seismic stratigraphic framework of the forearc basin off central sumatra sunda arc abstract new multichannel seismic reflection data provide information on the stratigraphic framework and geologic history of the forearc basin west of central sumatra we recognize six seismic stratigraphic sequences that reflect the cenozoic history and development of the outer continental shelf and forearc basin southeast of nias island these sequences indicate several episodes of uplift of the subduction complex and filling of the forearc basin early in the development of this margin paleogene slope deposits prograded onto the adjacent basin floor onlapping this assemblage are two units interpreted as younger paleogene trough deposits uplift associated with rejuvenation of subduction in the late oligocene led to erosion of the sumatra shelf and formation of a regional unconformity the early miocene was a period of significant progradation a miocene limestone unit partly downlaps and partly onlaps the older paleogene deposits it is characterized by shallow shelf and oblique progradational facies passing into basin floor facies a buried reef zone occurs near the shelf edge the cutting of an erosional unconformity on the shelf and slope in late miocene early pliocene time culminated this episode of deposition in the late pliocene a large flexure developed at the western boundary of the basin displacing the outer arc ridge upward relative to the basin over 1 km of pliocene to recent sediment was deposited as a wedge in the deep western portion of the basin landward of the outer arc ridge these deposits are characterized by flat lying high amplitude continuous reflections that overstep the late miocene unconformity up to 800 m of shallow water limestone have been deposited on the shelf since mid pliocene time

backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

rapid water exchange between the lagoon and the open ocean at majuro atoll due to wind waves and tide current measurements and conductivity temperature depth surveys of the lagoon and ocean at majuro atoll the republic of the marshall islands were made from january 10 to 24 1997 a vertically integrated tidal current model reproduced qualitatively well tidal ellipses calculated from the observed current measurements by fourier transform a three dimensional robust diagnostic residual current model explained the major features observed in the current measurements averaged over the dominant tidal cycles we used the diagnostic model to examine the effects of wind stress radiation stress density gradients and tidal stress on the exchange of water between the lagoon and the ocean wind effectively mixes the lagoon water in approximately 2 weeks tidal flushing appears to be restricted to a small area near the main channels connecting the lagoon to the ocean cross reef flat currents induced by radiation stress and flowing to the deep channels in the center of the northern boundary form the dominant mechanism for exchange between the lagoon and the open ocean causing water to exchange completely with the ocean in about 15 days computer generated particles tracked through the lagoon showed that radiation stress is also the main forcing mechanism for particle export from the lagoon however the coupling of tidal exchange through the calalin channel and wind stress induced mixing in the lagoon could also provide a significant export mechanism particularly for particles originating uniformly inside the lagoon

forcing of resonant modes on a fringing reef during tropical storm man yi 1 open basin resonant modes have been observed at tidal frequencies on coastal shelves but their excitation on coral reef platforms has only been suggested the topography of and water depth over most fringing reefs correspond to resonant periods that are the order of tens of minutes and fall outside of the energetic part of the wave spectrum during tropical storm man yi low frequency near resonant oscillations dominated the variance of the sea surface elevation at the shoreline of ipan reef guam the excitation of the resonant modes resulted both from a large increase in water level over the reef due to wave setup which increased the resonant frequencies and the occurrence of wave group forcing with time scales commensurate with these increased resonant frequencies

modelling gravel barrier profile response to combined waves and tides using xbeach laboratory and field results abstract xbeach a process based numerical model capable of computing nearshore circulation and morphodynamics including overwash and breaching has been used in 1d mode to simulate erosion occurring on a gravel barrier beach d50 11 mm subjected to wave and tidal forcing during selected tests undertaken during the bardex experiments the model demonstrated good quantitative skill brier skill score bss typically 0 65 with respect to beach face erosion offshore accretion and morphological impacts of washover xbeach was also used to model storm impacts on a steep average tan 0 12 4 5 km long and 100 m 140 m wide macrotidal gravel barrier d50 2 mm to 10 mm located in sw england although the development of a storm berm was not reproduced well erosion was predicted with sufficient skill bss 0 60 to meet practical engineering requirements concerned with predicting storm impacts and threshold wave and water level conditions for gravel barrier overwashing

