the topography and morphology of low shields and associated landforms of plains volcanism in the tharsis region of mars abstract the morphology and topography of volcanic landforms provide critical information to the investigation of their tectonic setting and the physical characteristics e g rheology of their eruption products their investigation is also an important prerequisite for studies of comparative planetology e g the comparison between surface features of the earth and other planetary bodies numerous small and low shield volcanoes on mars and associated vents and lava flows have previously been compared to terrestrial plains style volcanism which is defined as being an intermediate style between flood basalts and the hawai ian shields this study investigates the topography and morphology of martian landforms associated with plains volcanism using mola moc themis hrsc and hirise data low shields and other landforms of plains volcanism on mars display similarities with terrestrial basaltic volcanic fields and we do not observe any features that do not have morphologic analogues on earth the most typical landforms of martian plains style volcanism are low shields defined as volcanic shields with diameters of typically less than 50 km heights of a few hundred meters only and extremely shallow flank slopes of less than 0 5 other surface features related to plains style volcanism on mars are craters fissure vents cinder and spatter cones lava flows that are commonly associated with lava channels and tubes lava inflation features and volcanic rift zones our results reconfirm the viking orbiter based conclusion that plains volcanism in the eastern snake river plains is perhaps the best terrestrial morphological analogue for these martian surface features icelandic shields distinct structures in hawai i and other basaltic landforms also show similarities to martian plains volcanism sinuous rilles previously not described in association with plains volcanism on mars are interpreted as evidence for high eruption rates the extremely shallow flank slopes of the low shields suggest the eruption of shield building lavas with low viscosity which might be the result of high eruption temperatures high effusion rates a low si and a high mg content along with a possible high fe content or a combination of these factors the spatial distribution of low shield clusters in tharsis does not show any obvious association with large scale tectonic features plains volcanism might represent a relatively recent type of volcanism on mars which is not related to mantle plumes but to a zone of partial melting in an anomalously warm mantle underneath a thickened crust schumacher and breuer geophys res lett 34 l14202 doi 10 1029 2007gl030083 2007

implication of seismic attenuation for gas hydrate resource characterization mallik mackenzie delta canada wave attenuation is an important physical property of hydrate bearing sediments that is rarely taken into account in site characterization with seismic data we present a field example showing improved images of hydrate bearing sediments on seismic data after compensation of attenuation effects compressional quality factors estimated from zero offset vertical seismic profiling data acquired at mallik northwest territories canada demonstrate significant wave attenuation for hydrate bearing sediments these results are in agreement with previous attenuation estimates obtained from sonic logs and crosshole data at different frequency intervals the application of an inverse q filter to compensate attenuation effects of permafrost and hydrate bearing sediments improved the resolution of surface 3d seismic data and its correlation with log data particularly for the shallowest gas hydrate interval compensation of the attenuation effects of the permafrost likely explains most of the improvements for the shallow gas hydrate zone our results show that characterization of the mallik gas hydrates with seismic data not corrected for attenuation would tend to overestimate thicknesses and lateral extent of hydrate bearing strata and hence the volume of hydrates in place

attenuation measurements from sonic waveform logs in methane hydrate bearing sediments at the nankai trough exploratory well off tokai central japan 1 i have used full waveform logs from the nankai trough exploratory well off central japan to estimate both compressional and shear attenuation in sediments containing methane hydrate mh the attenuation estimates are based on a median frequency shift to the amplitude spectrum of the recorded waveforms this paper is concerned with attenuation at sonic frequencies of 10 20 khz for compressional waves and 500 1000 hz for shear waves i observed that the presence of mh increases the host sediments seismic attenuation and shear attenuation may be more helpful than compressional attenuation in detecting or characterizing mh bearing sediments as compressional attenuation is affected by the presence of gas moreover the ratio of compressional to shear attenuation is found to be a more sensitive indicator of the presence of low saturation gas than the corresponding velocity ratio

