active strike slip faulting in el salvador central america several major earthquakes have affected el salvador central america during the past 100 yr as a consequence of oblique subduction of the cocos plate under the caribbean plate which is partitioned between trench orthogonal compression and strike slip deformation parallel to the volcanic arc focal mechanisms and the distribution of the most destructive earthquakes together with geomorphologic evidence suggest that this transcurrent component of motion may be accommodated by a major strike slip fault el salvador fault zone we present field geological structural and geomorphological data collected in central el salvador that allow the constraint of the kinematics and the quaternary activity of this major seismogenic strike slip fault system data suggest that the el salvador fault zone consists of at least two main e w fault segments san vicente and berlin segments with associated secondary synthetic wnw ese and antithetic nnw sse riedel shears and nw se tensional structures the two main fault segments overlap in a dextral en echelon style with the formation of an intervening pull apart basin our original geological and geomorphologic data suggest a late pleistocene holocene slip rate of 11 mm yr along the berlin segment in contrast with low historical seismicity the kinematics and rates of deformation suggested by our new data are consistent with models involving slip partitioning during oblique subduction and support the notion that a trench parallel component of motion between the caribbean and cocos plates is concentrated along e w dextral strike slip faults parallel to the volcanic arc

kinematic links between subduction along the hellenic trench and extension in the gulf of corinth greece a multidisciplinary analysis abstract rapid northeast vergent subduction along the hellenic trench at 35 mm yr exists in concert with widespread extensional and strike slip faulting within the upper plate lithosphere of western greece integration of regional geomorphic geologic seismic gps remote sensing and field data demonstrates that young and active deformation in the area extending from the hellenic subduction boundary near kephalonia to the gulf of corinth consists of an interconnected network containing highly localized zones of deformation these bound a series of crustal fragments with relatively little internal deformation these deformation zones merge to form triple junction like features at the western end of the gulf of corinth and in the amvrakikos gulf at the western end of the corinth gulf most of its 14 2 mm yr of extension is relayed to the northwest along a prominent zone of left slip and extension through lake trichonis and the amphilochia fault zone 11 2 mm yr the remaining displacement across the western gulf of corinth is relayed into 7 2 mm yr of right slip on the southwest striking achaia fault zone which traverses the northwestern margin of the peloponnesus a second triple junction like feature occurs in the amvrakikos basin where the left slip amphilochia fault zone the right slip kephalonia transform fault 15 2 mm yr and the convergent thrust front of northern hellenides 4 2 mm yr are joined thus the extensional deformation in the gulf of corinth can be shown to be connected to convergence and subduction along the hellenic trench through a series of discrete deformation zones that are not dissimilar from those observed in the global plate tectonic system

a 100 kyr periodicity in the flux of extraterrestrial 3he to the sea floor most of the helium 3 in oceanic sediments comes from interplanetary dust particles idps and can therefore be used to infer the accretion rate of dust to the earth through time 3he records from slowly accumulating pelagic clays indicate that the accretion rate varies considerably over millions of years probably owing to cometary and asteroidal break up events muller and macdonald have proposed that periodic changes in this accretion rate due to a previously unrecognized 100 kyr periodicity in the earth s orbital inclination might account for the prominence of this frequency in climate records of the past million years here we report variations in the 3 he flux to the sea floor that support this idea we find that the flux recorded in rapidly accumulating quaternary sediments from the mid atlantic ridge oscillates with a period of about 100 kyr we cannot yet say however whether the 100 kyr climate cycle is a consequence of a cause of or an effect independent of these periodic changes

geological and seismological analysis of the 13 february 2001 mw 6 6 el salvador earthquake evidence for surface rupture and implications for seismic hazard the el salvador earthquake of 13 february 2001 mw 6 6 caused tectonic rupture on the el salvador fault zone esfz right lateral strike slip surface rupture of the east west trending fault zone had a maximum surface displacement of 0 60 m no vertical component was observed the earthquake resulted in widespread landslides in the epicentral area where bedrock is composed of volcanic sediments tephra and weak ignimbrites in the aftermath of the earthquake widespread damage to houses and roads and the hazards posed by landslides captured the attention of responding agencies and scientists and the presence of surface fault rupture was overlooked additionally the tectonic context in which the earthquake took place had not been clear until mapping of the esfz was completed for the present study we identified several fault segments the distribution of surface ruptures the aftershock pattern and fault rupture scaling considerations that indicate the 21 km long san vicente segment ruptured in the 2001 event static coulomb stress transfer models for the san vicente rupture are consistent with both aftershock activity of the 2001 sequence and ongoing background seismicity in the region at mw 6 6 the 2001 earthquake was of only moderate magnitude yet there was significant damage to the country s infrastructure including buildings and roads and numerous deaths and injuries thus earthquake hazard and risk in the vicinity of the esfz which straddles the city of san salvador with a population of 2 million is high because even moderate magnitude events can result in major damage deaths and injuries in the region

