do transcurrent faults guide volcano growth the case of nw bicol volcanic arc luzon philippines this paper shows that although volcanoes can be aligned along transcurrent faults their growth can in reality be controlled by secondary structures that have a different orientation and kinematics assessing the relationships between local structures and volcano growth can be very useful for the evaluation of volcanic hazards and for economic purposes such as geothermal and hydrogeological exploration in the north western bicol volcanic arc luzon philippines the quaternary labo and caayunan volcanoes are aligned with the nw striking transcurrent philippine fault system they are exceptional examples of composite volcanoes mostly comprising lava domes where regional tectonic structures and hydraulic radial fracturing have alternated to control volcano growth field data and analogue modelling demonstrate that apical depressions affecting the domes reflect the geometry of magma feeding fractures these data together with alignments of domes and hot springs and other structural information suggest that magma rose at depth along the nw striking transcurrent faults but that in the uppermost crust magma rose along ne striking fractures parallel to the greatest principal stress

errors in bathymetric retrievals using linear dispersion in 3 d fft analysis of marine radar ocean wave imagery the phenomenon of ocean wave shoaling and the associated reduction of ocean wave phase speed with decreased water depth provides useful information for inferring water depth d bathymetry in coastal environments one strategy for relating d to phase speed c and wave vector k of long wavelength ocean waves involves using the one dimensional 1 d linear gravity wave dispersion relationship c sup 2 g tanh kd k in principle this approach has limitations because the approach is based on a wkb approximation so it cannot be applied when d varies appreciably over the wavelength of a shoaling wave also the approach is restricted to waves that have small wave height the author uses a set of marine radar image sequences and applies this linear approximation using a 3 d fft analysis of 88 sets of image sequences spaced half an hour apart the author inverts the dispersion relation to solve for d depths between 3 6 and 5 8 m were tested for root mean square rms wave heights offshore between 8 and 3 m the author shows that for low to moderate wave heights the approach does generally retrieve the correct depth in water depths of 5 m and greater for moderate wave rms heights however an increase in the rms wave height from 1 m to 3 5 m produced a much poorer depth estimate proving the need for an application of a nonlinear wave model to the problem the errors also increase with shallower depths as expected as the error dependence on depth and wave height is determined

contractional tectonics and magma paths in volcanoes abstract this study aims to contribute a possible explanation for magma migration within volcanoes located in contractional tectonic settings based on field data and physically scaled experiments the data demonstrate the occurrence of large stratovolcanoes in areas of coeval reverse faulting in spite of the widely accepted idea that volcanism can develop only in extensional transcurrent tectonic settings the experiments simulate the propagation of deformation from substrate reverse faults with different attitudes and locations into volcanoes the substrate fault splits into two main shear zones within the volcano a shallow dipping one with reverse motion propagates towards the lower volcano flank and a steeper dipping one with normal motion propagates upwards in plan view the reverse fault zone is arcuate and convex outwards whereas the normal fault zone is rectilinear structural field surveys at volcanoes located in contractional settings show similar features the plio quaternary trohunco and los cardos centinela volcanic complexes argentina lie above plio quaternary reverse faults the late pleistocene holocene el reventador volcano ecuador is also located in a coeval contractional tectonic belt these volcanoes show curvilinear reverse faults along one flank and rectilinear extensional fracture zones across the crater area consistent with the experiments these data consistently suggest that magma migrates along the substrate reverse fault and is channelled along the normal fault zone across the volcano

laboratory studies of radar sea spikes at low grazing angles we report on measurements of low grazing angle radar sea spikes in a laboratory wave tank using an x band radar with 10 cm range resolution and multiple range bin capability the sea spikes were observed to have a horizontal vertical polarization ratio much larger than unity this is similar to observations in earlier field experiments but is in contrast to a ratio of unity recently observed for moderate grazing angles jessup et al 1990 our experiment was designed to validate the low grazing field results and to establish design criteria for spatial temporal measurement instrumentation single point measurements currently used as surface truth cannot provide water surface profiles needed for modeling of radar scatter from deterministic surface features surface scattering features were generated using a paddle generated wave combined with wind waves for a series of wind speeds the surface feature responsible for the sea spike echo was visually identified as a short wave crest occurring on the windward side of the paddle wave driven to extreme steepness by the passage of the faster paddle wave crest long decorrelation times observed in the field are also seen in these studies of the order of 500 ms for horizontal polarization and less than 5 ms for vertical we suggest that rather than bragg scatter the occurrence of similar ubiquitous features on the ocean are responsible for horizontally polarized low grazing angle sea scatter at the x band for low to moderate winds and perhaps at other frequencies as well

