beach ridges and prograded beach deposits as palaeoenvironment records abstract beach ridges are landforms commonly developed on prograded coasts with beach shorelines a sequence of beach ridges coupled with their subsurface deposits can be regarded as a time series of coastal evolution methodological advances in field surveying and chronology applicable to beach ridges have led to detailed palaeoenvironmental reconstructions to be derived from such sequences this paper reconsiders the basic aspects of beach ridges and deposits which need to be properly understood for their comprehensive interpretation in a palaeo environmental context it also reviews case studies in which beach ridge sequences have been used to unveil past sea level history catastrophic events and climate changes proposed formative processes of beach ridges include 1 progradation of sandy beach and berm formations in relation to fairweather waves coupled with aeolian foredune accumulation 2 building of gravel ridges by storm waves 3 welding of longshore bars beach ridge formation through sea level oscillation is thought to be questionable and caution is suggested for this process when undertaking palaeoenvironmental reconstruction beach deposit stratification is known to dip either landwards or seawards but landward dips are uncommon seaward dipping stratification is formed in relation to beachface progradation and is usually dissected in places by erosion surfaces resulting from episodic beach retreat the boundary between the foreshore and the underlying shoreface is well defined only in the case that longshore bars lead to complex bedding structure relative to that of the foreshore reliable chronology of beach ridges can be determined by radiocarbon and optically stimulated luminescence osl dating radiocarbon dating of articulated shells which are considered not to be extensively reworked provides robust results but osl dating is more useful as it enables direct dating of sediment grains it is noted that there are restrictions in chronological resolution and continuity inherent to beach ridge and beach deposits the plan view geomorphic expression of beach ridges typically consists of ridge sets with multi decadal intervals whereas their internal sedimentary structures define shorter time scales records of beach sedimentation and erosion are likely to be reworked by episodic high magnitude beach retreat and the resultant record of the net progradation is likely to be sporadic and discontinuous the height of sandy beach ridges is often variable due to differing degrees of aeolian sand accumulation and they are thus not used as sea level indicators unless purely wave built gravel ridge height is a relatively reliable indicator of sea level but can vary in response to storminess fluctuations subsurface sediment facies boundaries are preferred as sea level indicators and those proposed include boundaries of aeolian beach foreshore shoreface and upper lower shorefaces catastrophic events are expressed in both erosional and depositional records erosion surfaces or scarp imprints revealed in a cross section of beach deposits indicate storm or tsunami events however erosional events are likely to rework previous records of sedimentation and even other erosional events and thus the apparent history decoded from the resultant deposits tends to be biased several attempts for estimating the frequency and intensity of prehistoric cyclones rely on assumed relationships between the level of coarse sand beach ridges and cyclone inundation the formative process of coarse sand ridges remains uncertain and needs to be clarified as it constitutes the fundamental basis of these attempts the growth rates of beach ridge systems are expected to reflect fluctuations in river sediment discharge to the coast and in aeolian sand flux due to onshore winds both of which are affected by climate change assessment of the growth rate is potentially improved by ground penetrating radar survey of subsurface structure and by detailed chronology orientation of beach ridges reflects long term trends in wave direction inferred relationships between beach ridges and cyclic fluctuations of sea level and climate rely on weak assumptions and are not substantiated by rigid chronological evidence and thus remain highly questionable correlation between the inherently decadal signal of beach ridge intervals and possible climate cycles such as sunspot activity is probably coincidental as it lacks causal explanations

a seismotectonic study of the burma and andaman arc regions using centroid moment tensor data abstract the concept of a mean slip angle is introduced this enables a classification of focal mechanisms in any region into predominantly strike slip thrust and normal categories based on this concept the harvard centroid moment tensor cmt data in the burma and andaman arc regions from 1977 to 1992 comprising 167 focal mechanism solutions are examined and categorized distinct trends on the surface and in depth sections emerge on examination of these categories for instance in the burmese arc there is a clear segregation along the slab between strike slip type mechanisms down to a depth of about 90 km and thrust events which occur exclusively below this depth in addition a study of p and t axis orientations indicates that the stress pattern in the subducted slab is different from that farther east whereas the p axes of the thrust and strike slip mechanisms in the slab show a predominantly nne trend commensurate with the direction of motion of the indian plate with respect to the eurasian plate those in the region to the east show interestingly an average e w oriented compressive regime the northern and southern parts of the andaman are region exhibit distinct tectonic patterns whereas in the southern part the disposition of focal mechanisms along the slab and the p and t axis orientations indicate active subduction the mechanisms in the northern part exhibit a peculiar segregation into clusters of thrust normal and strike slip types without conforming to the local trend of the arc a similar study in the western aleutian trench region indicates a much simpler subduction pattern