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

relations between deformation and upper crustal magma emplacement in laboratory physical models abstract this paper presents analogue models for the emplacement of granitic magmas in upper crustal levels with different mechanical layering during shortening extension and strike slip deformation in particular we investigated how a weak layer embedded in the upper brittle crust can control the level of magma emplacement the adopted experimental setup was used to examine the control of soft rocks on the movement of magma through a deforming brittle crust model results indicate that the occurrence of a weak soft layer embedded in brittle stiff material has an impact on the level of magma emplacement the level of emplacement during both extension and shortening was systematically deeper for models with a soft layer than for purely brittle models during strike slip deformation the magma pierced the surface in both purely brittle and brittle ductile models

the contribution by interplanetary dust to noble gases in the atmosphere of mars interplanetary dust particles accumulate significant concentrations of noble gases implanted by the solar wind and solar energetic particles while they are in space when these dust particles enter the atmosphere of a planet they are heated and decelerated depositing some of the implanted noble gases directly into the atmosphere and if the particle survives atmospheric entry emplacing the remainder on the surface of the planet where it can be released by particle decomposition or by episodic surface heating because of the low mass of the atmosphere of mars compared to the atmospheres of venus and earth the atmosphere of mars is more severely influenced by the addition of noble gases by interplanetary dust the contribution of noble gases to mars has been modeled using estimates of the interplanetary dust flux at mars and measurements of the noble gas contents of interplanetary dust collected from the atmosphere of the earth over the past 3 6 b y interplanetary dust particles have contributed quantities of 3he 4he 20ne and 22ne comparable to the current total atmospheric inventories of these isotopes in the present era the rates of addition of 3he 20ne 22ne 36ar and 38ar to the atmosphere of mars by interplanetary dust range from about 2 to 20 of the modeled rates of addition by planetary outgassing pepin 1994 krasnopolsky and gladstone 1996 and the isotopic compositions of the he and ne added by interplanetary dust are distinctly different from those assumed for planetary outgassing 3he 4he 2 8 10 4 in the interplanetary dust nier and schlutter 1992 versus almost pure 4he for outgassing of u and th decay products and 20ne 22ne 11 8 in the interplanetary dust niert 1994 versus 20ne 22ne 13 7 assumed for planetary outgassing in pepin s 1994 model since the actual outgassing rate of mars in the present era is not well constrained by observations the contributions of ne and ar to the atmosphere of mars by the interplanetary dust must serve at least as a lower limit on the current sources of these noble gases

neotectonic development of the el salvador fault zone and implications for deformation in the central america volcanic arc insights from 4 d analog modeling experiments the el salvador fault zone esfz is an active approximately 150 km long and 20 km wide segmented dextral strike slip fault zone within the central american volcanic arc striking n100 e although several studies have investigated the surface expression of the esfz little is known about its structure at depth and its kinematic evolution structural field data and mapping suggest a phase of extension at some stage during the evolution of the esfz this phase would explain dip slip movements on structures that are currently associated with the active dominantly strike slip and that do not fit with the current tectonic regime field observations suggest trenchward migration of the arc such an extension and trenchward migration of the volcanic arc could be related to slab rollback of the cocos plate beneath the chortis block during the miocene pliocene we carried out 4 d analog model experiments to test whether an early phase of extension is required to form the present day fault pattern in the esfz our experiments suggest that a two phase tectonic evolution best explains the esfz an early pure extensional phase linked to a segmented volcanic arc is necessary to form the main structures this extensional phase is followed by a strike slip dominated regime which results in intersegment areas with local transtension and segments with almost pure strike slip motion the results of our experiments combined with field data along the central american volcanic arc indicate that the slab rollback intensity beneath the chortis block is greater in nicaragua and decreases westward to guatemala

extension and rotation of crustal blocks in northern central america and effect on the volcanic arc sinistral displacements across the north american caribbean plate boundary in northern central america are distributed among major arcuate faults that have been active in neogene time south of the jocotan and motagua faults in guatemala extensional tectonics has accompanied rotation of the trailing edge of the caribbean plate around these faults segmentation of the volcanic arc in northern central america hitherto attributed to transverse breaks in the subducting cocos plate may instead be a result of this block rotation accompanying changes along the arc are observed in seismicity gravity anomaly patterns volume and composition of volcanic products and topography therefore the complex volcano tectonic geology south of the main boundary faults may be explained by interaction and rotation of crustal blocks in the overriding caribbean plate above the magma production zone along the downgoing cocos slab

cenozoic tectonics of the nicaraguan depression nicaragua and median trough el salvador based on seismic reflection profiling and remote sensing data lakes nicaragua and managua are the two largest lakes in central america and they cover a combined area of 9000 km 2 of the presently active nicaraguan depression and central america volcanic front as part of the subduction factory focus area of the u s national science margins program 1925 km of shallow geophysical data were acquired over lakes nicaragua and managua in may 2006 to establish their late quaternary structural and stratigraphic history and to better constrain regional models for active tectonics in western nicaragua the gulf of fonseca and the median trough in el salvador in order to investigate regional upper crustal deformation resulting from forearc sliver transport and or slab rollback of the cocos plate these new data were integrated with relocated earthquake epicenters earthquake focal mechanisms high resolution digital topography from the national aeronautics and space administration nasa shuttle radar topography mission srtm published global positioning system gps vectors onland geologic maps previous maps of lake bathymetry and bottom sediment types a previously unpublished regional aeromagnetic data set and multichannel seismic reflection profiles from the gulf of fonseca and pacific ocean these new data sets have improved bathymetric bottom sediment and recent fault maps for both nicaraguan lakes and can be used as new constraints on the regional geology and tectonics three regional structural cross sections across the nicaraguan depression indicate that the basin is a highly asymmetrical half graben bounded to the southwest by northeast dipping oblique slip normal faults late oligocene to holocene extension resulted in footwall uplift along the elevated and folded area of the nicaraguan isthmus and the area of greatest subsidence occurs on the footwall block underlying lake nicaragua in the southeast a similar but younger pattern of footwall uplift adjacent to the down thrown footwall block is present to the northeast beneath lake managua and the gulf of fonseca we interpret this structural pattern as a time transgressive rift opening where the oldest extension late oligocene early miocene began in the southeast and migrated to the northwest gps data indicate that this earlier phase of intra arc normal rifting is presently being superimposed by arc parallel right lateral shear related to the northwestward transport of the central america forearc sliver