continental arc volcanism and tectonic setting in central anatolia turkey abstract the neogene and quaternary volcanism of central anatolia represents the central sector of the anatolian volcanic arc related to continental collision between the afro arabian and eurasian plates it is closely associated with a complex system of tectonic depressions related to brittle deformations of transtentional type and which commenced in the late miocene the volcanism here considered can be divided into three main periods of activity separated by important deformative and erosive events the first period is represented by a mostly andesitic effusive activity the second period is characterized by the emplacement of a thick ignimbritic sequence and shows an areal distribution up to 11 000 km 2 seven ignimbrite units have been recognized the three main units were found at a distance of more than 100 km from the presumed source area geological and sedimentological data lead us to recognize the melendiz dag volcanic complex and the ciftlik caldera as the probable ignimbrites source vent during the third period great andesitic basaltic stratovolcanoes and a number of prevalently acid monogenic centres developed the relationship between the volcanic activity is clearly conditioned by the main transcurrent fault systems present in central anatolia the neogene quaternary volcanic activity prevalently developed along the ene wsw karaman sivas lineament most of the great central volcanoes developed at the intersection between the ene wsw trends and the ecemis and tuz golu transcurrent faults the structural interpretation of the quaternary monogenic centres is more difficult probably they are related to the very recent n s fault swarms which cross the anatolides and the taurus range

constituent material properties and clast size distribution of volcanic breccia abstract volcanic breccia is a representative heterogeneous rock as a preliminary study for determining its mechanical properties in laboratory and field experiments its material properties and structural features were described three dimensional clast size distribution was also estimated for the study site using the theory of stereology with this distribution determined a numerical simulation was performed to investigate the relationship between the volumetric proportion of clasts in a sample and the areal proportion and maximum length of clasts on a lateral surface of the sample the following observations were made 1 coefficients of variation for volumetric proportion are dependent on the size of the sample and became large when the size of the sample is small 2 samples with a small coefficient of variation can be selected at any size if a restriction is placed on the ratio dmax d0 of the maximum length of clasts to the size of the sample and 3 the volumetric proportion of clasts in each sample can be estimated from the observed areal proportions within an error range that depends on the whole volumetric proportion

measurements of sea spikes in microwave backscatter at moderate incidence previous authors have associated large events or sea spikes in the radar cross section of the sea surface with steep or breaking waves here we present a method based on an intensity threshold and the mean doppler frequency to identify and quantify sea spikes likely to be associated with large scale wave breaking this method is applied to microwave measurements of ku band made with a dual polarized doppler scatterometer at a moderate incidence angle 45 from a fixed platform in the north sea for vv and hh polarization the mean normalized radar cross section 0 the frequency of sea spike occurrence n and the sea spike contribution to the mean normalized radar cross section ss0 are correlated with direct measurements of the friction velocity u in the range 13 u 49 cm s 1 the friction velocity exponents for both n and ss0 indicate an approximately cubic dependence while the residual radar cross section res0 0 ss is approximately proportional to friction velocity for u greater than approximately 20 cm s 1 at high friction velocities u 40 50 cm s 1 these data show that the contribution of large sea spikes to the mean radar cross section for measurements between 25 and 45 relative to the wind is in the range of 5 10 for vv polarization and 10 20 for hh polarization

on ocean sar raw signal simulation simulation of synthetic aperture radar sar raw signal pertaining to an ocean scene is presented it embodies the distributed surface model the hydrodynamic modulation and the bragg dominant scattering mechanism simulation experiments are provided