decelerating holocene sea level rise and its influence on southwest florida coastal evolution a transgressive regressive stratigraphy abstract the ten thousand islands tti are a myriad of low relief mangrove islands that lie along the low energy subtropical southwest florida coast the region has been subjected to a relative rise in holocene sea level which has continuously decelerated to its present rate data derived from surface and subsurface sampling indicate that the holocene sediment package of the tti area consists of two sediment sequences the lower sequence is transgressive and was generated as coastal salt marsh and or terrestrial environments were submerged and replaced by a shallow coastal marine setting the upper sediment sequence consists primarily of 1 biogenic shallowing upwards sequences or 2 thickened mangrove peat layers reflecting island emergence and shoreline stabilization respectively island emergence compartmentalized the area further reducing wave and current energy and promoting the infilling of the protected bays through deposition of organic rich shelly mudstone and wackestone based on coastal stratigraphy and 14c dates the formation of his transgressive regressive sediment sequence is directly related to changing rates of holocene sea level rise reported to have occurred between 3 500 and 3 200 ybp continued regressive sedimentation could eventually generate a 5 10 m thick transgressive regressive sediment couplet with an aerial extent of over 300 km2 in roughly 10 000 years this externally forced sediment couplet compares remarkably well to individual small scale transgressive regressive cycles which repeat tens to hundreds of times throughout the geologic record see james 1984 the results of this study thus support the allocyclic mechanism as a viable working hypothesis for the generation of these small scale rock cycles

two dimensional water flood and mudflow simulation flo 2d is a two dimensional finite difference model that simulates clear water flood hazards mudflows and debris flows on alluvial fans and urban floodplains interactive flood or mudflow routing between channel street and floodplain flow is performed using a uniform grid system to describe complex floodplain topography a quadratic rheological model developed from field and laboratory mudflow data enables appropriate simulations of flooding conditions ranging from clear water to hyperconcentrated sediment flows computer aided design cad graphics of predicted time sequenced flood depths automates the delineation of flood hazards replication of the 1983 rudd creek mudflow in utah demonstrates the capability of the model

deciphering holocene sea level history on the u s gulf coast a high resolution record from the mississippi delta published holocene relative sea level rsl curves for the u s gulf coast are in mutual conflict with some characterized by a smooth rsl rise akin to widely accepted eustatic sea level curves versus others including several recent ones that are characterized by a conspicuous stair step pattern with prolonged millennium scale rsl stillstands alternating with rapid meter scale rises in addition recent work in texas and alabama has revitalized the notion of a middle holocene rsl highstand estimated at 2 m above present mean sea level an extensive sampling program in the mississippi delta louisiana focused on the collection of basal peats that accumulated during the initial transgression of the pre existing consolidated pleistocene basement we used stable carbon isotope ratios to demonstrate that many of these samples accumulated in environments affected by frequent saltwater intrusion in the 30 cm zone between mean spring high water and mean sea level and we selected plant macrofossils that were subjected to ams 14c dating nearly 30 sea level index points from a 20 km2 study area on the eastern margin of the delta suggest that rsl rise followed a relatively smooth trend for the time interval 8000 3000 cal yr b p thus questioning the occurrence of major rsl stillstands alternating with abrupt rises given the narrow error envelope defined by our data set any sea level fluctuations if present would have amplitudes of 1 m although a true middle holocene highstand never occurred in the mississippi delta the high level of detail of our time series enables a rigorous test of this hypothesis correction of our data set for a hypothetical tectonic subsidence rate of 1 1 mm yr 1 assuming a constant subsidence rate compared to the tectonically relatively stable adjacent coast of texas leads to sea levels near 2 m above present during the time interval 6000 4000 cal yr b p however this model also implies a rsl position near 2 m around 8000 cal yr b p which is inconsistent both with data of this age from texas as well as with widely accepted sea level data from elsewhere we therefore conclude that a middle holocene highstand for the u s gulf coast is highly unlikely and that the entire area is still responding glacio isostatically by means of forebulge collapse to the melting of the laurentide ice sheet