fault kinematics in northern central america and coupling along the subduction interface of the cocos plate from gps data in chiapas mexico guatemala and el salvador new gps measurements in chiapas mexico guatemala and el salvador are used to constrain the fault kinematics in the north america na caribbean ca and cocos co plates triple junction area the regional gps velocity field is first analysed in terms of strain partitioning across the major volcano tectonic structures using elastic half space modelling then inverted through a block model we show the dominant role of the motagua fault with respect to the polochic fault in the accommodation of the present day deformation associated with the na and ca relative motion the na ca motion decreases from 18 22 mm yr 1 in eastern guatemala to 14 20 mm yr 1 in central guatemala assuming a uniform locking depth of 14 28 km down to a few millimetres per year in western guatemala as a consequence the western tip of the ca plate deforms internally with 9 mm yr 1 of east west extension 5 mm yr 1 across the guatemala city graben alone up to 15 mm yr 1 of dextral motion can be accommodated across the volcanic arc in el salvador and southeastern guatemala the arc seems to mark the northern boundary of an independent forearc sliver ar pinned to the na plate the inversion of the velocity field shows that a four block na ca co and ar model that combines relative block rotations with elastic deformation at the block boundaries can account for most of the gps observations and constrain the overall kinematics of the active structures this regional modelling also evidences lateral variations of coupling at the co subduction interface with a fairly high coupling 0 6 offshore chiapas and low coupling 0 25 offshore guatemala and el salvador

relations between surface faulting and granite intrusions in analogue models of strike slip deformation analogue models investigating the emplacement of granitic magmas at upper crustal levels during strike slip deformation are presented in this paper variations in parameters such as ratio between horizontal displacement dv and vertical magma upraising iv and the number of feeding points were considered during the experiments model results show that a strong interaction between surface faulting and granite intrusions develops during deformation on one side magma intrusion influences the sequence of structures development and the final fault pattern on the other side magma emplacement is strongly influenced by the syn intrusion lateral displacement dt which in turn reflects variations in dv and iv for high dt values i e high displacement velocity or low injection rate the importance of the horizontal displacement with respect to the vertical magma upraising results in asymmetric and drop shaped plutons with a sheared tail elongating in the sense of the strike slip displacement in this case the intrusion long axis tracks the long axis of the strain ellipsoid for low dt values i e low displacement velocity or high injection rate the strike slip displacement is strongly subordinated to the vertical magma upraising and pluton emplacement is controlled by development of tensional shears at the surface in these conditions the long axis of the magmatic bodies lies in the quadrants of compression of the strike slip deformation and intrusions are apparently dragged in an opposite sense to that imposed by the transcurrent deformation

a comparative study of accumulation rates derived by he and th isotope analysis of marine sediments abstract we present a detailed down core analysis of helium isotope ratios and concentrations for bulk sediments from the central equatorial pacific that span the last two glacial interglacial cycles measured 3 he 4 he ratios range from 1 0 10 5 to 2 1 10 4 or 7 4 to 149 times the atmospheric ratio the 3he from interplanetary dust particles idps constitutes virtually all of the 3he measured within the sediment because carbonate accumulation rates are high in the equatorial pacific the measured 3he concentrations are lower than have been measured elsewhere and range from 4 7 10 13 to 3 0 10 12 cm3stp g 1 if the cosmic dust 3he flux is constant with time sediment mass accumulation rates can be determined from the 3he concentration in sediments the excess 230th technique is an entirely independent method for calculating sediment mass accumulation rates because its source is in situ decay of 234u in seawater to first order initial excess 230th activities correlate with 3he concentrations within this core based on the 230th results we estimate the 3he flux to the earth s surface as 9 6 2 0 10 16 cm3stp cm 2 a 1 if this flux has remained constant over extended periods of time it can be used to determine sediment accumulation rates beyond the 230th range 300 000 yr