emplacement of volcanic vents and geodynamics of central anatolia turkey abstract observations on synthetic aperture radar sar scenes of the european remote sensing ers satellite and digital elevation models dems complemented by field structural analysis permit a new understanding of relationships between tectonics and volcanism since the late miocene 10 ma in central anatolia volcanic edifices form elongate stratovolcanoes linear clusters and volcanic ridges they indicate emplacement on tension fractures and tail crack or horsetail features for instance the kara dag volcano is rooted on a tail crack which accommodates a horizontal left lateral throw component at a fault termination caldera complexes of cappadocia are associated with horsetail fault patterns the emplacement of volcanoes also benefits from larger scale tectonic structures the erciyes dag volcano is localized by the sultan saz releasing bend which opens along the sinistral strike slip ecemis fault deformation has been analysed from tension fractures which are perpendicular to the direction of extension and from field structural analysis on a regional scale the tectonic regime responsible for the distribution of volcanic vents in this area of convergence and lateral extrusion is not compression but extension the central taurus range is the thermally uplifted shoulder of the adana cilicia basin which is related to lithosphere thinning westward movements in the northwestern part of the studied area are influenced by the active back arc aegean extension situated to the west farther to the south the direction of motion turns southwest and south under the influence of the opening of the adana cilicia basin we interpreted that extension in the central anatolian plateau is related to crustal blocks moving above sub horizontal detachment surfaces located in the lower crust this is based on several facts the tuz golu fault zone is a within crust detachment the tuz golu basin does not affect the whole lithosphere because otherwise it would have been bordered by thermally uplifted shoulders movements change trend within a small 50 km region

volcanism in a compressional andean setting a structural and geochronological study of tromen volcano neuquen province argentina we document evidence for growth of an active volcano in a compressional andean setting our data are surface structures and 39ar 40ar ages of volcanic products on tromen volcano tromen is an active back arc volcano in the andean foothills of neuquen province argentina its volcanic products are unconformable upon mesozoic strata of the neuquen basin the volcano straddles a n s trending pop up which formed during e w shortening the main underlying structures are eastward verging thrusts their traces curve around the eastern foot of the volcano minor folds and faults also occur in the volcanic cover of tromen as a result of e w shortening new 39ar 40ar ages for these volcanic rocks are younger than 2 27 0 10 ma and show that tromen has been active almost continuously from the late pliocene to the holocene we conclude that volcanism and thrusting have been coeval and that magma must have reached the surface in a tectonic setting of horizontal compression our results have wider implications for magmatic processes in such settings

morphometry of late pleistocene holocene faulting and volcanotectonic relationship in the southern andes of colombia diffuse seismicity and the active galeras volcano characterize the southern andes of colombia whose inhabitants are prone to a high natural risk we used field geological structural and quantitative morphological data to define the geometry and kinematics of late quaternary faults and the volcanotectonic relationships in this area the main ne sw buesaco fault has a late pleistocene holocene 29 km rupture length offset of dated landforms yields 188 13 m of horizontal right lateral long term cumulative component of slip the vertical component of uplift of the nw block being 67 12 m the long term net slip rate is 1 48 0 12 mm yr the other main structure here named aranda fault also strikes ne sw and is 13 km long offset of landforms yields 160 10 m of horizontal right lateral long term component of slip and 3 9 1 9 m of vertical component of uplift of the nw block the long term net slip rate is 1 19 0 08 mm yr on the galeras edifice e w to nw se faults have normal motions and slip rates from 0 02 to 0 23 mm yr both the galeras and a late pleistocene pyroclastic cone lie along the ideal extension of the buesaco fault magma uprising in the crust can occur along the ne striking fractures when magma pressure mp is greater than the normal to fault component of tectonic 1 magma uprising in the uppermost crust i e in the cone can occur by rock cracking along e w planes with mp 3 up to nw se planes with mp greater than shear stress component of the horizontal greatest principal stress hmax

accretion rates on some debris slopes in the mt rae area canadian rocky mountains accretion of debris was monitored on six slopes in the mt rae area of the canadian rocky mountains for the period 1975 1982 monitoring was accomplished by measuring annually the accumulated debris on fabric mats which were placed on the long profile of the slopes annual rates of accretion were extremely variable between points on the same slope between slopes and between years the data do not show a systematic decrease in accretion downslope but indicate that an absence of accretion is more likely on the lower half of the slopes in general annual rates are comparable to previously published values from the rockies and the maximum rates of 30 to 40 mm per year are comparable to those from karkevagge in scandinavia extremely high rates of accretion were all related to extreme events such as debris flows or full depth snow avalanches the variation in accretion rates is not explained by single environmental factors such as lithology and slope exposure

