late miocene early pliocene reactivation of the main boundary thrust evidence from the seismites in southeastern kumaun himalaya india abstract tectonic history of the himalaya is punctuated by successive development of the faults that run along the boundaries between different lithotectonic terrains the main boundary fault defining the southern limit of the lesser himalayan terrain is tectonically most active a review of published literature reveals that the nature and age of reactivation events on the main boundary fault is one of the poorly understood aspects of the himalayan orogen by systematic outcrop mapping of the seismites this study identifies a late miocene early pliocene reactivation on the main boundary thrust in southeast kumaun himalaya relatively friable and cohesionless neogene sedimentary sequences host abundant soft sediment deformation structures in the vicinity of the main boundary thrust among a large variety of structures deformed cross beds liquefaction pockets slump folds convolute laminations sand dykes mushroom structures fluid escape structures flame and load structures and synsedimentary faults are common the morphological attributes the structural association and the distribution pattern of the soft sediment deformation structures with respect to the main boundary fault strongly suggest their development by seismically triggered liquefaction and fluidization available magnetostratigraphic age data imply that the seismites were developed during a late miocene early pliocene slip on the main boundary thrust the hypocenter of the main seismic event may lie on the main boundary thrust or to the north of the study area on an unknown fault or the basal detachment thrust

changes in the hydrothermal system at loihi seamount after the formation of pele s pit in 1996 sulfide and sulfate minerals recovered from loihi seamount during pisces v submersible dives in october 1996 are the first occurrence of high temperature hydrothermal mineralization documented for ocean island volcanoes millimeter to centimeter sized particles recovered in talus deposits on the north rim of the newly formed pele9s pit consist of aggregates of pyrite marcasite pyrrhotite wurtzite and barite the barite occurs as single clear euhedral crystals or as acicular radial crystal clusters on sulfide or rock fragments the mineral assemblage and the composition of the sulfide phases are similar to those of hydrothermal sulfide from mid ocean spreading centers the sulfide assemblage confirms the presence of high temperature fluids 250 c in the loihi hydrothermal system the sulfide samples are not pieces of broken chimney or basal mound structures or stockwork from lower levels in the hydrothermal system the euhedral crystals and homogeneous compositions suggest rapid precipitation from a high temperature plume injected into cold seawater along fractures and fissures that define the north wall of the new pit crater formed during the seismic swarm in july and august 1996

metamorphism melting and extension age constraints from the high himalayan slab of southeast zanskar and northwest lahaul abstract we present evidence for two distinct stages of tertiary metamorphism m1 and m2 in the high himalayan slab of southeast zanskar and northwest lahaul as well as evidence for an older pre himalayan metamorphism pre m1 the m1 was a regional barrovian type event related to crustal shortening and thickening of the indian plate margin while m2 was associated with crustal melting and the emplacement of the gumburanjon leucogranite into the zanskar shear zone at the top of the slab u pb dating of metamorphic and magmatic accessory phases constrains the timing of m1 between 30 and 37 ma and the crystallization and emplacement age of the gumburanjon leucogranite at 21 22 ma inherited accessory phases in metamorphic and magmatic samples suggest that the protoliths of the slab are at least lower paleozoic in age and that they experienced a major pre m1 thermal perturbation at ca 450 500 ma whether this was associated with a regional barrovian type metamorphism or whether it was a thermal event rel

sedimentary structures tentatively attributed to seismic events abstract fingerprints of earthquakes in the sedimentary record are as yet difficult to identify this is particularly true for failure phenomena dewatering slumping fault graded and turbidite structures that could have been triggered also by other kinds of energy input other promising candidates are strange biostratinomic features convex down stacking of shells current orientation in wave ripple accumulations as well as certain preservational features found in geosynclinal pelagic limestones but in every case verification of the seismic origin is still needed

boiling sand springs dismal river nebraska agents for formation of vertical cylindrical structures and geomorphic change boiling motion not temperature sand springs fed by ground water moving upward along distinct conduits occur along the dismal river in the nebraska sand hills the cylindrical conduits are developed in alluvium and are as large as 10 m in diameter and 44 m deep sides of the spring conduits are firm boiling sand is overlain in several places by a clear layer of water in the spring time varying wave patterns and boils can be observed along the interface between these layers sediment within the conduits is generally well sorted although some debris is present because of their large size firm sides and persistence in time the conduits have the potential for preservation as vertical cylindrical structures comparable to those reported in sedimentary rocks of several ages the discovery of preserved structures is evidence of paleo ground water discharge and therefore the structures are a useful mapping tool for determining position in landscape evolution

brunt vaisala frequency and rossby radii in the south atlantic abstract brunt vaisala frequency profiles have been calculated in the south atlantic and neighboring regions on a 5 5 grid between 30 n and 70 s 70 w and 70 e using the annual mean temperature and salinity profiles computed by levitus from data available from the national oceanographic data center nodc these brunt vaisala frequency profiles are used in the computation of the first and second internal rossby deformation radii the external rossby radius is computed for reference these results compare very well with those of emery et at for the equatorial atlantic very small values of the radii are evidenced in the extreme south less than 10 km in the antarctic circumpolar current for the first internal mode high vertical variability in brunt vaisala frequency profiles is also observed at the center of the subtropical gyre this behavior is probably due to the lack of deep data in this region

earthquake induced soft sediment deformation in the lower shyok river valley northern ladakh india abstract soft sediment deformation structures occur in the 150 m thick pliocene quaternary fluvio lacustrine sediments exposed around the khalsar and tirit areas in the lower shyok river valley northern ladakh and eastern karakoram india these structures vary in morphology and pattern and occur at different stratigraphic horizons they satisfy the criteria for attributing them to seismic events and it is proposed that these structures were formed as a result of earthquake induced liquefaction the deformation structures are thus interpreted to represent pre historic seismic events which occurred up to 5 ma ago and were associated with tectonic activity along the karakoram fault

modelling seismites with a digital shaking table abstract soft sediment deformation structures induced by seismically triggered liquefaction and or fluidization seismites have been reproduced with a digital shaking table we have used natural sediments with variable granulometric ranges clay silt and sand and an earthquake simulator that is able to reproduce real tridimensional seismograms acceleration and pore pressure recording allowed us to follow liquefaction and or fluidization processes in unconsolidated sediments during earthquakes different types of seismites were produced by variations in the tested sedimentary column

middle late miocene 10 ma formation of the main boundary thrust in the western himalaya three independent data sets from northwestern india and pakistan suggest initial displacement along 1000 km of the main boundary thrust prior to 10 ma at least 5 m y earlier than previously reported regionally extensive changes in the depositional characteristics and rates of the foreland basinfill between 11 and 9 5 ma are interpreted to reflect new hinterland loading due to the formation of the main boundary thrust sediment accumulation rates sandstone siltstone ratios and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 ma in well dated stratigraphic sections from pakistan to nepal across the indo gangetic foreland basin in the himachal pradesh reentrant of northwestern india a newly discovered 8 7 ma conglomerate derived from the hanging wall of the main boundary thrust indicates that source area uplift and denudation must have occurred prior to 9 ma and probably prior to 10 ma assuming a gravelprogradationrateof3cm yr threeapatitefission trackages fromstructuresattheleadingedgeofthemainboundarythrustin the kohat region of northwest pakistan indicate that rapid cooling below 105 c1 2 m y earlier these data indicate that the main boundary thrust in the western himalaya formed synchronously along strike in the middle late miocene has a displacement rate of 10 mm yr and has a displacement history that is coeval with late displacement on the main central thrust

quartz cemented breccias from the mid atlantic ridge samples of a high salinity hydrothermal upflow zone textural mineralogical chemical and fluid inclusion studies on a suite of quartz rich sulfide bearing greenstone breccias indicate that high salinity high fluid flow systems were recently active on the mid atlantic ridge south of the kane fracture zone these systems involved fluids with temperatures close to 300 c and salinities almost 3 times that of seawater rounded pebblelike fragments enclosed within a matrix of euhedral quartz crystals appear to have been cemented by quartz deposition from a moving fluid which attained velocities in excess of 1 m s homogenization temperatures and freezing point depression effects in fluid inclusions from texturally early and late quartz crystals indicate temporal evolution of fluid temperatures and nacl equivalent salinities from 290 c and 10 wt to 200 c and 4 7 wt during alteration and cementation of the breccias highly variable cobalt concentrations in the evolving hydrothermal solutions are indicated by fine scale oscillatory zoning in euhedral pyrite within the breccias a continuous supply of the high salinity fluids may be generated either by supercritical two phase separation at depths well below the level of inclusion entrapment or by retrograde dissolution of hydroxy chloride phases in the deeper portions of the hydrothermal system either model requires the initial temperature of the deeply circulating fluids to have been distinctly higher than 415 c and the fluid to have been cooled by conduction or by mixing with lower temperature seawater prior to entrapment within the inclusions

meander evolution on piecewise uniform quasi geostrophic jets abstract a variety of simple two layer representations of a quasi geostrophic jet along a vorticity front are initialized with a simple meander the subsequent evolution of the jet is followed and related to the observed behavior of the gulf stream front east of the new england sea mounts by varying the horizontal and vertical structure of the jet a range of behavior is obtained that includes shingle formation and eddy detachment these effects can be related to the linear stability properties of the jet the structure that exhibits the most realistic meander evolution is that of a jet associated with a simple baroclinic vorticity front separating two pools of vorticity in the upper layer and two weaker pools in the lower layer this picture is consistent with what one can inter from the known density structure of the gulf stream in this region

earthquake magnitude frequency epicentral distance and soft sediment deformation in sedimentary basins abstract empirical equations available for earthquake magnitude frequency and the maximum epicentral distance of liquefied sites versus magnitude are combined with the characteristic instantaneous sediment deposition rate at a station in a sedimentary basin to form a model for the local stratigraphic frequency of earthquake induced soft sediment deformation structures both the style of sedimentation and the characteristics of the earthquake source combine to determine the pattern of incidence of the structures in a basin the model is probably most directly applicable to the geodynamic interpretation of fluvial basins in which substantial volumes of readily liquefied sands accumulate at high local short term rates

genesis and diagnostic value of soft sediment deformation structures a review abstract soft sediment deformation structures are recognized as important diagnostic features in the rock record for determination of depositional environments and slope processes the diagnostic value of these structures is reevaluated by analysis of the parameters controlling sediment deformation soft sediment deformation is contemporaneous with deposition and occurs dominantly in course silt to fine sand the high depositional rate low permeability and low shear strength of grains within this sediment range maximize the occurrence of deformation the dominant mechanisms responsible for sediment deformation include 1 liquefaction or fluidization 2 reverse density gradation 3 slumping or slope failure and 4 shear stress in most cases a combination of these mechanisms occurs the processes function in a continuum producing features that are microscopic to megascopic in scale it is shown that the processes and thus the structures are not environment specific the true diagnostic value of the structures may be in defining hydrodynamic conditions and in interpreting paleocurrents and paleoclimatic and paleoseismic events ultimately for the best diagnostic results soft sediment deformation structures should be studied in association with all other available lithologic structural and paleontological information