closure to direct determination of rheological characteristics of debris flow by philippe coussot dominique laigle massimo arattano andrea deganutti and lorenzo marchi

coastal reconstruction of vista alegre an ancient maritime maya settlement abstract past coastal reconstruction is useful for understanding archaeological coastal settlements and predicting how coastal change might affect modern populations the ancient maritime maya inhabitants of vista alegre in the northeastern yucatan were active seafaring peoples however the past coastal landscape environmental history is unknown previous research concentrated on the fully terrestrial component of the site and did not approach the issue from an earth sciences sedimentological perspective in this study a sediment core campaign in the shallow offshore of vista alegre aimed to reconstruct the coastal and environmental changes that occurred over the past 3000 years and specifically identify the changes in sea level nine cores were analyzed using a multi proxy approach including a range of sedimentological parameters such as granulometry micropaleontology foraminifera radiocarbon dating and loss on ignition the sediment cores provided an archive of environmental changes related to sea level change anthropogenic influence and shifting microenvironments which can be associated with cultural time periods the environmental phases and shifts show some linkage to the archaeological chronology suggesting an association between the environmental conditions and human activities sea level changes and shifting shorelines have always been and still are a challenge for coastal settlements and ancient sites can be a harbinger of what to anticipate in the future in addition to this current natural and anthropogenic pressures on coastlines are placing archaeological sites at increasing risk and thereby threatening this important scientific and cultural archive therefore efforts to identify characterize and record natural and anthropogenic pressures prior to destruction are increasingly important

oblique plate convergence in the indo burma myanmar subduction region the indo burma myanmar subduction boundary is highly oblique to the direction of relative velocity of the indian tectonic plate with respect to the eurasian plate the area includes features of active subduction zones such as a wadati benioff zone of earthquakes a magmatic arc thrust and fold belts it also has features of oblique subduction such as an arc parallel strike slip fault sagaing fault that takes up a large fraction of the northward component of motion and a buttress the mishmi block that resists the motion of the fore arc sliver

stratigraphic and micropetrographic occurrences of pyrite in sediments at the confluence of carbonate and peat forming depositional systems southern florida u s a abstract this study was undertaken to investigate peat carbonate relationships in a modern coal forming setting with initial research on pyrite and sulfur contents relative to stratigraphic and petrographic conditions over 40 closely spaced cores were taken from transects from florida bay inland through mangrove swamps and freshwater everglades core stratigraphy confirms on overall transgressive sequence but new evidence supports a small regression within the overall transgression on average both total s and pyrite are lowest at inland sites highest near the coastline and intermediate in florida bay sediments total s is higher in marine than freshwater peats but freshwater peats overlain by marine peats are enriched in s in cores that start with peat at the top pyrite is generally absent or very low in the top 20 cm higher percentages of pyrite occur in carbonate rather than peat facies pyrite exists in three forms framboidal aggregates minute crystals and large solitary crystals most of the pyrite occurs as minute crystals m and framboidal aggregates the occurrence of large crystals 2 m is less common and generally restricted to the cavities of larger cells or cavities within other organics e g formminifera pollen grains fungal sclerotia a general relationship seem to exist in certain cases between the occurrence of spherical organic cell fillings and the occurrence of the framboidal form of pyrite