a temporal waterline approach to mapping intertidal areas using x band marine radar abstract mapping the morphology of intertidal areas is a logistically challenging time consuming and expensive task due to their large expanse and difficulties associated with access a technique is presented here that uses standard marine navigational radar operating at x band frequency the method uses a series of time exposure radar images over the course of a two week tidal cycle to identify the elevation of the wetting and drying transitions at each pixel in the radar images thereby building up a morphological map of the target intertidal area this temporal waterline method is applied to a dataset acquired from hilbre island at the mouth of the dee estuary uk spanning march 2006 to january 2007 the radar gathered data with a radial range of 4 km and the resulting elevation maps describe the intertidal regions of that area the results are compared with airborne lidar data surveyed over the same area and within the radar survey time period the residual differences show good agreement across large areas of beach and sandbanks with concentrations of poor estimations around points that are shadowed from the radar or likely to suffer from pooling water this paper presents the theoretical framework of the method and demonstrates its stability and accuracy the temporal waterline radar method is aimed at providing a useful tool for the monitoring and operational management of coastlines

extensional tectonics and volcano lateral collapses insights from ollague volcano chile bolivia and analogue modelling understanding the process of volcanic lateral collapse which may be governed by numerous possible factors can be significantly improved by identifying the role of each factor here we test the perturbation induced in a volcano by tectonic normal faulting in the basement magma inflation and multiple flank deformation based on fieldwork and analogue experiments ollague volcano chile bolivia provides a good example of a sector collapse with exposed substrate extensional tectonics and our experiments have been done with conditions consistent with field data results show that none of the possible tectonic conditions led to complete lateral failure in agreement also with the field data sector collapse of the model cone occurred only when a shallow magmatic intrusion within the volcano was reproduced we also found out that volcano flank deformation is enhanced if a previous flank destabilization occurred interleaved by new cone growth terra nova 18 282 289 2006

plutonism oblique subduction and continental growth an example from the mesozoic of california major episodes of mesozoic plutonism in california correlate with periods of oblique subduction and trench parallel transport of western california along intrabatholithic faults major episodes of plutonism occurred in the late jurassic during left oblique convergence and in the mid cretaceous during right oblique convergence in contrast a conspicuous lull in plutonism but continuation of volcanism in the earliest cretaceous coincides with a time when the north america farallon convergence vector although large in magnitude was oriented perpendicular to the trench this correlation suggests that plutonism is facilitated by strike slip faulting within the batholithic belt one explanation which helps to solve the plutonic room problem is that plutons are passively emplaced at releasing bends in the strike slip faults and volume is conserved by thrusting at the trench if this correlation is generally applicable then it implies that mid crustal plutonism is limited beneath areas in which the convergence vector is subperpendicular to the trench continental growth in such areas may occur dominantly by volcanism

volcanism in compressional tectonic settings is it possible 1 this study presents the first field evidence of emplacement of a huge stratovolcano within contractional tectonics in spite of the widely accepted idea that volcanism can develop only in extensional or transcurrent tectonic settings el reventador volcano ecuador started to grow 0 32 ma bp and is still active during pre el reventador times its substratum has been affected by n s to nne striking reverse faults during late pleistocene holocene some reverse faults with the same orientations were still active locally with a component of right lateral strike slip motions seismicity shows widespread events with intensity up to 6 9 mw and compression some of these faults cut the volcano creating differential uplift along n s to nne trending tectonic blocks hypocenter swarms and field data indicate east vergent basal thrusting under the volcano and n s trending fault propagation folds to the east a model for magma migration in this setting is proposed suggesting that similar cases of volcanism within a dominantly contractional tectonic regime may be more frequent than previously thought