records of the evolution of the himalayan orogen from in situ th pb ion microprobe dating of monazite eastern nepal and western garhwal abstract in situ th pb monazite ages from rocks collected along two transects the dudh kosi everest eastern nepal and the bhagirathi river garhwal himalaya india perpendicular to the main central thrust mct suggest a striking continuity of tectonic events across the himalaya the youngest age reported in this study 5 9 0 2 ma mswd 0 4 from matrix monazite grains collected beneath the mct in the garhwal region is consistent with several age data from rocks at similar structural levels in central nepal providing support for widespread late miocene mct activity the lateral parallelism of orogenic events is further manifested by the 20 7 0 1 ma age of a high himalayan leucogranite from an injection complex that outcrops along the dudh kosi everest transect resembling ages of these bodies reported elsewhere the youngest monazite grain analyzed along the dudh kosi everest transect is 10 3 0 8 ma the absence of 7 3 ma monazite ages in eastern nepal may reflect a different nappe structure which obscures the reactivated ramp equivalent exposed in the garhwal and central nepal garnets from the mct hanging wall greater himalayan crystallines and footwall lesser himalaya display different major element zoning and the patterns are useful for constraining the location of the thrust system that separates the two lithologies pressure temperature paths for two upper lesser himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone along the dudh kosi everest transect upper lesser himalayan monazite grains from three rocks record a clear signal at 14 5 0 1 ma mswd 8 4 and the 23 ma age that characterizes the hanging wall is notably absent monazites collected within a large scale greater himalayan crystallines fold yield the 14 ma age consistent with the structure forming due to mct related compression paleo mesoproterozoic 1407 35 ma matrix monazite grains are found within an augen gneiss unit located beneath the mct whereas cambro ordovician 436 8 548 17 ma inclusions are preserved within garnets of the greater himalayan crystallines the presence of 45 2 2 1 ma grains from lower structural levels of the greater himalayan crystallines indicates the unit realized conditions conducive for monazite growth during the eocene

early pleistocene uplift of the northeastern tibetan plateau evidence from the dunhuang basin nw china abstract the cenozoic uplift of the tibetan plateau plays an important role in global climate change basin mountain geomorphology formation and desertification in western china as an important component the early pleistocene uplift process and its influence on the northeastern tibetan plateau has been controversial for many years quaternary synorogenic sediments within the dunhuang basin on the northeastern tibetan plateau record the details of plateau uplift detailed grain size analysis rock magnetic properties mudstone 36cl dating and 40ar 39ar detrital mica dating were conducted on the quaternary alluvial fluvial lacustrine aeolian sediments of the dunhuang basin the aksay section in the southern dunhuang basin is dominated by conglomerate sandstone siltstone and mudstone detrital mica 40ar 39ar ages show major age populations of 200 160 ma 400 200 ma and 510 430 ma from the northeastern altyn mountains the results of 36cl ages and rock magnetic properties show that the altyn mountains rapidly uplifted at 1 16 ma from 1 16 to 0 80 ma the sedimentary environment was stable with two small scale uplift pulses at ca 1 03 ma and 0 98 ma after 0 80 ma the climate became arid and the kumtag desert formed in the dunhuang basin these multistage uplifts are also widely found in the northeastern tibetan plateau the early pleistocene uplift of the altyn mountains corresponding to the kunhuang movement resulted in the northeastern margin of the tibetan plateau reaching its present height and climate changeing towards drought in this region

rupture history and seismotectonics of the 1991 uttarkashi himalaya earthquake abstract the 19 october 1991 uttarkashi india earthquake occurred in the main thrust zone of the himalaya with a moment magnitude of 6 8 this event is characteristic of the present day motion on the thrust fault system we examine this earthquake using different sets of data in order to understand better the faulting process of a major earthquake in the himalayan region firstly the modeling of the teleseismic records indicates that the mechanism is similar to the published cmt and indicates a shallow between 10 and 15 km depth low angle thrust event in the vicinity of the source the earthquake was recorded by a network of accelerometers run by the university of roorkee six three component accelerometers were triggered within a radius of 60 km two of them were very close to the surface projection of the fault forward modeling of those records shows that the rupture propagated toward the west this forward modeling gives us the possibility to confirm the epicenter location and to evaluate the timing of the accelerograms the accelerogram records are inverted to obtain the distribution of slip on the fault plane the results show a complex rupture process the slip maxima 1 5 m occurred 10 km west and 15 km southwest of the hypocenter the slip source function obtained with near field data is similar to the function obtained from teleseismic records and shows a low moment release at the beginning of the rupture and a maximum rate of moment release 4 seconds after the relation between the slip distribution obtained by inversion isoseimals mapped faults and the aftershocks location is then discussed and we finally propose a seismotectonic interpretation of this earthquake the uttarkashi earthquake probably occurred along the detachment surface which coincides with the upper surface of the subducting indian lithosphere this detachment surface is gently dipping under the lesser himalaya and south of the vaikrita thrust the vaikrita thrust marks the line separating the very shallow dipping detachment along which big earthquakes like the uttarkashi earthquake could occur from the steeper dipping aseismic basement thrust this observation is important for correctly estimating the seismic hazard in the uttarkashi region

problems and progress in the research on soft sediment deformations abstract the study of soft sediment deformations has gradually evolved from describing them as remarkable features into a structural analysis that should contribute to the reconstruction of the sediments early diagenetic history it appears that most attention has been devoted to this field during a few phases of the development of sedimentology a third phase when interest was renewed started in the eighties but progress is still hampered by inefficient scientific communication due to the absence of an unambiguous terminology specific topics deserving of much more attention in the near future are the interrelationships between the various forces that may effect a sediment simultaneously the development of deformational structures during a number of subsequent stages the preservational potential of the structures and systematic experiments that deal with the influence of the physico chemical and physico mechanical properties of the sediment

soft sediment deformation structures in the earth s oldest seismites abstract the chaibasa formation in eastern india which was deposited between 2100 and 1600 million years ago shows deformations that must have formed when the sediments were not yet consolidated some of these deformation structures have never been described before here they are described depicted and their origin is analysed we show that they must be the result of shocks which can only be explained satisfactorily as triggered by earthquakes the layers containing these deformation structures are termed seismites they are among the earliest records of earthquakes known in the earth s history

seismically induced deformation structures in oligocene shallow marine and aeolian coastal sands paris basin abstract soft sediment deformation structures have been identified in the oligocene coastal sand body in the southern part of the paris basin stampian sables de fontainebleau formation these coastal barrier sediments are characterized by pure fine grained sands the deformed beds are observed over some 60 110 km2 and are present either in foreshore sands or aeolian sand dunes convoluted laminations of an average amplitude around 0 70 m were identified in the foreshore or interdune sediments these are interpreted as the result of liquefaction conversely deformed sets of aeolian dune sand often involve large amounts of sediment these large scale contorted laminae several metres thick with minor internal faulting suggest deformation by slope failure under wet conditions such large scale soft sediment deformations may have been generated by storm surges and earthquakes which have similar liquefaction potentials we propose that the deformation structures were created by a single triggering event because of the absence of superposition of deformed layers over the study area the earthquake triggering hypothesis is supported by the large amount of sand involved in the slope failure features and the persistence of the contorted laminations over a wide area

seismites in the lokapur subgroup of the proterozoic kaladgi basin south india a testimony to syn sedimentary tectonism abstract syn sedimentary deformation structures ssds can indicate disturbance caused by autogenic or allogenic processes affecting the depositional system such structures are prolific in the proterozoic shallow marine sediments of kaladgi basin especially in the basal part of the succession lokapur subgroup in both clastics and carbonates widespread distribution lateral traceability of the deformed horizons across several hundreds of meters their occurrence between undeformed beds and proximity to nw se to e w trending faults and shears are used to attribute a seismic origin to some of these ssds release of accumulated stresses in the basin floor undergoing extension caused the episodic seismic activity producing the seismites these seismites record the syn sedimentary tectonism that controlled facies distribution patterns in the mesoproterozoic bagalkot group in conjunction with sea level fluctuations on one hand this is a contribution towards recognition of seismic events during the mesoproterozoic a period from which few seismites have been recorded on the other hand recognizing that the crustal processes that operated in brittle regimes during the creation of this extensional supracrustal basin around 1800 ma were comparable to those operating in present day extensional systems opens a window of understanding how the proterozoic platform basins hosting thick shallow marine sequences evolved

earthquake induced structures in young sediments abstract soft sediment deformational structures have been identified in several plio quaternary horizons near burdur in southwest turkey these horizons satisfy criteria for correlating such structures with seismic events and it is proposed that the structures were formed by earthquake induced liquefaction the deformational structures are thus interpreted to represent seismic events which occurred up to 2 3 ma ago

deciphering the sedimentological imprint of paleoseismic events an example from the aptian codo formation northern brazil abstract a stratigraphic horizon bearing several styles of soft sediment structures occurs sandwiched within entirely undisturbed deposits in the lacustrine codo formation aptian located in the eastern part of the grajau basin northern brazil these deposits show a variety of sedimentary features distinguished according to their deformation style as follows from bottom to top a zone 1 consisting of widespread molar tooth structures i e fine grained spar filled cracks that are interconnected with small scale faults fissures and stylolites inclined at a high angle to bedding b zone 2 represented by complex convolute folds with lateral gradation of smooth synclines and anticlines into isoclinic folds and internal layers that retain the normal bedding features recumbent folds nappe folds with associated thrust faults pseudonodules and mound and sag structures c zone 3 consisting of normal faults and fissures that are vertical to near vertical show ragged morphology with small delicate peaks taper both downward and upward after a few centimeters and are associated with large up to 2 5 m long collapsed boulders and d zone 4 characterized by shales with irregular convolute folds these deformation zones are interpreted to reflect sediment rheology and alternating periods of sediment accumulation and deformation under the influence of synsedimentary seismic events as suggested mostly by 1 occurrence between undeformed deposits 2 lateral correlation for several kilometers of distance 3 similarity with other features formed in areas affected by seismic activity 4 presence of structures attesting to sediment deformation in an unconsolidated to semi consolidated state and 5 complex association of brittle and ductile deformation structures hence zone 1 records small scale cracking and faulting of early cemented cohesive deposits zone 2 is dominated by ductile deformation structures formed on water saturated unconsolidated sediments probably as a reflex of compressional stress zone 3 records a change in tectonic style to dominantly distension as evidenced by the horizon with normal faults finally zone 4 shows structures attributed to ductile deformation of water saturated muds which was probably formed during the same episode of deformation responsible for the faults and fissures in zone 3 this succession of deformation events appears to be consistent with transtensional and compressional shear stresses during the sedimentary evolution of the grajau basin