control of folding and faulting on fracturing in the zagros iran the kuh e sarbalesh anticline abstract the kuh e sarbalesh anticline fars region shows a strong and poorly studied curvature in the vicinity of the kazerun transfer zone here we investigated this structure by analyzing the orientation and distribution of fractures in the oligocene early miocene asmari formation a major reservoir rock of the zagros petroleum system and paleo burial conditions far away from the kazerun fault the orientation of fractures is similar to that observed in other folds in the zagros two tensional fracture sets f1 and f2 develop respectively parallel and perpendicular to the fold axis whereas two shear fracture sets dextral f3 and sinistral f4 develop at 30 from f2 closer to the kazerun fault f1 and f2 become less abundant whereas f3 fractures do prevail f3 dextral shear fractures are associated with dextral shear zones sz3 within sz3 sets of tensional fractures f5 occur f4 shear planes are less abundant than f3 sets and are sometimes associated with sinistral shear zones sz4 within sz4 sets of tensional fractures f6 occur the frequency of f3 and sz3 structures increases from south to north suggesting a control by dextral shear along the kazerun fault average fracture spacing is around 20 cm and fracturing saturation values is larger than 0 7 at 20 km distance from the kazerun fault whilst spacing is smaller than 1 cm and saturation is as low as 0 32 typical of well developed fracture systems close to the fault highlighting a strong strain gradient from south to north the orientation of f1 and f2 fracture sets changes from south to north according to the clockwise rotation of the northern part of the fold with respect to the southern part inorganic thermal parameters derived from x ray diffraction analysis of clay minerals indicate that the asmari formation was deformed at burial depths of 3 km assuming a geothermal gradient of 20 c km we suggest that the progressive differential advancement of the thrust front produced the curved shape of the kuh e sarbalesh anticline with associated rotation of the pre early folding f1 and f2 fracture sets observed in the field consistently with a dextral shear associated with the kazerun transfer fault

4d analogue modelling of transtensional pull apart basins abstract scaled sandbox models were used to investigate the 4d evolution of pull apart basins formed above underlapping releasing stepovers in both pure strike slip and transtensional basement fault systems serial sectioning and 3d volume reconstruction permitted analysis of the full 3d fault geometries results show that very different pull apart basins are developed in transtension compared to pure strike slip both types of models produced elongate sigmoidal to rhomboidal pull apart systems but the transtensional pull apart basins were significantly wider and uniquely developed a basin margin of en echelon oblique extensional faults dual opposing depocentres formed in the transtensional model whereas a single central depocentre formed in pure strike slip in transtension a distinct narrow graben system formed above the principal displacement zones pdzs cross basin fault systems that linked the offset pdzs formed earlier in the transtensional models sequential model runs to higher pdz displacements allowed the progressive evolution of the fault systems to be evaluated in cross section transtensional pull aparts initiated as asymmetric grabens bounded by planar oblique extensional faults with increasing displacement on the pdzs basin subsidence caused these faults to become concave upwards and lower in dip angle due to fault block collapse towards the interior of the basin in addition strain partitioning caused fault slip to become either predominantly extensional or strike slip the models compare closely with the geometries of natural pull apart basins including the southern dead sea fault system and the vienna basin austria

subglacial bedforms reveal an exponential size frequency distribution subglacial bedforms preserved in deglaciated landscapes record characteristics of past ice sediment flow regimes providing insight into subglacial processes and ice sheet dynamics individual forms vary considerably but they can often be grouped into coherent fields typically called flow sets that reflect discrete episodes of ice flow within these bedform size frequency distributions predominantly height width and length are currently described by several statistics e g mean median and standard deviation that arguably do not best capture the defining characteristics of these populations this paper seeks to create a better description based upon semi log plots which reveal that the frequency distributions of bedform dimensions drumlin mega scale glacial lineation and ribbed moraine plot as straight lines above the mode this indicates by definition an exponential distribution for which a simple and easily calculated yet statistically rigorous description is designed three descriptive parameters are proposed gradient the exponent characterising bedforms likely least affected by non glacial factors area normalised y intercept 0 quantifying spatial density and the mode below small features are less prevalent due to i measurement data sampling and mapping fidelity ii possible post glacial degradation or iii genesis not being created sub glacially this new description has the benefit of being insensitive to the impact of potentially unmapped or degraded smaller features and better captures properties relating to ice flow importantly using flow sets can now be more usefully compared with each other across all deglaciated regions and with the output of numerical ice sheet models applications may also exist for analogous fluvial and aeolian bedforms identifying the characteristic exponential and that it is typical of emergent subglacial bedforms is a new and potentially powerful constraint on their genesis perhaps indicating that ice sediment interaction is fundamentally stochastic in nature

cenozoic variations in the flux of interplanetary dust recorded by 3he in a deep sea sediment helium 3 concentrations and 3he 4he ratios in modern pelagic sediments are known to be far in excess of terrestrial values as a result of micrometeorite fallout here i report that extraterrestrial helium is easily detected in a pelagic clay core dating back more than 70 myr the remarkable preservation of the extraterrestrial signature arises from high retention of 3he within interplanetary dust particles coupled with loss of radiogenic 4he from terrestrial mineral grains the core provides a continuous record of the fallout of extraterrestrial helium for the cenozoic era this record suggests that there have been significant variations in the influx of interplanetary dust through time probably related to asteroidal breakup events or the passage of comets through the inner solar system the results also show 3he to be a far more sensitive tracer of the interplanetary dust flux than is iridium