field observations of radar backscatter from the ocean surface under low wind speed conditions unique simultaneous field observations of wind speed and images of vertically polarized radar backscatter from the ocean surface at low grazing angles during low wind and weakly stable stratification show interesting similarities and an important difference depending on whether the average wind is falling or rising when the wind speed begins to fall there is a delay before the area averaged radar backscatter begins to fall when the wind speed begins to rise the backscatter rises immediately this observation points to hysteresis in the relationship between radar backscatter and wind speed such that more wind is necessary to generate radar backscatter of a given intensity than is necessary to maintain it this likely arises from differences between the generation of wind driven gravity capillary waves and the maintenance of gravity capillary waves and or microbores also saturation was observed in the radar backscatter at wind speeds between 2 1 and 2 8 m s 1 for both rising and falling winds this saturation may be related to the energetics of the centimeter scale gravity capillary waves and or microbores that cause the backscatter during rising winds the backscatter may saturate because fine scale features of the ocean surface reach a maximum amplitude well before wind derived energy is transferred upscale to larger waves whose tilting effect would ultimately produce the familiar increase in radar backscatter with increasing wind during falling winds the backscatter may saturate because of residual momentum in the surface wave field finally for both rising and falling winds radar backscatter and wind speed are in general correlated above a wind speed of 1 7 m s 1 while below this value they are not this observation of the critical wind speed necessary to generate or maintain wind related backscatter is consistent with results from a recent field observation and with existing laboratory based observations

emplacement of dikes sills and crustal magma chambers at divergent plate boundaries abstract a model of dike emplacement at divergent plate boundaries is developed which predicts that below the uppermost 1 3 km of the oceanic crust the number and lengths of dikes in any particular swarm should increase but that the thickness should decrease with depth in the crust these predictions are supported by field observations it is proposed that rapid injections of dikes at divergent plate boundaries may temporarily alter the normal stress field in such a way that at a certain level in the crust subsequent dikes change into sills such sills may develop into crustal magma chambers either when many nearby sills combine into a large one or when an initial sill absorbs the magma of many dikes that enter it in a rapid succession while the sill is liquid for the fastest spreading ridges the initial thickness of a sill that is to develop into a crustal chamber need only attain 20 m whereas at the slowest spreading ridges the initial thickness must exceed 170 m because thin sills are much more likely to form than thick sills the probability of sills developing into crustal chambers increases with dike intrusion frequency hence with spreading rate furthermore the model predicts first that with increasing spreading rate the length of the chamber along the ridge axis should increase and second that the most likely site for magma chambers is at depths of 2 4 km both these predictions are supported by available data

a gravitational spreading origin for the socompa debris avalanche abstract socompa volcano arguably provides the world s best exposed example of a sector collapse derived debris avalanche deposit new observations lead us to re interpret the origin of the sector collapse we show that it was triggered by failure of active thrust anticlines in sediments and ignimbrites underlying the volcano the thrust anticlines were a result of gravitational spreading of substrata under the volcano load about 80 of the resulting avalanche deposit is composed of substrata formerly residing under the volcano and in the anticlines the collapse scar can be traced up to 5 km from the edifice truncating two spreading related anticlines which collapsed in the event outcrops near the volcano preserve evidence of edifice material being carried along on top of mobilised substrata on the north side of the scar the avalanche motion was initially at right angles to the failure edge structural relations indicate that immediately prior to collapse the substrata disintegrated became effectively liquidised and were ejected from beneath the edifice catastrophic mobilisation of substrata probably resulted from breakdown of ignimbrite clasts and cement it may have occurred through progressive rock fracture by high shear strain during spreading material ejected from under socompa formed a layer on which volcanic edifice debris was transported this interpretation of events explains the puzzling observation that avalanche units with the lowest gravitational potential energy moved the furthest it can also account for avalanche motion normal to the collapse scar walls ignimbrites and other rock types probably capable of similar behaviour underlie many other volcanoes identification of spreading at other sites could therefore be a first step towards assessment of the potential for this style of catastrophic sector collapse