analysis of soft sediment deformation structures in neogene fluvio lacustrine deposits of caybagi formation eastern turkey during the neogene both strike slip and extensional regimes coexisted in eastern turkey and a number of fault bounded basins associated with the east anatolian fault system developed the caybagi formation late miocene early pliocene deposited in one of these basins consists of fluvio lacustrine deposits numerous soft sediment deformation structures are encountered in this formation particularly in conglomerates medium to coarse grained tuffaceous sandstones and claystones folded structures slumps convolute laminations and simple recumbent folds water escape structures intruded sands internal cusps interpenetrative cusps and sand volcanoes and load structures load casts pseudonodules flame structures and pillow structures these structures are produced by liquefaction and or fluidization of the unconsolidated sediments during a seismic shock consequently the existence of seismically induced deformation structures in the caybagi formation and the association with a neogene intraformational unconformity growth faults and reverse faults in the caybagi basin attest to the tectonic activity in this area during the late miocene and early pliocene the east anatolian fault system in particular the uluova fault zone is the most probable seismogenic source earthquakes with a magnitude of over 5 in the richter scale can be postulated

the south atlantic current in this paper we use the historical hydrographic data base for the south atlantic ocean to investigate i the hydrographic boundary between the subtropical gyre and the antarctic circumpolar current acc the sub tropical front stf and ii the southern current band of the gyre which is called the south atlantic current sac the stf begins in the west in the brazil falkland malvinas confluence zone but at locations at and west of 45 w this front is often coincident with the brazil current front east of 45 w the stf appears to be a distinct feature to at least the region south of africa whereupon it continues into the indian ocean the associated current band of increased zonal speed is the sac which except for one instance is found at or north of the surface stf until indian ocean water from the agulhas retroflection is reached a reversal of baroclinicity in the stf is observed south of a highly saline agulhas ring causing the sac to separate from the stf and turn north into the benguela current zonal flow south of the stf is generally weak and serves to separate the south atlantic and circumpolar currents in the argentine basin the sac has a typical volume transport of 30 sv 1 sv 106m3s 1 in the upper 1000 m relative to a deep potential density surface 4 45 87 kg m 3 and can be as high as 37 sv it is thus comparable to or stronger than the brazil current in the cape basin the transport of the sac is reduced to about 15 sy before it turns north to feed the benguela current in late 1983 this flow was joined by about 8 sv of water from the agulhas current

dynamic fluvial systems and gravel progradation in the himalayan foreland although the large scale stratigraphy of many terrestrial foreland basins is punctuated by major episodes of gravel progradation the relationships of such facies to hinterland tectonism and climate change are often unclear structural reentrants provide windows into older and more proximal parts of the foreland than are usually exposed and thus provide key insights to earlier phases of foreland evolution our magnetostratigraphic studies show that although the major lithofacies preserved within the himachal pradesh structural reentrant in northwestern india resemble neogene facies in pakistan they have a much greater temporal and spatial variability from 11 5 to 7 ma major facies boundaries in himachal pradesh vary by as much as 2 3 m y across distances of 20 30 km and are controlled by the interference between a major southeastward flowing axial river and a major southwestward flowing transverse river a thick but highly confined middle to late miocene conglomerate facies includes the oldest extensive siwalik conglomerates yet dated 10 ma and implies the development of significant erosional topography along the main boundary thrust prior to 11 ma our studies document extensive syntectonic gravel progradation with conglomerates extending tens of kilometers into the undeformed foreland during a period of increased subsidence rate and within 1 2 m y of major thrust initiation overall gravel progradation is modulated by the interplay among subsidence sediment supply and the proportion of gravels in rivers entering the foreland

seasonal cycle of velocity in the atlantic north equatorial countercurrent as measured by surface drifters current meters and ship drifts this report describes the general circulation and seasonal variation of currents in the equatorial atlantic concentrating on the north equatorial countercurrent necc using data collected as part of the seasonal response of the equatorial atlantic and programme francais ocean et climat dans l atlantique equatorial experiments plus historical ship drifts during 1983 1985 the lagrangian circulation was studied by launching and tracking 30 freely drifting drogued buoys in the north equatorial countercurrent and 23 in the south equatorial current sec in addition continuously recording current meters were moored for 20 months at depths from 20 to 300 m near the center of the necc at 6 n 28 w these measurements are the longest series ever obtained in this current and provide information about its seasonal and interannual variations and zonal meridional and vertical structure the seasonal cycle of the necc is very regular from year to year each year the necc starts up in may june it flows eastward across the atlantic with surface speeds of up to 143 cm s in the west extending down to 350 m at 28 w and flowing into both the guinea current and north equatorial current it disappears or reverses from about january june west of 18 w some near surface water of the necc is inferred to downwell and flow equatorward toward the equatorial undercurrent the sec flows into the north brazil current which during spring continues up the coast into the caribbean during fall however the whole north brazil current retroflects or turns back on itself between 45 and 50 w forming the western necc the retroflection establishes a meander pattern in the necc that slowly propagates westward during fall at a speed of 4 cm s the meanders have a displacement of about 300 km in latitude a wavelength of 900 km and meridional velocity fluctuations of up to 100 cm s the swift currents and time dependent meanders in the western necc cause a high eddy kinetic energy 2400 cm2 s2 equivalent to that of the energetic part of the gulf stream

liquefaction and fluidization of lacustrine deposits from lahaul spiti and ladakh himalaya geological evidences of paleoseismicity along active fault zone abstract soft sediment deformational structures are important diagnostic features of paleoseismic studies these seismites result from local vigorous fluidization and hydroplastic deformation formed during liquefaction as a direct consequence of permanent deformation of ground surface due to earthquakes of large magnitude the region of eastern ladakh spiti basin and the upper sutlej river basin record a concentration of earthquakes in a well defined ne trending seismic zone sub parallel to the strike of leo pargil horst and the kaurik chango fault zone soft sediment deformational studies of the fluvial and fluvio lacustrine sequences near sumdo village along sangla valley of himachal pradesh and near saspol village of ladakh indicate that the kaurik chango fault zone is a seismogenic fault causing liquefaction and fluidization as the consequence of earthquake activity in the past thermal luminescence tl date of the deformational structures near the sumdo village constrains the paleoseismic activities of large magnitude between 90 000 years third activity to 26 000 years last geological evidence with a recurrence interval of approximately 10 000 years during the quaternary period

microearthquake activity in some parts of the himalaya and the tectonic model abstract microearthquake data from temporary permanent networks in different parts of the himalaya shed new light on understanding the earthquake generating processes and their relation to tectonic models of the region the microearthquake activity in arunachal pradesh northeastern himalaya is found to be pronounced at the main boundary thrust mbt and to its south the subcrustal earthquakes depth 50 80 km occur much below the plane of detachment of the tectonic models proposed by seeber et al 1981 ni and barazangi 1984 the mbt is not the seismogenic fault the earthquakes are generated by strike slip movement on deep seated hidden faults transverse to the mbt the high seismic activity in the shillong plateau about 200 km south of the mbt in the northeast region is due to the influence of himalayan collision tectonics to the north and burmese arc subduction tectonics to the east the activity in the plateau is not directly related to the himalayan thrust belt or seismic belt these are mostly crustal earthquakes depth 10 30 km and are caused by local active faults lineaments in the eastern himalaya in the sikkim and darjeeling area the seismic activity is found to be clustered mostly to the north of the mbt the earthquakes occur at a depth range 0 50 km the majority of them occur below the detachment plane by thrust faulting in the central part in the nepal himalaya lateral variations of the seismic activity are observed which represent lateral segmentation of the mbt by transverse tectonic features in the western himalaya however the tectonic models fit well with the microearthquake data in the himachal pradesh of the western himalaya the microearthquakes are mostly recorded in the mbt zone and the hypocentres depth 0 20 km are confined above the plane of detachment or on the basement thrust the earthquakes mostly occur to the south of the main central thrust mct which suggests that the mct is not seismogenic it is rather a dormant fault no single tectonic model explains the himalayan earthquakes

clastic dykes of the devono carboniferous horton bluff formation nova scotia storm related structures in shallow lakes abstract clastic dykes are common in the late devonian to early carboniferous horton bluff formation of nova scotia where they occur in wave dominated shoreline deposits of lacustrine shoaling upward cycles the dykes typically less than 25 cm wide and 40 cm in vertical extent project downward into clayshale from broad loads at the base of elongate isolated lenses of hummocky cross stratified siltstone ptygmatic folding of the dykes reflects a shale siltstone compaction ratio of 2 3 times and maximum depth of dyke penetration prior to compaction was 1 m dyke orientation parallels the orientation of wave ripple crests and the elongation direction of the siltstone lenses dyke formation was penecontemporaneous with sedimentation dyke and load formation was probably initiated in shallow water during storms which promoted sediment failure through cyclic wave loading microseisms and rapid deposition of hcs bearing siltstone lenses reversed density gradient and uneven sediment loading from silt lenses probably promoted downward emplacement loads reflect ductile failure of surficial water rich mud whereas dykes reflect downward injection of silt following brittle failure of underlying more compacted mud the depositional slope provided a tensional stress that controlled the fracture direction orthogonal to the slope of the failed sediment

eddy shedding and energy conversions in the east australian current 1 mesoscale variability and eddy shedding in the tasman sea particularly of the east australian current eac is studied through the analysis of remotely sensed observations and outputs from a global ocean model previous observations of the western boundary current separation from the coast showed strong variability at periods ranging between 90 and 140 days we show from satellite altimetric observations that rapid northward migration of the separation point of the eac follows the formation of large eddies at periods of 100 days after an eddy separation event the normally southward flowing current swiftly assumes a more zonal configuration near the separation latitude with a cyclonic circulation developing inshore the formation of large separation eddies is preceded by the southward propagation of sea level anomalies along the east australian continental slope from 25 s sea level anomalies grow as they travel south eventually being pinched off in the form of large anticyclones at 32 s in the current retroflection area energy conversion terms in a global ocean model and in altimetric data suggest both barotropic and baroclinic instability may account for the growth of these anomalies as they propagate south east of the main eac jet there is evidence that eddies may be feeding potential energy back to the mean flow