middle holocene sea level rise and highstand at 2 m central texas coast new data suggest a revised picture of middle holocene sea level change for the texas gulf of mexico coast and suggest reevaluation of coastal evolution first brackish marsh facies with calibrated radiocarbon ages of 7 7 to 7 8 ka have been recovered from depths of 8 5 to 9 m in a core from the ancestral colorado river delta and are interpreted to represent a sea level pinning point second a series of ridges along the copano bay margin farther south consist of shelly mud and fine sand with subtidal foram assemblages occur at elevations of 1 95 m above the modern intertidal zone and have produced calibrated radiocarbon ages on foram tests of ca 6 8 to 4 8 ka these ridges are interpreted to represent relict shallow subtidal to intertidal spits that provide minimum sea level positions for the middle holocene and are now emergent because of later sea level fall in aggregate these data show rates of sea level rise during this time period that are very comparable to or even lower than published eustatic rates and suggest a middle holocene sea level highstand for this non uplifting non rebounding and very slowly subsiding part of the north american coastline

the effect of sea level and climate change on the development of a mixed siliciclastic carbonate deltaic coastline suwannee river florida u s a abstract florida s mixed siliciclastic carbonate northwest central gulf of mexico coastline is characterized by a 300 km long low energy sediment starved open marine marsh system fronted by a broad shallow limestone shelf the only significant point source of sediment to this system is the suwannee river which forms a small 20 km2 delta at the center of this coastline late pleistocene eolian dune formation along the suwannee river coastline reflects slightly drier conditions and local sediment sources influencing the early geologic development of this region as sea level rose in the early to middle holocene dunes on the low gradient shelf were transgressed and provided the core for the modern offshore sandy shoals with decelerating sea level rise in the middle to late holocene and probable increased riverine sediment input coastal sedimentation switched from transgressive to aggradational paleo distributary channels seaward of the delta coastline at approximately 4 000 cal yr bp indicate initial deltaic formation at this same time oyster reefs from the delta southward were able to keep pace with sea level rise and began to form large 20 km2 offshore oyster bioherms away from the river mouth the coastline continued to transgress until 2 350 cal yr bp to 1350 cal yr bp when the shoreline stabilized and the modern marsh system south and north of the river mouth respectively began to aggrade the geologic development of this coastline and the relict eolian dunes preserved within the modern marsh system indicates that middle to late holocene sea level did not exceed current elevations along this portion of the gulf of mexico coastline this finding agrees with other studies from the low gradient west central florida coast but contrasts with evidence for middle to late holocene highstands along other gulf of mexico shorelines

the west central florida inner shelf and coastal system a geologic conceptual overview and introduction to the special issue abstract this paper provides an overview for this special publication on the geologic framework of the inner shelf and coastal zone of west central florida this is a significant geologic setting in that it lies at the center of an ancient carbonate platform facing an enormous ramp that has exerted large scale control on coastal geomorphology the availability of sediments and the level of wave energy in order to understand the holocene geologic history of this depositional system a regional study defined by natural boundaries north end of a barrier island to the apex of a headland was undertaken by a group of government and university coastal geologists using a wide variety of laboratory and field techniques it is the purpose of this introductory paper to define the character of this coastal inner shelf system provide a historical geologic perspective and background of environmental information define the overall database present the collective objectives of this regional study and very briefly present the main aspects of each contribution specific conclusions are presented at the end of each paper composing this volume

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

tectonic segmentation of the burmese indonesian arc abstract several lines of evidences which include change in trend and offset in arcs bathymetry and sedimentation faulting in the region change in composition and trend of the line of volcanoes spatial distribution of earthquakes and change in dip of the benioff zone suggest a tectonic segmentation of the burmese indonesian arc two transverse boundary zones viz the north andaman boundary zone and the sunda boundary zone separating segments of the arc are identified larger than usual thickness of the oceanic crust beneath the ninetyeast ridge reduces the average density of the lithosphere carrying the ridge the buoyancy effect caused by the reduced average density inhibits the subduction of an approximately triangular shaped lithosphere between the burmese indonesian arc and the ninetyeast ridge which in turn causes tectonic segmentation along the arc the model predicts and some available focal mechanism solutions confirm left lateral northeast faulting within the sunda boundary zone