studies of dual polarized low grazing angle radar sea scatter in nearshore regions reports on experiments featuring horizontally hh and vertically vv polarized x band marine radar sea scatter imagery in the coastal environments of bermuda and la jolla under light to moderate winds and the absence of long gravity waves these conditions allowed the study of fundamental scattering mechanisms from small scale roughness and short waves of a few meters wavelength or less shorter than the radar pulse while a large fraction of radar data collected revealed the presence of slick bands the authors analyze the radar echo of ambient background outside of the slicks sea scatter data were digitally recorded for 360 spl deg azimuthal coverage for grazing angles between 1 3 5 spl deg and were converted to normalized radar cross section nrcs or spl sigma spl deg images the hh and vv polarizations show quite different spatial texture with hh exhibiting a discrete character and vv being more nearly spatially homogeneous grazing angle dependencies are different for hh and vv upwind downwind differences of mean nrcs show ratios of just 4 6 db for vv but are equal or greater than the 16 db noise imposed limit for hh for the low wind conditions hh nrcs grazing angle characteristics are shown to correlate with biological activity indicators of the coastal waters with a fourth power dependence in relatively unproductive waters to a nearly quadratic in productive waters arguments are presented that suggest different scattering mechanisms for the two polarizations evenly distributed bragg scatter patches for vv and scatter from small asymmetric bore features for hh a multipath illumination model for small bore features is outlined and scale sizes for the small scale breakers inferred from the experimental results presented are between 2 and 4 cm in height with crest widths between 24 and 48 cm

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

volcanic facies architecture of an intra arc strike slip basin santa rita mountains southern arizona the three dimensional arrangement of volcanic deposits in strike slip basins is not only the product of volcanic processes but also of tectonic processes we use a strike slip basin within the jurassic arc of southern arizona santa rita glance conglomerate to construct a facies model for a strike slip basin dominated by volcanism this model is applicable to releasing bend strike slip basins bounded on one side by a curved and dipping strike slip fault and on the other by curved normal faults numerous very deep unconformities are formed during localized uplift in the basin as it passes through smaller restraining bends along the strike slip fault in our facies model the basin fill thins and volcanism decreases markedly away from the master strike slip fault deep end where subsidence is greatest toward the basin bounding normal faults shallow end talus cone alluvial fan deposits are largely restricted to the master fault proximal deep end of the basin volcanic centers are sited along the master fault and along splays of it within the master fault proximal deep end of the basin to a lesser degree volcanic centers also form along the curved faults that form structural highs between sub basins and those that bound the distal ends of the basin abundant volcanism along the master fault and its splays kept the deep master fault proximal end of the basin overfilled so that it could not provide accommodation for reworked tuffs and extrabasinally sourced ignimbrites that dominate the shallow underfilled end of the basin this pattern of basin fill contrasts markedly with that of nonvolcanic strike slip basins on transform margins where clastic sedimentation commonly cannot keep pace with subsidence in the master fault proximal end volcanic and subvolcanic rocks in the strike slip basin largely record polygenetic explosive and effusive small volume eruptions from many vents in the complexly faulted basin referred to here as multi vent complexes multi vent complexes like these reflect proximity to a continuously active fault zone where numerous strands of the fault frequently plumb small batches of magma to the surface releasing bend extension promotes small multivent styles of volcanism in preference to caldera collapse which is more likely to form at releasing step overs along a strike slip fault

late quaternary kinematics slip rate and segmentation of a major cordillera parallel transcurrent fault the cayambe afiladores sibundoy system nw south america abstract we describe the recent activity of the cayambe afiladores sibundoy fault casf and recognise it as one of the major potential active structures of northwestern south america based on field observations stereoscopic aerial photos of offset late pleistocene holocene deposits and landforms and crustal seismic activity the casf runs for at least 270 km along the sub andean zone of northern ecuador and southern colombia we measured systematic latest pleistocene holocene right lateral strike slip motion and right lateral reverse motion consistent with earthquake focal mechanism solutions and estimated a 7 7 0 4 to 11 9 0 7 mm yr slip rate magnitudes of the earthquakes that could be generated by possible fault segment reactivation range up to m 7 0 0 1 the casf should be considered as a major source of possible future large magnitude earthquakes presenting a seismic hazard for the densely populated regions to the west the casf is part of the tectonic boundary of the north andean block escaping nne wards with respect to the stable south american plate