dynamics of separating western boundary currents abstract in this work pratt and stern s quasigeostrophic 1 layer infinite jet model is connected to a western boundary by a system of two converging boundary currents the model has a piecewise constant potential vorticity structure and the departing jet has a zonal cusplike profile in the ocean interior the relative strengths of the coastal jets can be varied and the coastline can be tilted relative to north the coastline tilt and the coastal current asymmetry cause an alongshore momentum imbalance that creates a spatially damped quasi stationary wave pattern the presence of the boundary favors the long waves in the model which behave fairly linearly in all study cases the effects of the coastline tilt and the coastal current asymmetry are varied to reinforce or cancel each other in the former case a retroflection type of boundary current separation like the one observed in most southern hemisphere western boundary currents is obtained in the latter case a much smoother separation results

earthquakes and deformational structures seismites in holocene sediments from the himalayan andaman arc india soft sediment deformational structures from the himalayan arc caused by episodic earthquake recurrences during the holocene recent are produced by the ongoing tectonic processes these small scale subtle structures occur along some major thrusts in the garhwal himalaya and vary from normal faults in the lower zone to compressional structures in the middle zone along the main central thrust in the tons valley the latter includes isoclinal angular kink and disharmonic folds with discrete displacements along their axial surfaces thrusts with decollement listric and overstep thrusts in duplex style and load slumping the upper zone is marked by very low angle thrusts with left lateral displacement and associated displacement shears d riedel shears r and thrust shears p as well as small scale sporadic kinks three zones reveal changing tectonic character from gravity tectonics in the lower zone to compression in both the middle and upper zones with the maximum principal stress 1 oriented n60 to n110 respectively along the tons thrust post depositional structures include drag along normal fault zone characterized by d r and p shears and conjugate fractures that provide localized orientation of stresses 1 at 75 n60 2 at 12 n280 and 3 at 12 n290 earthquake induced liquefaction and subtle variations in physiomechanical properties of soft sediments have primarily controlled the deformational pattern due to localized tectonic stresses ground and sediment deformations during two earthquakes in the himalayan andaman arc provide additional support for the seismogenic origin of such structures preserved in loose or consolidated sediments

structure and tectonics of the himalaya constraints and implications of geophysical data the himalayan range is a product of continental collision between india and eurasia and for several reasons it is a logical place to examine the physical causes and mechanisms of mountain building the exceptionally high mountains and the great length of the range alone single the himalaya out as a major structural feature the convergence rate between india and eurasia of about 50 mm yr 1 minster jordan 1978 requires rapid rates of slip and the occurrence of four major earthquakes with magnitudes in excess of 8 4 in the last 100 years attests to a high level of seismicity and a rapid rate of active deformation e g richter 1958 seeber armbruster 1981 the exposures of high grade metamorphic rocks in the greater himalaya may suggest large vertical transport of lower crustal rocks e g valdiya 1980a b and the occurrences of young postcollision granites imply recent large perturbations of the temperature structure of the range e g lefort 1981 moreover as sediments derived from the himalaya fill the broad ganga basin south of the range the mountain belt simul taneously seems to override the basin e g g d johnson et al 1979 n m johnson et al 1982 as such the processes occurring now and since the collision began in the early tertiary as well as the present structure of the himalayan belt are probably similar to those at different stages of collision and mountain building in other older orogenic belts such as the appalachians the scottish and scandinavian caledonides the alps and

mechanisms and controls on the formation of sand intrusions sandstone intrusions are found in all sedimentary environments but have been reported most commonly from deep water settings they also appear to be more frequently developed in tectonically active settings where applied tectonic stresses facilitate development of high fluid pressures within the sediments a variety of mechanisms have been cited as triggers for clastic intrusions these include seismicity induced liquefaction application of tectonic stresses excess pore fluid pressures generated by deposition related processes and the influx of an overpressured fluid from deeper within the basin into a shallow sand body the formation of sandstone dykes and sills is investigated here by considering them as natural hydraulic fractures when the seal on an unconsolidated overpressured sand body fails the resulting steep hydraulic gradient may cause the sand to fluidize the fluidized slurry can then inject along pre existing or new fractures to form clastic intrusions the scale and the geometry of an intrusive complex is governed by the stress state depth and pre existing joints or faults within the sedimentary succession as well as the nature of the host sediments for the simplest tectonic setting where the maximum stress in a basin is vertical gravitational loading small irregular intrusions commonly result in the formation of sills at shallow depths within a few metres of the surface whereas at greater depth dykes and sills forming clastic intrusion networks are more typical a simple relationship is derived to calculate the maximum burial depth at which a dyke sill complex forms as a function of the source bed to sill height the bulk density of the surrounding sediments and the ratio of the vertical to horizontal effective stresses k 0 when applied to three examples of large scale dyke sill complexes developed within paleocene and eocene deep water reservoir sand bodies of the north sea maximum burial depths in a range of 375 to c 500 m 450 700 m and 550 850 m are estimated for intrusion of each of the three complexes

new field and laboratory evidence for the origin of hyaloclastite flows on seamount summits new field observations with the submersible alvin and photographic evidence from a study of the summits of seamounts near the east pacific rise show that hyaloclastite deposits occur commonly hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m these new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies most of the hyaloclastite samples are of hydrovolcanic eruptive origin but a few show evidence of a predominantly sedimentary origin primarily from morphology we identify several vent areas from which hyaloclastite presumably erupted the surface appearance of the hyaloclastite deposits varies with distance to these vents leading us to propose a facies model for deep sea hyaloclastites on seamount summits hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding parallel alignment of platy shards they consist of blocky sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment the shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous we propose that the shards form mainly by cooling contraction granulation but cannot rule out the possibility of limited steam explosivity hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate promoting clastic dominated versus flow dominated eruptive behavior is the dominant control on hyaloclastite formation we propose that shard formation occurs during submarine lava fountaining gravitational instability of the resulting slurry of shards sea water and possibly steam causes gravity flow that carries the shards outward from the vent further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism eruption dynamics and deposition of deep sea hyaloclastites

multi sensor satellite and in situ measurements of a warm core ocean eddy south of the brazil malvinas confluence region abstract a warm core eddy was detected south of the brazil malvinas falkland confluence bmc region in satellite images of the southwestern atlantic ocean in late 2002 the structure was also sampled by in situ instruments deployed by a ship in 2 days of november 2002 an analysis of satellite data was performed to account for the lifecycle dimension surface temperature surface chlorophyll concentration surface height anomaly and displacement of the eddy since it was detached from the brazil current in september 2002 satellite data were derived from several sources such as the amsr e modis and radar altimeter the structure lasted 64 days at south of the brazil malvinas confluence region later being re assimilated by the brazil current return flow in situ data collected during 2 days showed that the eddy was about 150 m deep 5 5 c warmer and 1 practical salinity unit saltier than adjacent waters the salt anomaly associated to the eddy was estimated as 1 37 10 12 kg while its heat content was 4 24 10 18 j these are typical estimates for eddies present at the bmc region sea surface temperature sst chlorophyll concentration and sea surface height anomaly time series were analyzed for the eddy s center along its trajectory path throughout its lifetime mean translational velocities for the eddy during its lifetime were computed from visual interpretation and by using an adaptation of the maximum cross correlation mcc method on amsr e sst images the overall deviation between the two methods was 26 this suggests that the mcc method has a potential to be applied in monitoring programs to automatically account for the translational velocities of eddies in the bmc region

soft sediment deformation in a tectonically active area the plio pleistocene zarzal formation in the cauca valley western colombia abstract the plio pleistocene zarzal formation corresponds to fluvio lacustrine sediments deposited in an intramontane depression within the colombian andes associated with the cauca romeral fault system it crops out mainly in the cauca valley where numerous field sections have permitted the mapping of the vertical and lateral lithological variations lacustrine deposits of sands silts clays and diatomites are interbedded with fluvial sand and gravel beds and fluvio volcanic mass flows derived from the volcanic central cordillera numerous soft sediment deformation structures are encountered in this formation particularly in fine to medium grained sands silts and clays load structures load casts flame structures pseudonodules water escape structures water escape cusps dish and pillar and pocket and pillar structures soft sediment intrusions clastic sills and dykes disturbed laminites convolute laminations slumps and synsedimentary faulting deformation mechanisms and driving forces are related essentially to gravitational instabilities dewatering liquidization and brittle deformations field and regional geological data show that most of these deformations are related to seismicity and can be interpreted as seismites this area has a geological and recent seismic history and outcrops show both syn and post depositional faulting related to the transpressional regime of this part of the colombian andes which generates strike slip faults and associated local normal faults the drainage pattern within the zarzal formation shows the signature of neotectonics moreover the fine to coarse grained sands of the zarzal formation are lithologies prone to liquefaction when affected by seismic waves the intercalation of the deformed intervals within undisturbed strata confirms the catastrophic nature of the events finally the large areal extent of the deformations and the type of structures are compatible with seismites consequently the existence of seismites in the zarzal formation represents corroboration of tectonic activity in this area during the pleistocene earthquakes with a magnitude higher than 5 can be postulated based upon the proximity of active faults and the types of deformations

the drifting confluence zone abstract alongshore migration of a western boundary current separation is investigated with a nonlinear inviscid reduced gravity model on an f plane separation due to a collision with an opposing current is considered it is known that for such a case stationary collision and separation is possible only for boundary currents with balanced transports that is equal near wall depths the authors perturb this stationary solution with a small steplike variation of the opposing current transport and focus on the resulting time dependent flow two different analytical methods to compute the migration rate are used the first method involves integrated balances and the second involves the path equation for the separated flow using the fist approach it is found analytically that the flow consists of one current intruding into the area occupied by the other after an initial adjustment period the intrusion becomes steadily propagating the width of intrusion is much greater than the width of boundary curren

prehistoric earthquake deformations near masada dead sea graben earthquake inducedfluidizations and suspensions of lake sediments associated with syndepositionalfaults formapaleoseismicrecordinthedeadseagraben theassociation offluidized beds with surface faulting supports the recognition of mixed layers as reliable earthquake indicators and provides a tool for the study of very long term 70 kar seismicity along the dead sea transform the faults compose a fault zone that offsets laminated sediments of the late pleistocene lake lisan they exhibit displacements of as much as 2 m layers of massive mixtures of laminated fragments are interpreted as disturbed beds each formed by an earthquake the undisturbed laminated layers between these mixed layers represent the interseismic interval a typical vertical slip of about 0 5 m per event is separated by several hundred years of quiescence the fault zone lies within the dead sea graben 2 km east of masada where archaeology and historical accounts indicate repeated strong earthquake damage the distribution of strikes in the fault zone resembles that of the faults exposed in and around the graben including the seismogenic ones the excellent exposures over hundreds of metres allow an unprecedented temporal and spatial resolution of slip events on faults