recent glaciation at high elevations on arsia mons mars implications for the formation and evolution of large tropical mountain glaciers 1 the 166 000 km2 fan shaped deposit at arsia mons contains three characteristic facies ridged knobby and smooth which are interpreted as the depositional remains of a cold based glacier that was present on the west northwestern flanks in the late amazonian period of mars history here we consider several high elevation graben on the western flank of arsia mons that are interpreted as the source regions for late stage cold based glaciers that overflowed graben walls advanced tens to hundreds of kilometers downslope experienced subsequent retreat and left distinctive depositional features similar to those associated with cold based glaciers in the dry valleys of antarctica these new observations provide additional support for a cold based glacial interpretation of the tharsis montes fan shaped deposits morphological evidence suggests that the largest of these graben was the primary source region for the most proximal smooth facies lobes at arsia mons and map plane ice sheet reconstructions are consistent with these observations this relationship has significant implications for the relative ages of the individual facies within the fan shaped deposit including the possibility that the smooth facies represents several distinct phases of glaciation the high relief and large elevation differences between the upslope and downslope walls of these graben appear to be critical factors for ice accumulation and the generation collection of rockfall within glacial accumulation zones we suggest that rockfall was the dominant process responsible for debris cover formation on the glaciers sourced within these graben and consider the implications for the entire arsia fan shaped deposit new crater count data suggest that the fill material in the large arsia graben formed within the past 100 myr model age of 65 ma taken together these new observations provide additional evidence for several periods of late amazonian climate change on mars within the past few 100 myr resulting in episodes of tropical mountain glaciation finally mola topography reveals that several lobate features interpreted as remnant debris covered ice from the most recent phase of glaciation are presently hundreds of meters thick suggesting the possibility of long term near surface water ice survival in the equatorial regions of mars

centennial scale asian monsoon variability during the mid younger dryas from qingtian cave central china the regional climate correlation within the northern hemisphere in the cold dry mid younger dryas event yd remains elusive a key to unraveling this issue is sufficient knowledge of the detailed climate variability at the low latitudes here we present a high resolution 3 yr 18o record of an annually laminated stalagmite from central china that reveals a detailed asian monsoon am history from 13 36 to 10 99 ka the yd in this record is expressed as three phases characterized by gradual onsets but rapid ends during the mid yd the am variability exhibited an increasing trend superimposed by three centennial oscillations well correlated to changes in greenland temperatures these warming wetting fluctuations show a periodicity of 200 yr generally in agreement with centennial changes in cosmogenic nuclides indicated by the 10be flux from the greenland ice this relationship implies that centennial scale climate changes during the mid yd are probably caused by solar output and rapidly transported over broad regions through atmosphere reorganization

sonic waveform attenuation in gas hydrate bearing sediments from the mallik 2l 38 research well mackenzie delta canada 1 the mallik 2l 38 research well was drilled to 1150 m under the mackenzie delta canada and penetrated a subpermafrost interval where methane hydrate occupies up to 80 of the pore space a suite of high quality downhole logs was acquired to measure in situ the physical properties of these hydrate bearing sediments similar to other hydrate deposits resistivity and compressional and shear sonic velocity data increase with higher hydrate saturation owing to electrical insulation of the pore space and stiffening of the sediment framework in addition sonic waveforms show strong amplitude losses of both compressional and shear waves in intervals where methane hydrate is observed we use monopole and dipole waveforms to estimate compressional and shear attenuation comparing with hydrate saturation values derived from the resistivity log we observe a linear increase in both attenuation measurements with increasing hydrate saturation which is not intuitive for stiffening sediments numerical modeling of the waveforms allows us to reproduce the recorded waveforms and illustrate these results we also use a model for wave propagation in frozen porous media to explain qualitatively the loss of sonic waveform amplitude in hydrate bearing sediments we suggest that this model can be improved and extended allowing hydrate saturation to be quantified from attenuation measurements in similar environments and providing new insight into how hydrate and its sediment host interact