stability of dike intrusion along preexisting fractures we investigate the ability of magma to propagate along preexisting fractures oblique to the least compressive stress relaxation of the preexisting shear stress to zero over the portion of the fracture dilated by magma the dike results in slip for some distance along the closed portion of the fracture ahead of the dike tip and a stress concentration near the dike tip this could lead to the production of new tensile cracks oblique to the parent dike that could capture the flow if the shear stress resolved on the fracture plane is perpendicular to the fracture front mode i ii the front may deviate along its entire length if the shear stress is parallel to the fracture front mode i iii the front may splay into segments for mode i ii dikes the maximum tensile stress occurs at the dike tip and is parallel to the dike if the effective tensile stress exceeds the rock tensile strength then the intruding magma rather than dilating the existing fracture is expected to propagate into a self generated crack analogous to the wing cracks observed to form at the tips of pure mode ii fractures for mode i iii dikes the maximum tensile stress lies within the plane of the dike and is oriented at some angle that depends upon the far field boundary conditions even if the magma pressure exceeds the ambient normal stress it appears to be very difficult for dikes to intrude into preexisting fractures unless one or more of the following conditions is satisfied 1 the fracture is nearly perpendicular to the least compressive stress 2 the resolved shear stress on the fracture is small compared to the excess magma pressure i e the ratio of shear to opening of the dike walls is small 3 the effective ambient dike normal stress is small compared to the rock tensile strength this indicates that it may be quite difficult for dikes emerging from midcrustal to lower crustal depths to follow faults

infrastructure and mechanics of volcanic systems in iceland the divergent plate boundary in iceland consists of more than two dozen systems where most of the volcano tectonic activity takes place at the surface the volcanic systems are characterised by 5 20 km wide and 40 100 km long swarms of tension fractures 102 m long normal faults 103 m long and volcanic fissures the holocene fissure swarms are confined to less than 10 000 year old basaltic lava flows mostly pahoehoe occurring near the centre of the active rift zone in any particular swarm the number of tension fractures exceeds that of normal faults all tension fractures and normal faults are vertical at the surface indicating that the surface parts were generated by an absolute tension in addition to a fissure swarm many volcanic systems have a central volcano some of which have developed collapse calderas in the late tertiary and pleistocene lava pile of iceland extinct volcanic systems are represented by local sheet swarms and regional dyke swarms the sheet swarms are normally circular or slightly elliptical several kilometres in radius and are confined to the extinct central volcanoes many swarms are associated with large plutons exposed at 1 2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers and in short traverses up to 90 of the rock may consist of sheets the sheets have a very variable strike dip on average 45 65 mostly towards the centre of the associated volcano and have an average thickness of about 1 m the regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5 10 km wide in several kilometre long traverses commonly 1 5 of the rock consist of dykes but occasionally as much as 15 28 most regional dykes are subparallel and subvertical the average dyke thickness in the pleistocene swarms is less than 2 m but 4 6 m in the tertiary swarms while active the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8 12 km at the boundary between the crust and upper mantle the reservoirs are partially molten with totally molten top regions and of cross sectional areas similar to those of the volcanic systems that they feed some active volcanic systems especially those that develop central volcanoes high temperature areas and calderas have in addition to the deep seated magma reservoirs shallow crustal magma chambers located at 1 3 km depth which in turn are fed by the deeper magma reservoirs it is proposed that the regional dyke swarms are supplied with magma largely from the deep seated reservoirs whereas the local sheet swarms are mainly fed from the associated crustal magma chambers because the volume of a crustal chamber is less than that of its deeper source reservoir a single magma flow dyke intrusion from the reservoir may trigger tens of magma flows sheet intrusions from the chamber which is one explanation for the enormous number of sheets associated with many central volcanoes the sheets follow the stress trajectories of the local stress fields around the source chambers whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes it is suggested that many dykes develop as self affine some as self similar structures when applied to the krafla volcanic system in northern iceland model calculations suggest that a magma flow from the krafla reservoir with regional dyke formation should occur on average once every several hundred years and that tens of sheets might be injected from the shallow krafla chamber and triggered by a single magma flow from the krafla reservoir these results are in broad agreement with the available data

rise and emplacement of magma during horizontal shortening of the brittle crust insights from experimental modeling 1 magmatic activity tends to concentrate at plate margins at divergent margins extensional tectonics provide steep conduits for magma to reach the surface at rapidly convergent margins such as the andes one might imagine that horizontal compression prevents the rise of magma nevertheless volcanoes are also common in order to study the mechanisms by which magma rises in a compressional context we resorted to laboratory experiments in which a brittle crust was shortened while magma was intruding our model materials were 1 cohesive fine grained silica powder representing brittle crust and 2 molten low viscosity vegetable oil representing magma in general horizontal shortening and injection were coeval but independent processes thrust faults accommodated the shortening while overpressured oil formed hydraulic fractures in those experiments where there was no shortening injection resulted in a saucer shaped intrusive body in the other experiments where there was shortening oil formed a basal sill before rising along thrust faults once in place the sill lubricated the base of the model so that arcuate thrusts formed at the leading edge of a plateau uplift of the plateau promoted further intrusion of oil at depth in general the pattern of deformation and intrusion depended on the kinematic ratio r between rates of shortening and injection the lengths of the basal sill and plateau increased with decreasing r on the basis of these results we have reexamined two natural examples of magmatic complexes which were emplaced in compressional tectonic settings tromen volcano in argentina and the boulder batholith of montana