thrust geometries and deep structure of the outer and lesser himalaya kumaon and garhwal india implications for evolution of the himalayan fold and thrust belt the kumaon garhwal region of the himalaya lies near the center of the himalayan fold and thrust belt we have drawn two balanced cross sections 100 km apart through the outer and lesser himalaya the cross sections incorporate all the surface well log and earthquake seismic data currently available from the region two branch line maps showing trailing and leading branch lines and cutoff lines of the major thrusts in the region are also drawn the three dimensional deep structure of the outer and lesser himalaya is interpreted based on the balanced cross sections and the branch line maps deep structure of the higher and tethyan himalaya is extrapolated based on surface geology and is subject to revision as more surface and seismic data become available from these areas a sequential evolutionary model for the kumaon himalaya along the eastern pindari section is proposed according to this model the kumaon himalaya evolved by an overall forelandward progression of thrusting with some reactivation along the munsiari thrust mt the main boundary thrust mbt and the main central thrust mct we use structural stratigraphic and radiometric criteria to place time constraints on the motion of these thrusts earliest motion along the mbt may have occurred in early middle paleocene but the main episode probably started in late eocene and may still be continuing emplacement of the mt had occurred by middle late eocene whereas the mct shows activity around 20 ma thus exhibiting break back thrusting with respect to the mt shortening estimates are obtained from the pindari section minimum shortening in the sedimentary thrust sheets of the outer and lesser himalaya is 161 km or 65 as a first approximation we have also restored the crystalline sheets in order to obtain shortening estimates for the entire himalaya minimum shortening for the himalaya after restoring the mct sheet varies from 354 76 to 421 km 79 these estimates were further combined with published data from the area between the mct sheet and the indus tsangpo suture zone itsz minimum shortening between the indo ganga foreland and the itsz thus obtained lies in the range 687 754 km or 69 72 we compare our shortening estimates with those available from the pakistan and nepal himalaya

an early india asia contact paleomagnetic constraints from ninetyeast ridge odp leg 121 new paleomagnetic results from sedimentary rock and basement of the ninetyeast ridge ocean drilling program leg 121 sites 756 758 detail the northward movement of the indian plate for the past 80 m y analysis of the combined paleolatitude age profile indicates a distinct reduction in india9s northward movement rate at 55 ma interpreted as completion of suturing of greater india and asia india9s northward motion slowed from 18 19 5 cm yr to 4 5 cm yr for the location of site 758 comparison of this profile with paleomagnetic data from the wider himalayan region indicates that initial contact between northwestern greater india and southern asia was already established by cretaceous tertiary time this supports a possible causal link between the india asia convergence and the deccan traps extrusion

seismically induced soft sediment deformation structures in the palaeogene deposits of the liaodong bay depression in the bohai bay basin and their spatial stratigraphic distribution abstract soft sediment deformation structures ssds have been identified from well cores in the palaeogene deposits of the liaodong bay depression in the bohai bay basin china these deposits formed as interbedded sand and mud at a delta front or on the slope toe of the prodelta according to criteria proposed by previous research we established that these ssds were induced by earthquakes and that they can be divided into two groups ductile deformation structures plastic intrusions ball and pillow structures flame structures boudinage structures irregular convolute stratifications and synsedimentary faults and folds and brittle deformation structures sand dykes and autoclastic breccias based on their level of deformation size and complexity the ssds were divided into three groups from weak to strong to reflect the intensity of palaeo earthquakes with consideration of the palaeo sedimentary environment we proposed a model to account for the production and preservation of these ssds according to the classification adopted in this study and the spatial stratigraphic distribution of the ssds the tectonic activities of the tan lu faults in the bohai bay basin were investigated the a and b oilfields assumed names are located in the tectonically active zones of the west and east branches of these faults respectively the extension tectonic activities in the a oilfield region exhibit a sharply decreasing trend from e2s3 to e2s1 and increase again in e3d2 whereas the strike slip tectonic activities in the b oilfield region exhibit an increasing trend from e2s3 to e2s1 and finally reach a maximum to e3d3 the results of this study show that the method of analysis of the spatial stratigraphic distribution of ssds is suitable for determining the evolution of tectonic activity and thus it can provide a new perspective for basin analysis

experimental soft sediment deformation structures formed by the liquefaction of unconsolidated sands and some ancient examples the effects of liquefaction in saturated sand bodies under a variety of driving forces are described from shaking table experiments and structures from the geological record are presented which are analogous to the experimental structures the collapse of sloping heaps of cross bedded sand under a gravitational body force generates low angle essentially uncontorted stratification a basal zone of shearing may be present with steepened and folded foresets stretching of foresets may be accommodated on normal faults and bottomsets may be contorted into inclined folds in natural systems the substrate may also liquefy causing deformation driven by an unevenly distributed confining load stratification in the surface bedform is flattened and stratification in the substratum contorted experiments failed to produce relative displacement at the interface between stacked sand bodies liquefaction of gravitationally unstable systems in sands generates load structures comparable to those from sand mud systems recumbent folded deformed cross bedding is formed by current shear over a liquefied bed as has been inferred from field and theoretical analyses shear of nonliquefied sand forms angular folds other deformation mechanisms such as fluidization or seepage may generate structures similar to all of these local water escape structures driven by fluidization occur in the upper parts of some liquefied sand bodies they include cusps sand volcanoes and clastic dykes transient cavities formed in some experiments and seemed to be preserved as breached cusps although the experiments tried to isolate individual driving forces driving forces may operate together and there may be a continuum between deformation driven by water escape and deformation driven by loading different structures from those described here may form where liquefaction develops in a buried layer as opposed to at the sediment surface

microearthquakes at the main boundary thrust in eastern himalaya and the present day tectonic model abstract two microearthquake surveys were carried out at the main boundary thrust mbt eastern himalaya temporal as well as spatial variation in microseismicity was observed the vertical section of the hypocentres shows subcrustal earthquakes down to a depth of 80 km to the south of the mbt and composite focal mechanisms show strike slip solutions or transverse tectonics which do not agree with the conceptual tectonic model

sinistral transpression along the main boundary thrust in amritpur area southeastern kumaun himalaya india abstract outcrop mapping and structural analyses in a 100 m thick fault damage zone elucidate geometry and kinematics of the main boundary thrust in the study area the north dipping fault damage zone is sandwiched between the proterozoic amritpur granite in the hangingwall and the neogene siwalik sediments in the footwall the amritpur granite is overlain by the volcanic sedimentary sequence along a top to the south thrust the footwall sediments are deformed into gentle upright and steeply plunging folds by contrast the hangingwall granite is generally massive the km scale folds in the sub himalayan sediments are geometrically discordant to the folds in the lesser himalayan volcanic sedimentary sequence the damage zone contains both the sub himalayan sediments and the lesser himalayan amphibolite it consists of an imbricate structure tectonic wedges and pressure solution bands the imbricate structure is made up of seven fault bounded lithotectonic domains that record the imprints of coaxial refolding and subsequent modification of the folds into sheath like structures the geometry and kinematics of mesoscopic structures and the results of paleostress analysis reveal a sinistral wrench accompanied n s to nne ssw directed subhorizontal shortening in the damage zone the aforesaid evidences also help distinguish between pure contractional pure wrench contraction dominated and wrench dominated deformation in the different domains and the tectonic wedges the sinistral transpression in the damage zone is ascribed to the occurrence of an oblique lateral ramp along the main boundary thrust in the study area

petrography and geochemistry of achnelithic tephra from las herrerias volcano calatrava volcanic field spain formation of nephelinitic achneliths and post depositional glass alteration abstract we present the results of a petrographic and geochemical study carried out on a layer of achnelithic tephra outcropping at the base of the volcanic cone of las herrerias miocene quaternary volcanic region of campo de calatrava spain the tephra with a composition of nephelinite and ash 3 of all major elements and notable increase in h 2 o content the elements liberated by this process formed smectite with an average structural formula comprised between those of beidellite and nontronite end terms na 0 01 k 0 03 ca 0 18 mg 0 22 fe 0 16 2 fe 0 48 al 1 02 3 ti 0 18 4 si 3 58 al 0 42 o 10 oh 2 the degree of palagonitization in each achnelith was likely related to the amount of water incorporated by individual clasts at the moment of their deposition in a volcanic maar lake afterwards there was no more water circulation through the achnelithic tephra which was sealed from water by overlying hydrovolcanic tuff deposits it was this isolation that made possible the preservation of glass to the present day

soft sediment deformation structures in the late quaternary sediments of ladakh evidence for multiple phases of seismic tremors in the north western himalayan region abstract the exposed quaternary sections of ladakh show evidence of seismicity during the late quaternary multiple levels of soft sediment deformation structures seismites are recorded from the quaternary sediments of the spituk leh along indus suture zone isz and the khalsar palaeolakes along shyok suture zone ssz and karakoraum fault kf the studied sections are a part of two major tectonically formed palaeolakes at 35 000 yr bp nine levels of seismites from spituk leh and eight levels from khalsar sections are recorded the deformed sediments comprise of clay silts and sand and are restricted to a single stratigraphic layers bounded by undeformed beds suggesting synsedimentary deformation the various deformational structures identified are simple and complex convolutes pinch and swell bedding microfolds and microfaults flame like structures pseudonodules clay diapirs ball and pillow structures pillar structures sedimentary dykes and mud lenses the release of stress along the isz ssz and kf may have been responsible for inducing seismicity in the area during the late quaternary times which may have caused liquefaction as a direct consequence of permanent deformation of ground surface due to earthquakes of large magnitudes 5 intensity the chronological data suggests release of stresses along isz ssz and kf in the form of earthquakes between 35 000 yr bp to approximately the last glacial maxima lgm as evident form the available chronology