neotectonic development of western nicaragua this study presents a reconstruction of the neotectonic evolution of western nicaragua by means of structural analysis of outcrop scale joints faults and bedding planes integrated with macroscale structures the results indicate three clearly separable deformation phases which are closely related to specific volcanic events and account for the major tectonic features of nicaragua the first deformation phase was a late miocene to early pliocene shortening that produced the pacific plain anticlines and controlled the contemporaneous extrusion of the el coyol group volcanic rocks the second phase active between the pliocene and the pleistocene resulted in extensional structures such as the 500 km long and 50 km wide nw se trending nicaragua depression this phase was accompanied by a gradual trenchward migration of the volcanic activity responsible for the formation of the las sierras group the third deformation phase is still active and is responsible for the present shallow focus seismic activity associated with n s trending volcanic vents and the n s managua pull apart graben the neotectonic development of western nicaragua as interpreted here is closely related to the contemporaneous evolution of the neighboring areas of guatemala south of the jocotan fault honduras el salvador and costa rica north of the valle central the valle central of costa rica represents the southern boundary of this structurally homogeneous domain and may indicate the southern boundary of the neogene chortis block changes in deformation style and the trenchward migration of volcanism which occurred during the pliocene may indicate an increase in the subduction angle of the cocos plate and may also be correlated to pliocene shortening of jamaica and hispaniola

size distribution of submarine landslides along the u s atlantic margin abstract assessment of the probability for destructive landslide generated tsunamis depends on the knowledge of the number size and frequency of large submarine landslides this paper investigates the size distribution of submarine landslides along the u s atlantic continental slope and rise using the size of the landslide source regions landslide failure scars landslide scars along the margin identified in a detailed bathymetric digital elevation model dem have areas that range between 0 89 km2 and 2410 km2 and volumes between 0 002 km3 and 179 km3 the area to volume relationship of these failure scars is almost linear inverse power law exponent close to 1 suggesting a fairly uniform failure thickness of a few 10s of meters in each event with only rare deep excavating landslides the cumulative volume distribution of the failure scars is very well described by a log normal distribution rather than by an inverse power law the most commonly used distribution for both subaerial and submarine landslides a log normal distribution centered on a volume of 0 86 km3 may indicate that landslides preferentially mobilize a moderate amount of material on the order of 1 km3 rather than large landslides or very small ones alternatively the log normal distribution may reflect an inverse power law distribution modified by a size dependent probability of observing landslide scars in the bathymetry data if the latter is the case an inverse power law distribution with an exponent of 1 3 0 3 modified by a size dependent conditional probability of identifying more failure scars with increasing landslide size fits the observed size distribution this exponent value is similar to the predicted exponent of 1 2 0 3 for subaerial landslides in unconsolidated material both the log normal and modified inverse power law distributions of the observed failure scar volumes suggest that large landslides which have the greatest potential to generate damaging tsunamis occur infrequently along the margin

successive orthogonal and oblique extension episodes in a rift zone laboratory experiments with application to the ethiopian rift small scale modeling was performed to examine the effects of the superposition of two successive extensional phases from orthogonal to oblique type 1 and from oblique to orthogonal type 2 in both the type 1 and type 2 models faults produced during the first stage strongly control fault development during the second stage in type 1 models the oblique faults developed during the second oblique phase are confined within a first phase graben whereas in type 2 models the oblique faults produced during the first phase continue to develop during orthogonal extension and connect with each other to give sigmoidal fault blocks type 1 models are compared with the structural setting of the ethiopian rift the evolution of the rift is related to a recent extensional event whose principal direction of stretching trends at around 50 to preexisting major normal faults type 1 laboratory models are fairly comparable to the northern sector of the ethiopian rift referred to here as mer they account for both the development of the en echelon oblique faults of the wonji fault belt and the sinistral shear gradient running parallel to the eastern border of the mer which formed during an oblique rifting extension the statistical analysis of the whole ethiopian rift fault pattern by reference to the experimental data allows the determination of a n100 n110 mean direction of stretching

the potential of crinoids as u th sm he thermochronometers abstract u th sm he dating of crinoid fossils from late paleozoic rocks from a variety of sites in north america suggest that this material can be used as a useful low t thermochronometer under certain conditions available data suggest that common he is present in crinoids and at the low production rates typical of calcite this common he will be sufficient to produce spurious ages in crinoid samples of any age heating of samples to temperatures in excess of the opening temperature will expel all he common and radiogenic with all subsequent he accumulation being exclusively radiogenic such samples have potential for being robust thermochronometers sensitive to temperatures in the range of 60 110 c without common he analysts can take advantage of the large size of crinoid columnals and produce reliable ages even when young miocene or with low production rates eu 0 1 ppm samples not subjected to post depositional heating above the opening temperature burial to depths of 2 to 4 km depending on geothermal gradient sedimentation rate and residence time at peak t will likely retain common he and be unsuitable for thermochronologic studies

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

an assessment of the meteoritic contribution to the martian soil the addition of meteoritic material to the mars soils should perturb their chemical compositions as has been detected for soils on the moon anders et al 1973 and sediments on earth kyte and wasson 1986 using the measured mass influx at earth and estimates of the mars earth flux ratio we estimate the continuous planet wide meteoritic mass influx on mars to be between 2700 and 59 000 t yr if distributed uniformly into a soil with a mean planetary production rate of 1 m b y consistent with radar estimates of the soil depth overlaying a bouldered terrain in the tharsis region christensen 1986 our estimated mass influx would produce a meteoritic concentration in the mars soil ranging from 2 to 29 by mass analysis of the viking x ray fluorescence data indicates that the mars soil composition is inconsistent with typical basaltic rock fragments but can be fit by a mixture of 60 basaltic rock fragments and 40 meteoritic material clark and baird 1979 the meteoritic influx we calculate is sufficient to provide most or all of the material required by the clark and baird 1979 model particles in the mass range from 10 7 to 10 3 g about 60 200 m in diameter contribute 80 of the total mass flux of meteoritic material in the 10 13 to 106 g mass range at earth hughes 1978 on earth atmospheric entry all but the smallest particles generally 50 m in diameter in the 10 7 to 10 3 g mass range are heated sufficiently to melt or vaporize mars because of its lower escape velocity and larger atmospheric scale height is a much more favorable site for unmelted survival of micrometeorites on atmospheric deceleration we calculate that a significant fraction of particles throughout the 60 1200 m diameter range will survive mars atmospheric entry unmelted thus returned mars soils may offer a resource for sampling micrometeorites in a size range which is not collectable in unaltered form at earth