radar scattering behavior of estuarine outflow plumes we present results of dual polarized radar scattering measurements of the chesapeake bay outflow plume near unity polarization ratios ratios of horizontally polarized radar echoes over vertically polarized ones are observed in large incidence angle 60 spl deg to 80 spl deg radar echoes from the outflow plume and its frontal boundary normally referred to as a front under strong surface current convergence 0 008 0 02 s sup 1 suggesting the existence of steepened and breaking waves in the regions cumulative distribution functions of the horizontally polarized radar returns from the front show approximately 90 of the radar echoes are from steepened and breaking waves vertically polarized echoes do not show this effect these experimental results substantiate early modeling investigators speculation of featured scattering contributing to horizontally polarized radar signatures of oceanic fronts our results also suggest that horizontal radar polarization can be used to remotely sense additional hydrodynamic processes such as wave trapping blocking and breaking near oceanic fronts better than what is possible with only vertical polarization

eruption of alkali basalts during crustal shortening in southern california the common occurrence of alkali basalts in continental rifts has led to the widespread assumption that late cenozoic alkali basalts of the western united states indicate an extensional tectonic regime however field relations of late cenozoic alkali basalt flows and cinder cones in the mojave desert area including flows that bear mantle xenoliths indicate that many were erupted in contractional or transpressional tectonic settings these observations call into question models of extensional magmatism in which lithospheric extension facilitates transport of magmas to the surface and suggest that an association of alkali basalts and coarse clastic sedimentary rocks need not indicate rifting

shear zones and magma ascent a model based on a review of the tertiary magmatism in the alps 1 the alpine oligocene plutons are spatially and temporally associated with the activity of the periadriatic fault system pfs an orogen parallel crustal scale transpressive mylonitic belt excellent three dimensional exposure combined with a wealth of structural seismic petrological geochronological geochemical and paleomagnetic data collected over the last decades help to constrain the relationships between deformation ascent and emplacement of the plutons magmas were channeled from the base of the thickened continental crust into the narrow mylonitic belt of the periadriatic fault system which was used as ascent pathway to cover vertical lengths of 20 to 40 km therefore the linear alignment of the plutons at the surface is not the expression of a linear source region at depth ascent of the melts is controlled by the mylonitic foliation of the pfs which forms the only steep anisotropy continuously traversing the entire alpine crust in contrast the flow direction is not influenced by the specific kinematics of the faults final emplacement of the plutons occurred by extrusion from the periadriatic fault system into the adjacent country rocks the transition from ascent to final emplacement is favored by partitioning of transpressive deformation

evolution and volcanic hazards of taapaca volcanic complex central andes of northern chile taapaca volcanic complex is a large dacitic volcano 35 km 3 located in the western border of the active zone of the central andes of northern chile apart from early poorly preserved silicic andesites taapaca volcanic complex has generated remarkably similar porphyritic hornblende biotite dacites with distinctive sanidine megacrysts for at least 1 5 ma the main products of the volcano are dacite lavas and domes with associated block and ash flow deposits there have also been several sector collapses to generate debris avalanches which are closely associated with volcanic blasts and episodes of dome growth four stages of evolution are recognized with volcanism occurring in short bursts between much longer periods of dormancy volcanism has built a substantial stratovolcano and has migrated 4 5 km towards the sw with time late pleistocene to holocene activity has involved at least three sector collapses of the hydrothermally altered flanks and domes volcanic blasts block and ash flows debris avalanches and lahars have been distributed down the southwestern flanks these areas are the main populated part of the chilean altiplano and the location of the main road between bolivia and the pacific ocean coast a future eruption will threaten these areas and the regional economy