thermobarometric constraints on the thermal history of the main central thrust zone and tibetan slab eastern nepal himalaya the main central thrust mct south of mt everest in eastern nepal is a 3 to 5km thick shear zone separating chlorite bearing schist in the lower plate from sillimanite bearing migmatitic gneiss in the overlying tibetan slab the metamorphic grade increases through the mct zone toward structurally higher levels previous workers have suggested that either post or synmetamorphic thrust movement has caused this inversion of metamorphic isograds in an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault four well calibrated thermobarometers were applied to pelitic samples collected along two cross strike transects through the mct zone and tibetan slab results show an increase in apparent temperature up section in the mct zone from 778 k to 990 k and a decrease in temperature to 850 k in the lower tibetan slab which is consistent with synmetamorphic thrust movement a trend in calculated pressures across this section is less well defined but on average decreases up section with a gradient of 28mpa km resembling a lithostatic gradient pressure temperature paths for zoned garnets from samples within the mct zone modelled using the gibbs method show a significant decrease in temperature and a slight decrease in pressure from core to rim which might be expected for upper plate rocks during synmetamorphic thrust movement samples from the uppermost tibetan slab yield higher temperatures and pressures than those from the lower tibetan slab which may be evidence for later resetting of thermobarometers by intrusion of the large amounts of leucogranite at that structural level

soft sediment deformation structures from the paleoproterozoic damtha group of garhwal lesser himalaya india abstract soft sediment deformation ssd structures are observed in the paleoproterozoic damtha group in the garhwal lesser himalaya for a lateral extent of over 110 km in comparable litho sections the ssd structures are primarily observed in medium to fine grained sandstones siltstones and shale at sand mud interfaces that were deposited in an inter tidal to supra tidal setting of the shallow marine platform the ssd structures are conspicuously absent in horizons within facies dominated by a single grain size the ssd structures include load structures namely drops pseudonodules and ball and pillow structures along with contorted beds slumps cusps and water escape structures of varying geometry and dimensions the ssd structures are invariably bounded by undeformed beds and their dimensions depend on the thickness of bed affected by liquefaction the observed ssd structures show distinct similarities with liquefaction induced deformation structures in geological records and those produced in the laboratory under reverse and normal density gradient conditions the recurring nature of ssd structures in the stratigraphic column over large spatial distances points toward a trigger mechanism with recurrent activity that affected the basin on a regional scale such as prolonged rifting of the damtha basin during the paleoproterozoic this would have caused frequent earthquakes and shakes of the basin floor producing recurrent ssd structures in view of the similarity with seismites and absence of a viable alternative rift related seismicity is preferred as the trigger mechanism for the observed ssd structures in the damtha group

cooling rates of hyaloclastites applications of relaxation geospeedometry to undersea volcanic deposits glass fragments from three different hyaloclastites have been used to evaluate the range of cooling rates experienced by undersea volcanic deposits we found that the glass fragments retain structures with a range of apparent quench rates from 25 to 0 15 k min 1 the most rapid cooling rates are interpreted to be those resulting from cooling of the lava near the water interface simple conductive cooling models produce a range of quench rates comparable to those of the more rapidly cooled samples the very slow apparent quench rates are unlikely to result from simple linear cooling through the glass transition because of the onset of crystallization instead they are indicators of a more complex thermal history that involves the annealing of glasses at temperatures within the glass transition interval for a dwell time sufficient to allow the relaxation of the glass to lower temperature structures the thermal history recorded in these samples illustrates the complexity of eruptive processes and demonstrates that quench rates for natural glasses retain information relevant to more complex cooling models

thermal evolution of the greater himalaya garhwal india the hanging wall of the main central thrust mct in garhwal india roughly 79 n 80 e 30 n 31 n exhibits an inverted metamorphic gradient sillimanite potassium feldspar assemblages near the top of the hanging wall or greater himalayan sequence are underlain by kyanite grade rocks near the fault textural relationships in pelitic samples from the alaknanda and dhauli river valleys indicate that the inversion is the product of two distinct metamorphic events an early harrovian event m1 which affected the entire greater himalayan sequence and a later buchan event m2 the effects of which are most obvious in the upper part of the sequence rim thermobarometry garnet inclusion thermobarometry and thermodynamic modeling of garnet zoning reveal that the basal portions of the metamorphic sequence experienced peak m1 conditions of 900 k and 960 mpa roughly 36 km depth before following an erosion controlled uplift path e g england and richardson 1977 m2 metamorphic temperatures in the upper part of the sequence also exceeded 900 k but maximum pressures 317 523 mpa indicate paleodepths of only 12 19 km calculated pressure temperature paths indicate that m2 was characterized by temperature increases of 80 k and roughly 5 km of tectonic burial we attribute m1 to tectonic burial of the greater himalayan sequence during the early stages of india eurasia collision we believe that the uplift and cooling path of the sequence was interrupted in late oligocene miocene time by a second burial and heating event m2 related to thrust imbrications in southern tibet this burial was coincident with the generation of leucogranites which are abundant near the top of the greater himalayan sequence but are virtually absent near the mct field relations do not constrain whether the leucogranites were derived from some presently unexposed portion of the greater himalayan sequence and were injected at their present structural level or were melted in situ if the granites were injected then they may have provided some of the heat necessary for m2 metamorphism although our data suggest no direct relationship between the main central thrust as mapped in garhwal and metamorphism in the greater himalaya anatectic melting of an unexposed portion of the greater himalayan sequence could have been associated with movement along a blind thrust with characteristics similar to the mapped mct in central nepal cf le fort 1981 if the granites were produced by in situ m2 melting then we must appeal to a heat source within the upper part of the greater himalayan sequence such as locally high concentrations of heat producing elements cf pinet and jaupart 1987

effusive and explosive volcanism on the ultraslow spreading gakkel ridge 85 e 1 we use high definition seafloor digital imagery and multibeam bathymetric data acquired during the 2007 arctic gakkel vents expedition agave to evaluate the volcanic characteristics of the 85 e segment of the ultraslow spreading gakkel ridge 9 mm yr 1full rate our seafloor imagery reveals that the axial valley is covered by numerous small volume order 1000 m3 lava flows displaying a range of ages and morphologies as well as unconsolidated volcaniclastic deposits with thicknesses up to 10 cm the valley floor contains two prominent volcanic lineaments made up of axis parallel ridges and small cratered volcanic cones the lava flows appear to have erupted from a number of distinct source vents within the 12 15 km wide axial valley only a few of these flows are fresh enough to have potentially erupted during the 1999 seismic swarm at this site and these are associated with the oden and loke volcanic cones we model the widespread volcaniclastic deposits we observed on the seafloor as having been generated by the explosive discharge of co2 that accumulated in possibly deep crustal melt reservoirs the energy released during explosive discharge combined with the buoyant rise of hot fluid lofted fragmented clasts of rapidly cooling magma into the water column and they subsequently settled onto the seafloor as fall deposits surrounding the source vent

magma water interactions in subaqueous and emergent basaltic in the subaqueous growth and emergence of a basaltic volcano clasts are formed by one or a combination of 1 explosive release of magmatic volatiles 2 explosive expansion and collapse of steam formed at magma water contact surfaces 3 explosive expansion of steam following enclosure of water in magma or entrapment of water close to magma and 4 cooling contraction these processes named respectivelymagmatic explosivity contact surface steam explosivity bulk interaction steam explosivity andcooling contraction granulation can be enhanced by mutual interaction and feedback the first three explosive processes are limited at certain water depths hydrostatic pressures and become increasingly vigorous at shallower levels the depth of onset of magmatic explosivity depends largely on juvenile volatile content it is up to 200 m for tholeiitic magmas and up to 1 km for alkalic magmas at the depth where formation of clastic deposits becomes predominant over effusion of lavas magmatic explosivity is subordinate to steam explosivity as a clast forming process the upward transition to accumulation of dominantly clastic deposits is not simply related to the onset of substantial exsolution of magmatic volatiles and can occur without it contact surface explosivity commonly requires initiation by a vigorous impact between magma and water and although no certain depth limit is known likelihood of such explosivity decreases rapidly with depth clast generation by bulk interaction explosivity appears to be restricted to depths much shallower than that of the critical pressure of water which in sea water is at about 3 km cooling contraction granulation can occur in any depth of water but at shallow levels may be replaced by contact surface explosivity during continuous eruption under water tephra can be ejected and deposited within a cupola of steam such that rapid quenching does not occur emergent volcanoes are characterized by distinctive steam explosive activity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry of clastic material water and steam the water gets into the vent by flooding across or through the top of the tephra pile and violent explosions cease when this access is sealed the eruptions during emergence of surtsey and capelinhos typify the distinctive explosive activity the style and controls of which are different from those of maar volcanoes

seismites in a proterozoic tidal succession singhbhum bihar india abstract early proterozoic metasediments of the chaibasa formation galudih ghatsila dhalbhumgarh region singhbhum bihar india comprise a number of cyclic fining upward prograding successions of tidalites the tidalites show indications for earthquakes in the form of synsedimentary deformation features apart from the structures due to high energy wave action deformed cross bedding convolute laminations synsedimentary faults graben like structures sandstone dykes pseudonodules and slump folds record the seismic activity a gradual decline in the frequency of seismites and tsunami related depositional features in combination with an upward increase in thickness of the tidal cycles are attributed to gradual diminishing of tectonic activity within the basin

seismically induced soft sediment deformation structures seismites around sumdo in the lower spiti valley tethys himalaya abstract soft sediment deformation structures were observed in 120 m thick fluvio lacustrine sediments exposed in the sumdo area in the lower spiti valley tethys himalaya himachal pradesh india these structures vary in their morphology and pattern and occur at eight stratigraphic levels s1 s8 spaced at irregular intervals the deformation structures include over steepened stratification diapiric flame like intrusions of clayey material or complex type recumbent folding of flame like nature pseudonodules large scale soft sediment folding faulting small scale associated with folding and sand dykes these structures are restricted to single stratigraphic layers separated by undeformed beds and can be correlated over a large distance the deformation of the soft sediments is attributed to the combined effects of fluidization liquefaction and shear stress and can be related to past seismic activity in the area recently on 19 january 1975 a major earthquake kinnaur earthquake of magnitude 6 8 caused extensive damage including ground rupturing in the sumdo kaurik area this earthquake has been related to the activation of the kaurik chango fault the deformed layers seismites reported here lie adjacent to this fault zone and can be related with episodic seismicity along this fault it is evident from this study that the kaurik chango fault has been re activated a number of times and that the area suffered at least nine major earthquakes with a magnitude of more than 6 during late pleistocene holocene time including the 1975 kinnaur earthquake