very high energy sedimentation supratidal hurricane deposits and mid holocene highstand on carbonate platforms andros bahamas an alternative view abstract twenty eight 14 c dates were obtained from 90 cores drilled through the tidal flats of western andros island a large area 2000 km 2 mostly covered by mangroves and stromatolitic deposits these dates are correlated with location of the samples sedimentological description position of the samples at the time of deposition and their present day sedimentological position measurements were made on bulk algal sediments and location of the relative paleo sea level is inferred from paleobathymetric reconstructions on the basis of 14 c dates i conclude that the whole area was flooded about 5000 years bp and then was covered by the subtidal greyish mud facies rich in peneroplids in the area of deep creek north of the study area pelican creek and wide opening southeast of the study area the level of this paleo great bahama bank sea continued to rise until 3500 years bp and then slowly fell asymptotically toward the present day level thick hurricane marks aragonitic lime mud with didemnids spicules several cm thick previously defined as hurricane trails can be observed throughout the cores from 2000 years bp onwards from 2000 years bp onwards it seems that the modern physiography was already established including tidal estuaries channels and stromatolite bearing swamps paleogeographic maps and sections have been drawn and accumulation rates calculated in the various sedimentological zones of the tidal flats of western andros these vary between 0 5 to 0 04 mm yr the modern topography of the tidal flats was established by 2000 years bp similarly the modern geological features controlled by a tropical climate strictly dependent on hurricanes were already established by 2000 years bp and seem to have been initiated between 2500 and 2000 years bp

earthquake occurrence processes in the indo burmese wedge and sagaing fault region abstract earthquakes in the indo burmese wedge and sagaing fault regions occur in response to the partitioning of the india sunda motion along these two distinct boundaries under the accretionary wedge of the indo burmese arc majority of the earthquakes occur in the depth range of 30 60 km and define an eastward gently dipping seismicity trend surface that coincides with the indian slab the dip of the slab steepens in the east direction and earthquakes occur down to a depth of 150 km though the slab can be traced up to the 660 km discontinuity although these features are similar to a subduction zone the nature of the earthquakes and our analysis of their focal mechanisms suggest that these earthquakes are of intra slab type which occur on steep plane within the indian plate and the sense of motion implies a northward relative motion with respect to the sunda plate thus these earthquakes and the stress state do not support active subduction across the indo burmese arc which is also consistent with the relative motion of india sunda plates the absence of inter plate earthquakes lack of evidence of the occurrence of great earthquakes in the historical records and non seismogenic nature of the plate interface under the accretionary wedge suggest that seismic hazard due to earthquakes along the plate boundary may be relatively low however major intra slab earthquakes at shallow and intermediate depths may still cause damage in the sediment filled valley regions of manipur and cachar in india and chittagong and sylhet regions of bangladesh in the sagaing fault region earthquakes occur through dextral strike slip motion along the north south oriented plane and the stress state is consistent with the plate motion across the sagaing fault

how the state of stress varies in the wadati benioff zone indications from focal mechanisms in the wadati benioff zone beneath sumatra and java summary this study concentrates on the detailed determination of the state of stress in the wadati benioff zone in the lithosphere subducting under sumatra and java the orientations of p and t axes of earthquake focal mechanisms hcmt are used to define a system of domains in which a uniform state of stress is assumed the method of gephart forsyth is then used to determine the state of stress in each domain we succeeded in delimiting eight domains three si siii in the sumatra region and five ji jv in the java region domains with similar states of stress occur in both regions in similar positions the maximum compression 1 is perpendicular to the trench in domains si sii and jii that is within the depth range 0 165 km the orientation of 1 is almost parallel to the trench in domains siii and jiii depth range 25 225 km the boundary between domains sii and siii and jii and jiii is not distinct covering the depth range 40 165 km it seems that the focal mechanisms belonging to domains sii and siii and similarly to domains jii and jiii occur in different stress layers and that we observe an overlap of earthquakes with different focal mechanisms from two different stress state layers parallel to the wadati benioff zone in domains jiv and jv the states of stress correspond to the stress pattern observed in other subduction zones slab dip parallel extension is observed in domain jiv depth range 225 315 km and slab dip parallel compression is observed in domain jv deeper than 400 km