fluid circulation in the upper brittle crust thickness distribution hydraulic transmissivity fluid inclusion and isotopic data of veins hosted in the oligocene sandstones of the macigno formation in southern tuscany italy abstract we present structural analysis fluid inclusion data on calcite and quartz and isotopic composition of calcite forming veins occurring in the upper crustal level and hosted in oligocene sandstone in southern tuscany italy the veins have been analysed in two sites few kilometres apart along well exposed coastal cliffs and in an abandoned quarry these two sites were at a different depths at the time of the vein formation with a h 100 m structural analysis of veins provided estimations of stress ratio 2 3 1 3 driving stress ratio r pf 3 1 3 and fluid overpressure po pf 3 at the time of vein formation the estimated po is in the range of 42 103 mpa 0 24 and r 0 45 indicating that fluid pressure was higher than the intermediate principal stress at the time of veins formation the veins thickness t shows a clear power law distribution d 1 8835 and r2 0 9762 in the lowermost site coast and a negative exponential distribution a 0 6943 and r2 0 9921 in the uppermost site abandoned quarry the vein thickness distributions have been used to compute the average transmissivity of the veins in the two sites the computed transmissivity for the vein formation is 10 4 m2 s 1 with higher values attained by the veins having negative exponential thickness distribution fluid inclusions studies highlighted that in both calcite and quartz water rich inclusions with salinities of 2 2 4 3 wt nacl equiv and methane rich inclusions were coevally trapped during fluid un mixing processes thermogenic origin from thermal maturation of organic matter present in the macigno formation is proposed for methane whereas the similarity between the 18o from 14 9 to 17 4 and 13c from 0 4 to 2 4 data of representative calcite veins and the isotopic composition 18o 16 1 13c 1 0 of host rock carbonate component indicates that the fluid which formed calcite was in isotopic equilibrium with the carbonates present in the oligocene sandstones the calculated pressure temperature conditions during the formation of these inclusions are prevalently within the 40 145 mpa and 160 260 c ranges the highest pressure values approximate the lithostatic pressure 120 mpa computed from geological data and are coherent with a geothermal gradient ranges of 35 45 c km whereas the lower pressure values are comparable with hydrostatic pressure conditions the pressure range indicated by fluid inclusion data is also comparable with the fluid pressure estimated from structural analysis the considerable pressure range can be related to pressure cycling between lithostatic and hydrostatic conditions as a consequence of fault valve actions and rock fracturing with subsequent pressure recover due to self sealing process

analogue modelling of multiphase rift systems abstract analogue models of multiphase rift systems show that structures produced by the first phase of extension exertsignificant control on the geometries of the later phase extensional faults two experimental series series 1 and series 2 used different modelling media subjected to two phases of extension series 1 models used a simple homogeneous sandpack whereas series 2 models used a sand polymer multilayer in each series kinematic scenarios included orthogonal extension followed by oblique extension or vice versa extension directions for oblique rift kinematics varied from 75 to 30 to the axis of rifting in all models where oblique rifting superseded earlier orthogonal rift structures the second deformation phase produced segmented fault systems and reorientation of fault tips into high angles with respect to the extension vector higher angles of obliquity resulted in increased fault segmentation orthogonal extension superposed on earlier oblique rifting leads to coalescence of segmented faults although remnants of the inherited initial rift structure remained even for low angles of obliquity the sand polymer multilayer models produced more complex fault patterns than those in the simple homogeneous sandpack showing the possible complexities that can be generated during polyphase rifting persistent first phase rift faults caused boundary fault segmentation and salients and embayments along the boundary faults during the second deformation phase the natural occurrence of similar structural styles particularly in map view may indicate that previously unrecognised multiphase rifting has occurred in these natural examples

mechanical behaviour of granular materials used in analogue modelling insights from grain characterisation ring shear tests and analogue experiments abstract the mechanical behaviour of several dry granular materials is investigated through ring shear tests grain characterisation and simple analogue experiments analysed by x ray computed tomography an improved knowledge of granular materials is essential to determine their suitability as analogues for upper crustal rocks in experimental models and to compare analogue and numerical experiments the ring shear tests show that the granular materials have an elastic frictional plastic behaviour with strain hardening preceding failure at peak strength followed by strain softening until a dynamic stable value is reached this is similar to the behaviour exhibited by experimentally deformed rocks the physical characteristics of the grains determine the amount of diffuse deformation before failure the percentage of strain softening and act on the thickness of the shear zones before broadening initial shear zone width in extensional and contractional experiments is between 11 and 16 times the mean grain size the angle of internal friction defining one of the mechanical properties of granular materials and thus fault dip is not only related to physical characteristics of the grains and to the handling technique used e g sieving or pouring but also to the overburden and to the experimental setup used