recognition of trigger mechanisms for soft sediment deformation in the pleistocene lacustrine deposits of the santʻarcangelo basin southern italy seismic shock vs overloading abstract the lacustrine deposits of the san lorenzo cycle belong to the infill succession of the sant arcangelo basin a pliocene to pleistocene satellite basin developed close to the front of the south apennines thrust belt the lower to middle pleistocene lacustrine succession is made up of siltstone and claystone interbedded with sandstone carbonate and volcaniclastic beds arranged in fining upward sequences the upper to middle part about 50 m thick of the succession about 200 m thick has been investigated in detail in the depocentral sector of the lacustrine basin here soft sediment deformation structures occur in fine grained sandstone and claystone alternations and show a wide morphological variability deformed laminations slumps load structures large vertical water escape structures and neptunian dykes their formation occurred during and after sedimentation with different mechanisms of deformation some structures are related to liquefaction and fluidization processes viscous fluid behaviour while others occurred when sediment had already undergone lithification and its behaviour was plastic and or brittle facies analysis and detailed morphologic study of the soft sediment deformation structures indicate that the main trigger agents for deformation were seismic shocks and overloading induced by sudden deposition of coarser sediments on clays due to the arrival of density currents thus the soft sediment deformation structures provide a continuous record of the tectonic and sedimentary processes that acted in the lacustrine basin from sedimentation until diagenesis

climate and seismicity in the higher central himalaya during 20 10 ka evidence from the garbayang basin uttaranchal india abstract in the trans himalaya the trans himadri fault thf controlled the extent of past glaciers the rivers were impounded by glacial debris resulting in the formation of lakes that recorded regional climatic and seismic events one such lake formed at garbayang where the upper 28 m lacustrine succession spanned an age range of 20 10 ka magnetic susceptibility and sedimentological data suggest that the region experienced high amplitude low frequency millennial scale climate changes during the period 20 3 and 13 2 ka followed by low amplitude high frequency changes from 13 2 11 1 ka a sudden drop in magnetic susceptibility and elemental concentration of layers dated at 11 12 ka suggests cooling associated with the younger dryas occurrence of faulted strata seismites during 20 17 and 14 13 ka suggests recent activation of the thf this is noteworthy as the thf has been considered dormant since its last reactivation at 11 ma further the luminescence dates suggest that radiocarbon ages on organic carbon are grossly overestimated due to a hard water effect caused by the dominance of limestone in the lake catchment this implies that earlier radiocarbon based palaeoenvironmental reconstruction in the region will need revalidation

dynamics of the north brazil current retroflection region from the western tropical atlantic experiment observations hydrographic and velocity observations of the north brazil current nbc retroflection region during the 1990 1991 western tropical atlantic experiment westrax are examined with the intent of extracting dynamical information about the nbc eddy shedding a comparison is performed between the depth structure of empirical orthogonal functions and the dynamical normal modes of the nbc retroflection region on a beta plane centered on 5 n the barotropic and first two baroclinic modes account for about 75 of the vertical structure of the nbc flow thus a three layer model of the nbc region is suitable for dynamical studies of the nbc retroflection in terms of flow structure the upper layer represents the retroflection of the surface layers of the nbc to feed the north equatorial countercurrent the middle layer represents the separating subthermocline waters of the nbc which feed the north equatorial undercurrent the third layer represents a weak meandering flow that may be thought of as the deep western boundary current signature in the three layer ocean in terms of pv a well defined front separates the nbc waters from the north equatorial current in the upper layer both upper and middle layers present closed pv contours associated with the eddy in the process of pinching off from the retroflecting nbc the nbc separation region although equatorial complies reasonably well with the basic quasigeostrophic qg assumptions therefore qg methods are applied to investigate the nbc meander growth by isolating the effect of pv anomalies in each of the layers on each of the layers baroclinic growth is verified to occur during the nbc eddy shedding

the growth and propagation of synsedimentary faults abstract offset of each syn fault horizon across a synsedimentary i e growth fault records the throw subsequent to its deposition allowing successive growth stages of a fault to be determined this technique has been applied to synsedimentary faults mapped on 3d seismic datasets to distinguish between faults which propagated laterally during growth faults which did not propagate laterally for significant growth periods and faults in which the lateral dimensions of the active surface decreased during their growth these different fault tip line behaviours result in characteristic lateral and dip parallel fault displacement variations fault propagation rates have been measured for 10 fault tips the maximum propagation rate is 15 km my for gravity driven faults and 3 km my for tectonically driven faults rates of fault propagation decrease with elapsed time from the initiation of faulting with tip line stasis and retreat becoming dominant in the later stages of fault growth static and retreating lateral tip lines are associated with overlap of the fault strain field with that of a neighbouring fault most often with the formation of a fault relay zone

chronology of tectonic events in the crystalline core of the himalaya langtang national park central nepal the main central thrust mct is an important intracontinental subduction zone that accommodated a significant proportion of shortening between india and asia during tertiary himalayan orogenesis argon 40 argon 39 geochronology indicates at least two distinct periods of thrust movement on the mct in the langtang national park region of central nepal ductile deformation and associated mylonitization characterizing the earlier event are constrained by 40ar 39ar dating of muscovites to have occurred sometime before 5 8 ma a later period of brittle deformation resulting in the juxtaposition of rocks of different lithology within the mct zone occurred at approximately 2 3 ma based on 40ar 39ar dating of neoblastic muscovites from the brittle fault zones the hanging wall of the mct contains amphibolite to upper amphibolite grade gneisses and small leucogranite bodies assigned to the greater himalayan sequence argon 40 argon 39 cooling ages of muscovite and biotite from the gneisses range from 4 6 to 9 7 ma these dates contrast with previously obtained 16 21 ma u pb monazite and zircon ages for metasedimentary rocks from the same structural levels parrish et al 1992 indicating relatively slow cooling over the early to middle miocene interval for much of the mct hanging wall however one 19 3 ma biotite 40ar 39ar cooling age for a sample from the uppermost portion of the hanging wall is only slightly younger than u pb monazite ages for nearby anatexites possibly suggesting rapid cooling of the uppermost greater himalayan sequence by tectonic denudation associated with the structurally higher south tibetan detachment system the general consistency of 40ar 39ar ages throughout the 11 km thick greater himalayan sequence suggests rapid cooling in late miocene time probably due to an increase in erosion rate related to ramping on the structurally lower main boundary thrust an alternative possibility would be massive hydrothermal resetting in late miocene time of the entire sequence

chlorophyll variability and eddies in the brazil malvinas confluence region abstract ocean color data from sea viewing wide field of view sensor seawifs have been used to investigate temporal and spatial variability of chlorophyll a concentration in the brazil malvinas confluence bmc region 30 50 s and 30 70 w our analysis is based on 60 monthly averaged and 230 weekly averaged images 9 9 km 2 resolution that span from october 1997 to september 2002 in the southwestern atlantic ocean a nonlinear model was used to fit the annual harmonic of the chlorophyll concentration anomalies in the region analysis of the spatial variability of model parameters has shown the existence of a marked annual cycle at several locations in the study region we also have observed shorter period oscillating features on smaller spatial scale associated with the bmc dynamics these mesoscale features have periods of about 9 12 weeks and have a marked northward propagation with higher chlorophyll values relative to the surrounding waters further investigation of these mesoscale features with an advanced very high resolution radiometer thermal infrared and topex poseidon t p altimeter data has unveiled interesting eddy like surface structures in the bmc region these multiple eddies are clearly visible on both thermal and color imagery off southern brazil furthermore the seawifs images have shown a decrease in chlorophyll a concentration as these eddies propagate northward due to mixing with chlorophyll poor tropical waters carried southward by the brazil current several rings along the shelf region which are probably generated by shear instabilities also have been detected by the ocean color images

hyperquenched volcanic glass from loihi seamount hawaii abstract explosive submarine basaltic eruptions occurring in water depths of several kilometres commonly result in the formation of layered volcaniclastic deposits in order to quantify the cooling of such volcaniclastics we have identified and separated two types of glassy fragments from these deposits on loihi seamount 1 fine 30 80 m limu o pele bubble wall fragments and 2 coarser 0 8 1 2 mm dense angular fragments these pristine basaltic glasses have been subjected to differential scanning calorimetry dsc from room temperature up to temperatures above their glass transition heat capacity c p in j g 1 k 1 data reveal glassy and liquid regimes separated by clear hysteresis behaviour within the glass transition the transient c p in the glass transition interval exhibits a deep trough before the peak indicating very high cooling rates it is a classic feature of so called hyper quenching having been observed in dsc experiments on glasses produced using the splat quench d b dingwell p courtial d giordano and a r l nichols viscosity of peridotite liquid earth and planetary science letters 226 1 2 127 138 2004 and cascade fibre spinning y z yue j d christiansen and s l jensen determination of the fictive temperature for a hyperquenched glass chemical physics letters 357 1 2 20 24 2002 techniques the trough is deepest for the limu o pele fragments energy matching methods developed for the estimation of cooling rates for such glasses yield a rate of 10 5 31 k s 1 for the cooling of the fine sieve fraction of limu glass on the sea floor at loihi thus limu o pele volcaniclastics believed to be formed during mild submarine pyroclastic eruptions have experienced the fastest cooling of any natural volcanic glass measured to date such extreme cooling rates are likely to impact on many chemical and physical aspects of the stability of these glasses on the sea floor as well as their use as proxies for field variables of the earth s physical state and as monitors of the efficiency of chemical lithosphere hydrosphere exchange

seismotectonics of the great 1905 kangra earthquake meizoseismal region in kangra chamba nw himalaya abstract three great earthquakes with magnitude greater than 8 have struck the himalayan belt within a span of 50 years and the 1905 kangra earthquake is one of them the seismotectonics of the meizoseismal region of this earthquake in the kangra chamba region are analyzed on the basis of the major structures and the present day seismic activity the microseismicity recorded by a network of 12 stations for a period of 16 months spanning 4 years 1993 1996 shows the activity is mainly concentrated over the main boundary thrust mbt and panjal thrust pt zones and the chamba nappe region a ne sw vertical section reveals that the majority of events are concentrated within a depth range of 5 to 18 km and the majority of the foci lie between 12 and 18 km the base of the microseismicity zone is interpreted as representing a contact or a decollement plane between the top of the indian plate and the himalayan rock formations this decollement plane is interpreted as the northern continuation of the detachment observed in the sub himalaya in reflection profile the southward propagation of the chamba nappe as well as the pt and mbt zones over the detachment causes accumulation of strain energy in this region

why does the north brazil current regularly shed rings but the brazil current does not abstract both the north brazil current nbc and the brazil current bc are western boundary currents wbcs that separate from the western atlantic coast the nbc retroflects and sheds several rings per year at the retroflection region whereas the bc rarely sheds rings near its separation point traditionally the difference between these two wbcs has been attributed to the malvinas current mc whose momentum flux opposes the poleward momentum flux of the bc thus preventing rings shedding at the point where the current leaves the coast even in the absence of the mc rings from the separating bc would have never been regularly generated because of the relatively large slant of the coastline relative to the zonal direction using the recently proposed theory of zharkov and nof it is demonstrated that the large inclination of the coastline between 20 and 45 s approximately 50 lies within the regime that does not allow the bc a continuous shedding of rings in contrast the inclination of the co