recognition classification and mechanical description of debris flows abstract various types of flow or mass movement involving water and sediments occur on steep slopes in mountainous areas among them debris flows are peculiar events during which a large volume of a highly concentrated viscous water debris mixture flows through a stream channel throughout the world these phenomena cause considerable damage but remain poorly understood although a basic knowledge is already available concerning their recognition and propagation firstly a synthesis of the useful practical criteria of recognition is proposed debris flows must be seen as intermediate phenomena between hyperconcentrated flows intense bed load transport and landslides separated from them by sharp transitions of some characteristics celerity deposit nature and flow type two parameters solid fraction and material type thought to be appropriate for a sound and practical classification are brought out and the corresponding complete classification of flow and mass movements in mountain areas is presented two extreme debris flow types are thus distinguished muddy debris flows and granular debris flows a critical review of recent advances in debris flow dynamic is then proposed it is pointed out that adequate work must be carried out in the field of non newtonian fluid mechanics in particular one fundamental rheological property of debris flow materials is the yield stress which explains thick deposits on steep slopes and can be inferred from field measurements furthermore it can be used to estimate viscous dissipation within the bulk during flow relevant models predicting muddy debris flow dynamics are already available whereas further progress is needed concerning granular flows

constraints on 2004 sumatra andaman earthquake rupture from gps measurements in andaman nicobar islands the 26 december 2004 sumatra andaman earthquake mw 9 0 9 3 is the greatest earthquake of the modern seismological era the rupture characteristics of the earthquake particularly in the andaman nicobar region are not well resolved from seismological or far field geodetic data here in this article we present campaign mode global positioning system gps measurements of coseismic displacements at 13 sites in the andaman nicobar islands before and after the 2004 sumatra andaman earthquake these measurements provide improved estimates of rupture characteristics in the region coseismic horizontal ground displacement of 1 5 6 5 m towards the southwest and coseismic vertical displacement mostly subsidence of 0 5 2 8 m occurred along the andaman nicobar islands with maximum displacements in the nicobar islands we estimate coseismic slip under the andaman and nicobar islands as 3 8 7 9 m and 11 15 m respectively the length of the rupture is estimated to be about 1500 km with a width varying from 120 km under middle andaman island to 160 km under great nicobar island gps measurements during january 11 22 2005 from port blair suggest rapid afterslip in the postseismic period limited gps data available from 1995 measurements at two sites in andaman provide evidence of strain accumulation that varied significantly in the 10 yr preceding the earthquake d 2005 elsevier b v all rights reserved

properties of debris flow deposits and source materials compared implications for debris flow characterization in the alps debris flow deposits generally contain 5 clay size particles and the role of the surface charged 2 mu m particles is often neglected although these particles may have a significant impact on the rheological properties of the interstitial fluid the objective of this study was to compare debris flow deposits and parent materials from two neighbouring catchments of the swiss alps with special emphasis on the colloidal constituents the catchments are small in area 4 km 2 2 5 km long similar in morphology but different in geology the average slopes are 35 40 the catchments were monitored for debris flow events and mapped for surface aspect and erosion activity debris flow deposits and parent materials were sampled the clay and silt fractions extracted and the bulk density 2 mm fraction bulk density particle size distribution chemical composition cation exchange capacity cec and mineralogy analysed the results show that the deposits are similar to the parent screes in terms of chemical composition but differ in terms of i particle size distribution and ii mineralogy reactivity and density of the 2 mm fraction in this fraction compared with the parent materials the deposits show dense materials enriched in coarse monocrystalline particles of which the smallest and more reactive particles were leached the results suggest that deposit samples should not be considered as representative of source or flow materials particularly with respect to their physical properties