an inverse cascade explanation for the power law frequency area statistics of earthquakes landslides and wildfires frequency magnitude statistics for natural hazards can greatly help in probabilistic hazard assessments an example is the case of earthquakes where the generality of a power law fractal frequency rupture area correlation is a major feature in seismic risk mapping other examples of this power law frequency size behaviour are landslides and wildfires in previous studies authors have made the potential association of the hazard statistics with a simple cellular automata model that also has robust power law statistics earthquakes with slider block models landslides with sandpile models and wildfires with forest fire models a potential expla nation for the robust power law behaviour of both the models and natural hazards can be made in terms of an inverse cascade of metastable regions a metastable region is the region over which an avalanche spreads once triggered clusters grow primarily by coalescence growth dominates over losses except for the very largest clusters the cascade of cluster growth is self similar and the frequency of cluster areas exhibits power law scaling we show how the power law exponent of the frequency area distribution of clusters is related to the fractal dimension of cluster shapes

the caribbean north america cocos triple junction and the dynamics of the polochic motagua fault systems pull up and zipper models the polochic motagua fault systems pmfs are part of the sinistral transform boundary between the north american and caribbean plates to the west these systems interact with the subduction zone of the cocos plate forming a subduction subduction transform triple junction the north american plate moves westward relative to the caribbean plate this movement does not affect the geometry of the subducted cocos plate which implies that deformation is accommodated entirely in the two overriding plates structural data fault kinematic analysis and geomorphic observations provide new elements that help to understand the late cenozoic evolution of this triple junction in the miocene extension and shortening occurred south and north of the motagua fault respectively this strain regime migrated northward to the polochic fault after the late miocene this shift is interpreted as a pull up of north american blocks into the caribbean realm to the west the pmfs interact with a trench parallel fault zone that links the tonala fault to the jalpatagua fault these faults bound a fore arc sliver that is shared by the two overriding plates we propose that the dextral jalpatagua fault merges with the sinistral pmfs leaving behind a suturing structure the tonala fault this tectonic zipper allows the migration of the triple junction as a result the fore arc sliver comes into contact with the north american plate and helps to maintain a linear subduction zone along the trailing edge of the caribbean plate all these processes currently make the triple junction increasingly diffuse as it propagates eastward and inland within both overriding plates

constraints from finite element modeling on the active tectonics of northern central america and the middle america trench 1 we have developed an elastic finite element model in order to study the role of the different forces acting on the northwestern part of the central american volcanic arc and the chortis block we present synthetic focal mechanisms maps of tectonic regime and strain crosses to analyze the results the models show that to achieve the observed state of stress on the volcanic arc the arc must be modeled as a lithospheric weak zone also the forces related to the eastward drift of the caribbean plate must be higher than those related to the subduction of the cocos plate the coupling on the subduction interface must be low with or without slip partitioning due to the obliquity of the subduction at the trench at guatemala the western edge of the chortis block is pinned against north america even with low trench normal forces making the triple junction between the cocos north american and caribbean plates a zone of diffuse deformation the extension in the western part of the chortis block from guatemala to the honduras depression is explained by the geometry of the north american caribbean plate boundary and the direction of motion of the caribbean plate with respect to north america the direction of extension in the chortis block is always e w regardless of the magnitude of the applied forces and the main part of the deformation is absorbed between the ipala graben and the honduras depression both features being consistent with our models

interaction of ascending magma with pre existing crustal fractures in monogenetic basaltic volcanism an experimental approach 1 magma transport through dikes is a major component of the development of monogenetic volcanic fields these volcanic fields are characterized by numerous volcanic centers each typically resulting from a single eruption therefore magma must be transported from source to surface at different places which raises the question of the relative importance of 1 the self propagation of magma through pristine rock and 2 the control exerted by pre existing fractures to address this issue we have carried out a series of analogue experiments to constrain the interaction of a propagating dike through a medium with pre existing fractures the experiments involved the injection of air into an elastic gelatin solid which was previously cut into its upper part to simulate pre existing fractures the volume of the dikes their distance from the fractures and the ambient stress field were systematically varied to assess their influence on potential dike fracture interactions the results show that distance and angle between dikes and fractures influence these interactions and the dike trajectory dike geometry and dynamics are also affected by both the presence of the fractures and the dike volume dikes propagating in between fractures tend to decelerate in nature interactions are expected for dikes and fractures separated by less than about 200m and dikes with a volume less than about 10 2 km 3 would experience a velocity decrease these results highlight the influence of pre existing fractures on the mechanics and dynamics of dikes these heterogeneities must be considered when studying the transport of magmas within the crust

extension induced by volcanic loading in regional strike slip zones volcanoes and strike slip pull apart structures are commonly found together we use theoretical predictions and analogue modeling to evaluate the effect of volcanic loading in strike slip zones and the converse effect of regional strike slip faults on volcano structure theoretical predictions indicate that regional stress is reoriented by volcano loading to cause extension analogue models indicate that volcanoes in strike slip zones develop extensional pull apart structures a graben typically forms normal to the minimum regional stress axis 3 at a cone cones emplaced onto preexisting faults increase that fault9s tensional component if the substratum is weak enough to permit volcano spreading thrusts form normal to the maximum stress axis 1 a feedback situation can arise i e loading related extension allows more magma influx more eruption and thus increased loading this process can create large volcano tectonic depressions volcano structures may also be used as indicators of regional strike slip movements