use of liquefaction induced features for paleoseismic analysis an overview of how seismic liquefaction features can be distinguished from other features and how their regional distribution and properties of source sediment can be used to infer the location and strength of holocene paleo earthquakes abstract liquefaction features can be used in many field settings to estimate the recurrence interval and magnitude of strong earthquakes through much of the holocene these features include dikes craters vented sand sills and laterally spreading landslides the relatively high seismic shaking level required for their formation makes them particularly valuable as records of strong paleo earthquakes this state of the art summary for using liquefaction induced features for paleoseismic interpretation and analysis takes into account both geological and geotechnical engineering perspectives the driving mechanism for formation of the features is primarily the increased pore water pressure associated with liquefaction of sand rich sediment the role of this mechanism is often supplemented greatly by the direct action of seismic shaking at the ground surface which strains and breaks the clay rich cap that lies immediately above the sediment that liquefied discussed in the text are the processes involved in formation of the features as well as their morphology and characteristics in field settings whether liquefaction occurs is controlled mainly by sediment grain size sediment packing depth to the water table and strength and duration of seismic shaking formation of recognizable features in the field generally requires a low permeability cap above the sediment that liquefied field manifestations are controlled largely by the severity of liquefaction and the thickness and properties of the low permeability cap criteria are presented for determining whether observed sediment deformation in the field originated by seismically induced liquefaction these criteria have been developed mainly by observing historic effects of liquefaction in varied field settings the most important criterion is that a seismic liquefaction origin requires widespread regional development of features around a core area where the effects are most severe in addition the features must have a morphology that is consistent with a very sudden application of a large hydraulic force this article discusses case studies in widely separated and different geological settings coastal south carolina the new madrid seismic zone the wabash valley seismic zone and coastal washington state these studies encompass most of the range of settings and the types of liquefaction induced features likely to be encountered anywhere the case studies describe the observed features and the logic for assigning a seismic liquefaction origin to them also discussed are some types of sediment deformations that can be misinterpreted as having a seismic origin two independent methods for estimating prehistoric magnitude are discussed briefly one method is based on determination of the maximum distance from the epicenter over which liquefaction induced effects have formed the other method is based on use of geotechnical engineering techniques at sites of marginal liquefaction in order to bracket the peak accelerations as a function of epicentral distance these accelerations can then be compared with predictions from seismological models

downstream development of the gulf stream from 68 to 55 w abstract two ctd sections across the gulf stream at 68 and 55 w were acquired in late march of 1988 within 11 days of one another as part of an effort to look at downstream changes in the current using complementary current meter measurements sections of total barotropic and geostrophic baroclinic velocity are constructed and used to calculate transport in potential density classes potential vorticity sections are presented for both locations including the effects of planetary stretching and relative vorticity the data are also used to examine the core properties of recently formed 18 water at the two sections it is found that 1 water parcels in the exposed surface layers experience downstream density and potential vorticity changes consistent with surface forcing 2 thermocline gulf stream transport is conserved downstream and below the exposed layers is conserved within individual density classes 3 subthermocline gulf stream transport increases modestly at levels above the sill depth of th

age of initiation of collision between india and asia a review of stratigraphic data abstract the collision of india with asia is perhaps the most profound tectonic event to have occurred in past 100 ma it is responsible for the uplift of the himalayas and tibetan plateau and has been argued to have been responsible for geological geochemical and climatological consequences of global extent yet the age of initiation of this collision remains poorly constrained the literature is replete with estimates that range from the late cretaceous 65 ma to latest eocene

paleontological view of the ages of the deccan traps the cretaceous tertiary boundary and the india asia collision volcanism in the deccan traps of india occurred over at most three magnetic chrons centered on a main reversed chron paleontological data indicate that this reversed chron must coincide with 29r which contains the cretaceous tertiary k t boundary recent 40 ar 39 ar data may therefore provide one of the best estimates of the absolute age of the k t boundary 65 7 2 0 ma moreover the eurasian character of k t boundary terrestrial faunas in india suggests that this was also close to the time of india asia collision this is significantly earlier than generally recognized and implies about 4000 km of continental shortening in the collision zone probably absorbed jointly by crustal thickening and strike slip extrusion

no depth limit to hydrovolcanic limu o pele analysis of limu from lō ihi seamount hawai i deep sea limu o pele are shards of basaltic glass commonly described as bubble walls when first identified they were inferred to form in submarine fire fountains but were then reinterpreted as the products of hydrovolcanic volcanism formed when submarine lava flows entrapped and vaporised seawater limu discovered below the c 3 km critical depth of seawater where superheated water exists as a supercritical fluid instead of a vapour led to the hydrovolcanic model of limu o pele formation being discarded in favour of a magmatic co2 driven strombolian like model this revised magmatic mechanism has been widely accepted by the scientific community we describe a newly discovered limu o pele rich deposit at 1 052 mbsl on the northeast summit plateau region of l ihi seamount hawai i the limu at this site is concentrated in a chemically monomict ash lens interbedded with thin lava sheets that are separated from overlying volcaniclastic material by a discontinuity the geometry and geochemistry of the deposit provide compelling evidence for a hydrovolcanic sheet flow related origin the exceptional abundance and preservation of limu at this site allows 4 morphologic subtypes of limu thin film plateau border convex film and pele s hair to be identified and linked to portions of the isolated rupturing bubbles from which they are derived we extend our discussion to beyond this new l ihi deposit by including a review of limu o pele occurrences and thermodynamic considerations that demonstrate the hydrovolcanic model of limu formation to be more tenable than the magmatic model at all depths including below the critical depth of seawater

constraints on himalayan deformation inferred from vertical velocity fields in nepal and tibet spirit leveling data from the nepal himalaya between 1977 and 1990 indicate localized uplift at 2 3 mm yr in the lesser himalaya with spatial wavelengths of 25 35 km and at 4 6 mm yr in the greater himalaya with a wavelength of 40 km leveling data with significantly sparser spatial sampling in southern tibet between 1959 and 1981 suggest that the himalayan divide may be rising at a rate of 7 5 5 6 mm yr relative to central tibet we use two dimensional dislocation modeling methods to examine a number of structural models that yield vertical velocity fields similar to those observed although these models are structurally nonunique dislocation models that satisfy the data require aseismic slip rates of 2 7 mm yr on shallow dipping faults beneath the lesser himalaya and rates of 9 18 mm yr on deep thrust faults dipping at 25 n near the greater himalaya unfortunately the leveling data cannot constrain long wavelength uplift 100 km across the himalaya and unequivocal estimates of aseismic slip in central nepal are therefore not possible in turn the poor spatial density of leveling data in southern tibet may inadequately sample the processes responsible for the uplift of the greater himalaya despite these shortcomings in the leveling data the pattern of uplift is consistent with a crustal scale ramp near the greater himalaya linking shallow northward dipping thrust planes 3 6 beneath the lesser himalaya and southern tibet aseismic slip on the potential rupture surface of future great earthquakes beneath the nepal himalaya south of this ramp appears not to exceed 30 of the total convergence rate between india and southern tibet resulting in an accumulating slip deficit of 13 8 mm yr

soft sediment deformation structures interpreted as seismites in lacustrine sediments of the prebetic zone se spain and their potential use as indicators of earthquake magnitudes during the late miocene lacustrine formations of late miocene age in the prebetic area se spain show several types of deformational structures that are interpreted as seismites they are present in both marginal and deep lacustrine facies seismites formed in marginal lake environments comprise sand dikes pillows and intruded and fractured gravels in deep lacustrine facies the seismites are represented by pseudonodules mushroom like silts protruding into laminites mixed layers disturbed varved lamination and loop bedding the measured orientations of these structures are consistent with the orientations of the main faults limiting the basins showing that their origin is clearly related to the tectonic stress field that prevailed in the region during the late miocene the magnitudes of the earthquakes that deformed the sediments have been estimated after published data from both ancient and recent lake deposits accumulated in tectonic active regions elsewhere a rank of earthquake magnitudes with two end members i e the lowest magnitudes recorded by loop bedding in laminites and the highest magnitudes represented by intruded and fractured gravels is proposed

accumulation rates of volcaniclastic deposits on loihi seamount hawaii ams radiocarbon age dating of planktonic foraminifera in volcaniclastic deposits on loihi seamount yields ages ranging from 590 years before present y bp at 10 cm depth to 5 880 y bp at 1 007 cm depth in an 11 m thick section exposed along inward facing caldera bounding faults on the eastern side of loihi s summit the accumulation rate of the deposit was about 0 37 cm y from 5 880 to 3 300 y bp and it consisted of subequal amounts of alkalic and tholeiitic fragments the rate slowed dramatically at about 3 300 y bp to an average 0 04 cm y and the particles that have accumulated since consist mostly of alkalic glass fragments the decrease in accumulation rate could indicate a decrease in volcanic activity at loihi beginning about 3 300 y bp this lower level of activity appears to be continuing today

earthquake induced soft sediment deformation structures in late pleistocene lacustrine deposits of issyk kul lake kyrgyzstan abstract this paper discusses the composition and distribution of soft sediment deformation structures induced by liquefaction in late pleistocene lacustrine terrace deposits on the southern shore of issyk kul lake in the northern tien shan mountains of kyrgyzstan the section contains seven deformed beds grouped in two intervals five deformed beds in the upper interval contain load structures load casts and flame structures convolute lamination ball and pillow structures folds and slumps deformation patterns indicate that a seismic trigger generated a multiple slump on a gentle slope the dating of overlying subaerial deposits suggests correlation between the deformation features and strong earthquakes in the late pleistocene

lava bubble wall fragments formed by submarine hydrovolcanic explosions on lō ihi seamount and kīlauea volcano glassy bubble wall fragments morphologically similar to littoral limu o pele have been found in volcanic sands erupted on l ihi seamount and along the submarine east rift zone of k lauea volcano the limu o pele fragments are undegassed with respect to h2o and s and formed by mild steam explosions angular glass sand fragments apparently form at similar and greater depths by cooling contraction granulation the limu o pele fragments from l ihi seamount are dominantly tholeiitic basalt containing 6 25 7 25 mgo none of the limu o pele samples from l ihi seamount contains less than 5 57 mgo suggesting that higher viscosity magmas do not form lava bubbles the dissolved co2 and h2o contents of 7 of the limu o pele fragments indicate eruption at 1200 300 m depth 120 30 bar these pressures exceed that generally thought to limit steam explosions we conclude that hydrovolcanic eruptions are possible with appropriate pre mixing conditions at pressures as great as 120 bar