does shallow dike intrusion and widening remain a possible mechanism for graben formation on mars shallow dike intrusion and widening was proposed several decades ago as a cause of surface faulting and graben formation on earth this hypothesis was subsequently applied to the spectacular linear and or radial graben systems visible on mars however a recent study has suggested that shallow dike intrusion on mars results in contractional folding and uplift adjacent to dike walls rather than extensional faulting and subsidence above the dike tip even in the presence of concurrent regional extension here discrete element numerical modeling is used to re examine the typical style of deformation above the shallow tips of widening dikes on mars the dikes are embedded in a frictional cohesive material representing the martian crust in the experiments presented here subsidence and extensional faulting graben formation are produced above the dike tips even with modest amounts of widening for suggested depths to dike tips on mars an overlying graben is produced in all cases no upright detachment style folds are produced results indicate that dike widening does indeed remain a possible mechanism for graben formation on mars implications for the interpretation of deformation associated with shallow dike intrusion on earth and distant planetary surfaces are discussed

dike induced faulting in rift zones of iceland and afar geodetic data and field observations demonstrate that the emplacement of dikes in volcanic rift zones frequently generates normal faulting and graben subsidence at the earth9s surface elastic modeling of the vertical ground surface displacements above dikes and faults indicates that the extent of graben subsidence can be achieved only if fault slip extends virtually to or beyond the dike plane at depth a mechanical model that includes dikes and frictional faults shows that dike opening tends to compress and lock faults located to either side of the dike therefore slip extending into or beyond the dike cavity must occur either 1 on faults that intersect the dike near its top above the zone of dike induced compression or 2 on faults that slip ahead of the dike as it propagates laterally data from iceland indicate that slip occurred on deep faults that presumably slipped in advance of the laterally propagating dike

sedimentation stratigraphy and petroleum potential of krishna godavari basin east coast of india the krishna godavari basin is located in the central part of the eastern passive continental margin of india the structural grain of the basin is northeast southwest exposures of upper cretaceous sedimentary rocks demarcate the basin margin toward the north west where the northwest southeast trending pranahita godavari graben abuts the basin the basin contains thick sequences of sediments with several cycles of deposition ranging in age from late carboniferous to holocene a major delta with a thick argillaceous facies that has prograded seaward since the late cretaceous is a hydrocarbon exploration target magnetic and gravity data predicted the basin architecture which was subsequently confirmed by a multichannel seismic sur vey the basin is divided into subbasins by fault controlled ridges sediments accumulated in subbasins more than 5 km thick above the basement ridges thin sediments are found until the jurassic period sediments were deposited in the rift valley and in topo graphic lows this sequence is completely overlain by a lower cretaceous transgressive sedimentary wedge later continued delta progradation characterized basin sedimentation with an areal extent of approximately 45 000 km2 this proven petroliferous basin has potential reservoirs ranging in age from the permian to the pliocene exploratory drilling of more than 350 wells in more than 160 structures has resulted in the discovery of 42 oil and gas bearing structures good source rocks are known from sequences ranging in age from permian carboniferous to early miocene because the reservoir sand bodies have limited lateral variation understanding the stratigraphy and depositional sub environments in different sequences is essential to decipher the favorable locales for reservoir sands tilted fault blocks growth faults and related rollover anticlines provide the structural traps begin page 1624

evidence of paleo cold seep activity from the bay of bengal offshore india 1 we report evidence of paleo cold seep associated activities preserved in methane derived carbonates in association with chemosynthetic clams calyptogena sp from a sediment core in the krishna godavari basin bay of bengal visual observations and calculations based on high resolution wet bulk density profile of a core collected on board r v marion dufresne may 2007 show zones of sharp increase in carbonate content 10 55 vol within 16 20 meters below seafloor mbsf the presence of calyptogena clam shells chimneys shell breccias with high mg calcite cement and pyrite within this zone suggest seepage of methane and sulfide bearing fluid to the seafloor in the past highly depleted carbon isotopic values 13c ranges from 41 to 52 vpdb from these carbonates indicate carbon derived via anaerobic oxidation of methane extrapolated mean calendar age 58 7 ka b p of the clastic sediments at a depth of 16 mbsf is close to the upper limit of the u th based depositional age 46 2 3 7 and 53 0 1 6 ka of authigenic carbonates sampled from this level thereby constraining the younger age limit of the carbonate deposition methane expulsion events the observed carbonate deposition might have resulted from the flow of methane enriched fluids through the fracture network formed because of shale diapirism