constraints for the recent tectonics of the el salvador fault zone central america volcanic arc from morphotectonic analysis abstract we have used hypsometric analysis to improve our understanding of the current tectonic deformation and structure of el salvador fault zone a n90 e oriented strike slip fault zone that extends 150 km through el salvador central america our results indicate an important amount of transtensive strain along this fault zone providing new data to understand the tectonic evolution of the salvadorian volcanic arc we have defined kilometric scale tectonic blocks and its relative vertical movements length of segments with homogeneous vertical motions and lateral relay of active structures we have identified and quantified slip rate variations along strike of the el triunfo fault within el salvador fault zone ranging from 4 6 mm year in its central parts to 1 mm year towards the tips of the fault this study supports the hypothesis of a recent rotation in the maximum shortening direction and the accommodation of the current deformation through the reactivation of pre existing structures inherited from a previous tectonic regime

intermediate depth earthquakes linked to localized heating in dunite and harzburgite earthquakes can occur at great depths in the earth within subducting tectonic plates deformation experiments suggest these deep earthquakes can be triggered by localized heating of the slabs under high pressures

analogue models of caldera collapse in strike slip tectonic regimes regional scale faulting particularly in strike slip tectonic regimes is a relatively poorly constrained factor in the formation of caldera volcanoes to examine interactions between structures associated with regional tectonic strike slip deformation and volcano tectonic caldera subsidence we made scaled analogue models tabular sill like inclusions of creamed honey in a sand gypsum mix replicated shallow level granitic magma chambers in the brittle upper crust lateral motion of a base plate sited below half the sand gypsum pack allowed simulation of regional strike slip deformation our experiments modelled 1 strike slip deformation of a homogeneous brittle medium 2 strike slip deformation of a brittle medium containing a passive magma reservoir 3 caldera collapse into sill like magma reservoirs without regional strike slip deformation and 4 caldera collapse into sill like magma reservoirs after regional strike slip deformation our results show that whilst the magma chamber shape principally influences the development and geometry of volcano tectonic collapse structures regional tectonic strike slip faults riedel shears and y shears may affect a caldera s structural evolution in two main ways firstly regional strike slip faults above the magma chamber may form a pre collapse structural grain that is exploited and reactivated during subsidence our experiments show that such faults may preferentially reactivate where tangential to the collapse area and coincident with the chamber margins in this case volcano tectonic extension in the caldera periphery tends to localise on regional tectonic faults that lie just outside the chamber margins in addition volcano tectonic reverse faults may link with and reactivate pre collapse regional tectonic faults that lie just inside the chamber margins secondly where regional tectonic strike slip faults define corners in the magma chamber margin they may halt the propagation of volcano tectonic reverse faults the experiments also highlight the potential difficulties in assessing the relative contributions of volcano tectonic and regional tectonic subsidence processes to the final caldera structure seen in the field disruption of the pre collapse surface by regional tectonic faulting was preserved during coherent volcano tectonic subsidence to produce a caldera floor of differentially subsided fault blocks without definitive evidence for syn eruptive growth faulting thickness changes in caldera fill across such regional tectonic fault blocks in nature could be mistaken as evidence for piecemeal volcano tectonic collapse

accretion rate of extraterrestrial matter iridium deposited 33 to 67 million years ago iridium measured in 149 samples of a continuous 9 meter section of pacific abyssal clay covering the time span 33 to 67 million years ago shows a well defined peak only at the cretaceous tertiary boundary in the rest of the section iridium ranges from a minimum concentration near 0 35 nanograms per gram in the paleocene to a maximum near 1 7 in the eocene between 63 and 33 million years ago the mean iridium accumulation rate is approximately 13 nanograms per square centimeter per million years correction for terrestrial iridium leads to an extraterrestrial flux of9 3 nanograms of iridium per square centimeter per million years and an estimated annual global influx of 78 billion grams of chondritic matter consistent with recent estimates of the influx of dust meteorites and crater producing bodies with masses ranging from 10 13 to 1018 grams combining the recent flux of objects ranging in mass from 106 to 107 grams with the flux of 1014 to 1015 gram objects indicates that the number of objects is equal to 0 54 divided by the radius in kilometers to the 2 1 power periodic comet showers should increase the cometary iridium flux by a factor of 200 to 600 on a time scale of 1 to 3 million years the predicted iridium maxima more than 30 times background are not present in the intervals associated with the cretaceous tertiary boundary or the tektiteproducing late eocene events

fractal analysis for natural hazards in the earth sciences the concept of fractals and scale invariance is well recognized in many natural objects however the use of fractals for spatial and temporal analyses of natural hazards has been less used and accepted in the earth sciences this book brings together 12 contributions that emphasize the role of fractal analyses in natural hazard research including andslides wildfires floods catastrophic rock fractures and earthquakes a wide variety of spatial and temporal fractal related approaches and techniques are applied to natural data experimental data and computer simulations these approaches include probabilistic hazard analysis cellular automata models spatial analyses temporal variability prediction and self organizing behaviour the main aims of this volume are a to present current research on fractal analyses as applied to natural hazards and b to stimulate the curiosity of advanced earth science students and researchers in the use of fractals analyses for the better understanding of natural hazards