithstanding the kizilkaya ignimbrite is a single cooling unit its magnetic properties suffered substantial variations laterally and vertically within the deposit the kizilkaya case shows that thick pyroclastic deposits should be sampled using a stratigraphic approach at different sites and different stratigraphic heights at each individual sampling location otherwise under sampling may significantly affect the paleomagnetic results when sampling is performed on a short duration or on very poorly preserved deposits we recommend drilling the lower central portion in the most strongly welded and devitrified facies such sampling strategy avoids complications arising from the potential presence of a pervasive secondary crm masking the original chrm