basin architecture and petroleum system of krishna godavari basin east coast of india krishna godavari basin is a peri cratonic passive margin basin on the east coast of india figure 1 the onland part consists of 28 000 km2 and is mostly alluvium covered krishna and godavari are the two major river systems which drain the area and discharge in the bay of bengal the offshore basinal area covers 24 000 km2 to the isobath of 200 m however the basin extends into deeper water and covers a much larger area the basin s characteristic feature is its en echelon horst and graben system which is filled with a thick pile of sediments of permian to recent age and emerging as one of india s most promising petroliferous areas commercial accumulation of hydrocarbons occurs in sediments from the permian to as young as the pliocene

seismic imaging of a fractured gas hydrate system in the krishna godavari basin offshore india gas hydrate was discovered in the krishna godavari kg basin during the india national gas hydrate program nghp expedition 1 at site nghp 01 10 within a fractured clay dominated sedimentary system logging while drilling lwd coring and wire line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500 m transect three dimensional 3d seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures a system of two fault sets was identified part of a typical passive margin tectonic setting the lwd derived fracture network at hole nghp 01 10a is to some extent seen in the seismic data and was mapped using seismic coherency attributes the fractured system around site nghp 01 10 extends over a triangular shaped area of 2 5 km2 defined using seismic attributes of the seafloor reflection as well as seismic sweetness at the base of the gas hydrate occurrence zone the triangular shaped area is also showing a polygonal nearly hexagonal fault pattern distinct from other more rectangular fault patterns observed in the study area the occurrence of gas hydrate at site nghp 01 10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source the triangular shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits additionally the fault associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone it is proposed that there are additional locations in the kg basin with possible gas hydrate accumulation of similar tectonic conditions and one such location was identified from the 3d seismic data 6 km nw of site nghp 01 10

igneous dikes on mars revealed by mars orbiter laser altimeter topography the origin of martian grabens has remained controversial for decades given the nonuniqueness of graben morphology and conflicting interpretations of dike induced or faultingbased formation processes we use measurements from the mars orbiter laser altimeter to identify the characteristic topographic signature at the surface of an igneous dike beneath a martian graben our results suggest that this subtle but diagnostic signature can be used to test competing interpretations of the formation of grabens and associated fissures and collapse depressions

dike indicators in the hadriaca patera promethei terra region mars abstract the eastern hellas basin rim has been highly modified by a multitude of both volcanic and fluvial processes it is covered by a large part of the circum hellas volcanic province consisting of the volcanic plains of hesperia planum and western promethei terra as well as the two central volcanoes hadriaca and tyrrhena patera and their associated lava flows large outflow channels of dao niger harmakhis reull and teviot valles dissect the wrinkle ridged plains postdating the major volcanic events the origin of these outflow channels is still ambiguous as the mobilization mechanism for the underground volatile reservoirs has yet to be revealed due to the proximity of the volcanic structures many previous studies have suggested via models that the released ice or ground water was mobilized due to endogenic heating from magmatic activity in the region to find evidence of this we conducted a systematic search for any magmatic intrusions at 50 m pixel resolution this paper provides detailed mapping of several areas where we found features which we interpret to be dikes and shows the overall distribution of such structures in the eastern hellas rim region the hadriaca patera volcano flanks exhibit a concentric pattern of linear ridges and fractures indicative of a vast ring dike system the dikes occupy mostly the area near the summit and the hellas facing flank extending there for over 400 km away from the caldera a hadriaca radial dike swarm pattern is observed extending southwards from the volcano composed of isolated parallel linear ridge group outcrops and cut by the valles canyons an additional isolated dike ridge group exists south of lower reull valles possibly controlled by the tyrrhena patera volcano some 1300 km away all dikes within the study region are roughly late hesperian in age either predating the valles formation or being roughly concurrent with them in total 500 dike segments were identified with a combined length of 2500 km since 90 of the dikes are ridges they suggest that the entire region has undergone large scale erosion of at least a few tens of meters to expose the dikes the dike orientations were most probably controlled by the volcanic centers and possibly partly by a hellas concentric fracture pattern