seismo tectonics of the andaman nicobar islands abstract the epicentres of 345 earthquakes that occurred between january 1967 and december 1982 in a region bounded by 4 n to 16 n latitudes and 90 e to 98 e longitudes were plotted in order to study the seismicity of this region most of the earthquakes in this region are shallow focus the greatest focal depth being about 250 km in the northern part of this region north of 10 5 n tne earthquake foci are aligned along two branches which strike northeastwards no intermediate depth earthquakes are associated with the eastern branch south of 10 5 n the foci are located along a zone striking north northwest focal mechanism solutions are reported for 24 earthquakes that occurred in this region during this period the first motions of p and pkp phases were used to determine the mechanism solutions five of these events occurred in northern sumatra and the rest occurred in the andaman nicobar islands the mechanism solutions of the shallow focus earthquakes in northern sumatra are characterized by a predominant component of thrust and strike slip faulting the inferred orientation of slip vectors deduced from mechanisms of shallow focus events is nearly northeast mechanism solutions of 13 shallow focus events that occurred in the andaman nicobar islands are characterized by thrust normal and strike slip faulting the orientation of slip vectors deduced from thrusting mechanisms is consistent with the underthrusting of the indian ocean under the andaman nicobar islands in a northeasterly direction normal faulting mechanism solutions of these islands suggest that they are tectonically similar to island arcs the results reported in this study show for the first time using mechanism solutions the existence of underthrusting along the andaman nicobar islands for which there is ample geological evidence

siliciclastic to carbonate transition on the inner shelf embayment southwest florida abstract the inner shelf embayment adjacent to southwest florida is a low energy system which displays a distinct and demonstrable transition from dominantly siliciclastic sediments in the north to virtually pure biogenic carbonates in the south sediment sources are littoral transport from the north autochthonous biogenic debris a modest input from land derived runoff and a small amount from reworking of older strata siliciclastic sediments are well sorted fine quartz sand and terrigenous and biogenic mud which is concentrated along the coast of the southern half of the 90 km coast where it accumulates from adjacent terrigenous sources biogenic carbonate ranges from mud to gravel in size and is dominated by sand and gravel the transition zone between the siliciclastic and carbonate sediments covers only about 10 km and trends nnw sse with a south extending lobe of siliciclastic material along the coast this detailed documentation of regional change in sediment texture and composition along with the geographic distribution provides data for application to the stratigraphic record such siliciclastic carbonate transitions are quite important for example in the cambro ordovician of the upper mississippi valley and the cretaceous of central texas

pop up tectonics of the shillong plateau in northeastern india insight from numerical simulations article i nfo the shillong plateau in northeastern india represents one of the most seismically active pop up structures within the peninsular shield area in order to constrain the role of the inferred oldham fault in the northern boundary of the plateau we performed 2 d finite element method fem simulations for convergent displacement caused by northeastward movement of the indian plate with respect to the eurasian plate various rock properties density poisson s ratio young s modulus cohesion and angle of internal friction and the mohr coulomb failure criterion are used to evaluate failure and faulting patterns two plane strain models with appropriate boundary conditions were also calculated the predicted maximum compressive stress 1 shows a preferred orientation that helps explain the tectonic environment and the fault pattern the best fit model suggests that a compressive stress regime is dominant in the study area everywhere except for the uppermost part of the crust where extensional stress dominates with increased progressive convergent displacement the modeled 1 are predicted to rotate counterclockwise around the fault zones the simulation results suggest that the oldham fault does not have a significant role in the development of stress and deformation distribution in the area we also infer that the tectonically induced deformation in both the plateau and the adjoining areas is restricted to mainly within the crust b30 km

seismicity and tectonic stress field of a part of the burma andaman nicobar arc abstract the nature of instrumentally recorded seismicity for a part of the burma andaman nicobar arc bounded by latitudes 2 and 24 n and longitudes 91 and 99 e was investigated based on the spatial distribution of the earthquakes and their focal mechanism solutions four tectonic units were identified from the south going northward these are the sumatra trench region characterized by strike slip and thrust faulting the andaman spreading ridge characterized by normal and a few strike slip faults the andaman nicobar ranges where faulting is mostly thrust type and the indo burman ranges region dominated by thrust faulting the stress orientations in these segments and their changes with depth were studied in relation to the tectonics of the region the direction of maximum compression in the burma andaman nicobar region is ne sw to n s compatible with the postulated motion of the indian plate changes in stress orientations are attributable to local tectonics