seismic wave attenuation in methane hydrate bearing sediments vertical seismic profiling data from the nankai trough exploratory well offshore tokai central japan 1 data from two vertical seismic profiles vsps from the nankai trough exploratory well offshore tokai central japan are used to estimate compressional attenuation in methane hydrate mh bearing sediments at seismic frequencies of 30 110 hz we compare spectral ratio and centroid frequency shift methods for measuring attenuation to isolate intrinsic attenuation from total attenuation attenuation is computed from multiples using one dimensional synthetic vsp data from sonic and density logs the use of two different measurement methods and two vsps recorded at just 100 m separation provides an opportunity to validate the attenuation measurements no significant compressional attenuation was observed in mh bearing sediments at seismic frequencies macroscopically the peaks of highest attenuation in the seismic frequency range correspond to low saturation gas zones in contrast high compressional attenuation zones in the sonic frequency range 10 20 khz are affected by the presence of methane hydrates in the same well locations thus this study demonstrated the frequency dependence of attenuation in mh bearing sediments mh bearing sediments cause attenuation in the sonic frequency range rather than the seismic frequency range

tessera terrain ribbon fabrics on venus reviewed could they be dyke swarms abstract endogenic and exogenic models have been developed over the past 25 years to explain the development of extensive pervasive high aspect ratio troughs and graben like structures ribbons and short to long wavelength folds in tessera terrains commonly associated with crustal plateaus on venus many of the models are complex and appear to be both internally contradictory and inconsistent with well established geological principles they are based on what amount to assumptions regarding i the secular evolution of crustal thickness ii crustal mechanical homogeneity and isotropy iii the mechanics of ribbon formation and iv the relative timing of ribbons in the structural geological sequence of tessera terrains on the basis of both theoretical and observational considerations this contribution will suggest that coupling the formation of tessera ribbons to the emplacement of dyke swarms relatively late in the structural development of tessera terrains may provide a resolution for the apparent inconsistencies in those published models

volcanic rifting at martian grabens 1 a large fraction of surface extension on mars occurred at segmented grabens having width length ratios akin to oceanic rifts on earth association with volcanic landforms such as pit craters clearly suggests interconnection between tectonic and magmatic processes a martian rift evolution model is proposed on the basis of new geomorphological and structural interpretations of imagery high resolution digital elevation models dems scaled experimental modeling and three dimensional boundary element modeling of magmatic and tectonic processes and a comparison with terrestrial rifts the dems were obtained from mars observer laser altimeter viking orbiter stereo images or a combination of both comparison of terrestrial rifts included afar iceland and the east pacific rise the ambient extensional stress field induced by regional body forces is combined at depth with decompression melting and mantle plume thermal anomaly resulting in emplacement of elongated magma reservoirs along the grabens injection of dikes above the reservoirs and flood basalt eruption result in a reservoir underpressurization of up to hundreds of mpa and induces surface collapse each collapse event is associated with an eruption of volcanic volumes akin to those of individual flow eruptions in large terrestrial igneous provinces the geometry and mechanisms of graben formation and surface collapse are described and used to infer reservoir depth and width we conclude from this study that giant dike swarms akin to typical giant terrestrial dike swarms are unlikely to underlie the volcanic martian grabens on the basis one graben one dike rather every volcanic graben segment appears to own a local dike swarm perhaps analogous to the dike swarms in the icelandic fissure zones index terms 5475 planetology solid surface planets tectonics 8149 5480 planetology solid surface planets volcanism 8450 6225 planetology solar system objects mars 8121 tectonophysics dynamics convection currents and mantle plumes 8010 structural geology fractures and faults keywords rifting graben pit crater magma chamber dyke

displacement length scaling for terrestrial and martian faults implications for valles marineris and shallow planetary grabens the displacement length ratio for the central composite graben in valles marineris inferred from topography and likely stratigraphic offsets to be d max l 4 6 x 10 3 is consistent with the scaling relations derived from terrestrial faults and grabens that also demonstrate a systematic variation in displacement with position along the structure the large 10 km depth in central valles marineris is therefore not unusual but rather should be expected for a fault system 2240 km long in contrast existing measurements of throw along the numerous shallow radial grabens in tharsis while apparently inconsistent with the scaling relations demonstrate the importance of recognizing mechanical segment length along linked or echelon graben sets faults on other planetary surfaces should scale with their terrestrial counterparts as long as they extend substantially deeper than the gravity dependent near surface rock mass zone 1 km for earth 2 5 km for mars

dike intrusions beneath grabens south of arsia mons mars 1 the existence of igneous dikes beneath grabens in tharsis remains contentious due to conflicting observations of martian geomorphology we examine the topographic signatures of a tharsis radial graben array in 3 zones south of arsia mons we identify dikes beneath grabens in all of the zones with fewer dike induced grabens closer to arsia mons we infer that a single radial swarm of dikes outside the magmatic source area central tharsis is unlikely our observations instead favor a long and varied magmatic history throughout tharsis with several episodes of dike injection

episodic basin wide fluid expulsion from geopressured shale sequences in the north sea basin a proposed general model for the episodic dewatering of thick shale successions is based on the recognition of a pervasive polygonal extensional fault network developed in the dominantly fine grained lower tertiary of the north sea basin seismic data show that the faults are arranged in stratigraphically bound structural units tiers that are delimited vertically by almost undeformed condensed sections and are restricted in distribution to the lowest permeability slope and basin floor facies i propose an episodic three stage mechanism to explain the fault genesis involving 1 the development of basin wide overpressured compartments 2 a density inversion between the overpres sured units and the overlying seal and 3 natural hydraulic fracturing pressure bleed off and resealing of the pressure compartment leading to a repeat of the cycle

surface deformation and shallow dike intrusion processes at inyo craters long valley california the inyo craters are the two largest of four phreatic craters that lie within a 2 5 km long 500 to 700 m wide n s trend of faults and fissures at the south end of the inyo volcanic chain in eastern california the alignment of these features with dike fed silicic volcanic centers of the same age a few kilometers to the north suggests that they were produced during intrusion of a dike at about 650 550 yr b p e w extension south of south inyo crater is ten to several tens of meters suggesting that the dike is at least that thick to understand how the faults and fissures developed we mapped and studied the fault and fissure pattern used a theoretical boundary element model to determine the surface strain profile above a shallow dike in a purely elastic medium and conducted physical model experiments of fault and fissure growth results of the field studies and experiments indicate that deformation develops according to the following sequence 1 extension fractures and perhaps other inelastic deformation develop immediately above the dike top and on the limbs of a shallow syncline which forms above the dike 2 fissures form along two parallel trends at the surface on opposite sides of the dike plane leaving a relatively unfractured region in between 3 dip slip movement on subsurface fractures and linkage of these fractures with surface fissures produces inward facing normal fault scarps that bound a nested graben above the dike experimental graben widths are up to several times narrower than predicted from the locations of theoretical strain maxima above a dilating crack in a linearly elastic medium this fact is apparently due to the growth of a zone of inelastic deformation above the experimental dike top in the subsurface if the ratio between the depth to the dike top and the distance between the outermost surface fissures at inyo craters is within the range of ratios measured experimentally then the depth to the dike top is between 250 m and several times that a slant hole drilled in the summer of 1987 by the department of energy office of basic energy sciences intersected three breccia bodies of possibly intrusive origin directly below the center of south inyo crater between 550 m and 610 m below the crater rim no intact juvenile igneous rock was intersected suggesting that the hole passed above the magma filled dike top perhaps passing through the zone of inelastic deformation which is inferred to exist from our model results

tectonostratigraphy and depositional patterns in krishna offshore basin bay of bengal the krishna godavari basin along the east coast of india covers the deltaic and interdeltaic areas of the krishna and godavari rivers and extends into the offshore figure 1 this basin is situated along the shoulder of a rifted passive continental margin which developed during the separation of india from antarctica in the late jurassic this rifting episode is superimposed on an earlier northwest southeast gondwana graben trend such as the pranhita godavari graben figure 2a which extends from onshore into the krishna godavari basin this is an earlier prerift depositional episode onto precambrian crystalline basement which commenced in the early permian and continued through the middle jurassic the late jurassic rift episode caused mechanical extension along the eastern margin leading to the formation of northeast southwest horst and graben features based on bouguer gravity data murty and ramakrishna 1980 recognized different tectonic elements figure 2b such as krishna graben bapatla horst west godavari subbasin including the median kaza kaikalur horst tanuku horst and east godavari subbasin including narasapur yanam high