crustal structure of the central sunda margin at the onset of oblique subduction summary the convergent margin of the central sunda arc in indonesia was the target of a reflection and refraction seismic survey conducted in 1998 and 1999 along two seismic lines across the subduction complex off southern sumatra and off sunda strait coincident multichannel and wide angle data were collected complemented by two refraction strike lines in the forearc basin off sumatra the combined analysis of the acquired data allows us to present a detailed model of the subduction zone where initiation of strain partitioning occurs due to the onset of oblique subduction the dip of the subducted plate is well defined along both dip lines and a lateral increase from 5 to 7 from beneath the outer high off sumatra to sunda strait is supported by complementary gravity modelling the downgoing slab is traced to a depth of more than 30 km on both reflection dip lines a clearly developed backstop structure underlying a trench slope break defines the landward termination of the active accretionary prism and separates it from the outer high active subduction accretion is supported by laterally increasing velocities between the deformation front and the active backstop structure seismic velocities of the outer high are moderate along both lines 5 8 km s 1 at 20 km depth suggesting a sedimentary composition reduced reflectivity beneath a rugged top basement traced along the outer high of both dip lines supports a high degree of deformation and material compaction several kilometres of sediment has accumulated in the forearc domain although a distinct morphological basin is only recognized off southern sumatra and is not developed off sunda strait the bathymetric elevation of the java shelf that is encountered in the southern sunda strait corresponds to increased velocities of a basement high there and is connected to extensional structures of the sunda strait transtensional basin differences observed in the morphology of the forearc domain are also reflected in the lower crustal structure off southern sumatra the velocity depth model clearly indicates a continental type crust underlying the forearc basin whereas lower velocities are found beneath the sunda strait forearc domain off sumatra some 3 d constraint on the upper plate structure is gained from the refraction strike lines which in addition is supported by synthetic data modelling

stratigraphic signatures of forearc basin formation mechanisms tectonic deformation of accretionary wedges is often interpreted using stratigraphic patterns observed in forearc basins however similar stratigraphic patterns could evolve from different deformational processes making it difficult to uniquely reconcile stratigraphic patterns and the geodynamic history of natural accretionary wedges therefore it is important to test the dynamic consistency of interpreted deformation histories one approach is to compare synthetic stratigraphy of forearc basins generated in numerical accretionary wedge models to the stratal patterns observed in natural forearcs we present a simple method to simulate synthetic stratigraphy in numerical accretionary wedge models comparable to the stratigraphic patterns observed in seismic reflection data as calibration we use reflection seismic and borehole data for the kumano forearc basin in nankai we observe that the stratigraphy in retro forearc basins remains predominantly undisturbed and unaffected by wedge deformation sediment stabilized wedge top basins in wedges with trenchward surface slope typically form landward of an active out of sequence thrust leading to a landward tilting of the basin these stratigraphic characteristics are diagnostic of the forearc basin forming mechanism we also infer that sedimentation in the trench can potentially influence the generation reactivation of megasplay fault activity in the nankai accretionary wedge activity on the megasplay fault helps create accommodation space landward to it that leads to the formation of kumano forearc basin

a framework for understanding drag parameterizations for coral reefs 1 in a hydrodynamic sense a coral reef is a complex array of obstacles that exerts a net drag force on water moving over the reef this drag is typically parameterized in ocean circulation models using drag coefficients cd or roughness length scales z0 however published cd for coral reefs span two orders of magnitude posing a challenge to predictive modeling here we examine the reasons for the large range in reported cd and assess the limitations of using cd and z0 to parameterize drag on reefs using a formal framework based on the 3 d spatially averaged momentum equations we show that cd and z0 are functions of canopy geometry and velocity profile shape using an idealized two layer model we illustrate that cd can vary by more than an order of magnitude for the same geometry and flow depending on the reference velocity selected and that differences in definition account for much of the range in reported cd values roughness length scales z0 are typically used in 3 d circulation models to adjust cd for reference height but this relies on spatially averaged near bottom velocity profiles being logarithmic measurements from a shallow backreef indicate that z0 determined from fits to point measurements of velocity profiles can be very different from z0 required to parameterize spatially averaged drag more sophisticated parameterizations for drag and shear stresses are required to simulate 3 d velocity fields over shallow reefs in the meantime we urge caution when using published cd and z0 values for coral reefs

the michael gabbro and other mesoproterozoic lithospheric probes in southern and central labrador the michael gabbro 1426 ma and the shabogamo gabbro 1459 ma represent two iarge diabase intrusive episodes that affected the northwestern margin of the northeastern grenville province both have sustained variable effects of subsequent grenvillian metamorphism other broadly contemporaneous magmatic activity in the region includes the formation of elssnian anorthosite granitoid complexes 1 46 1 29 ga that intruded churchill province and nain province rocks to the north the harp dykes 1 27 ga and mealy dykes 1 25 ga petrologic and geochemical data show that the michael gabbro and shabogamo gabbro are similar with the latter displaying more cumulate rock characteristics and the former having compositions closer to those of rapidly cooled magma both have compositions comparable to those of other continental diabases and to some continental flood basalts sr and nd initial isotopic compositions of michael gabbro 1426 ma 4 7 to 6 0 1 1426 ma 8 7832 0 7w and shabogamo gabbro i459 ma 4 0 to 5 5 1 1459 ma 8 7020 0 7060 are alike and overlap suggesting similar sources and processes of development comparisons with other nearly contemporary mafic suites in central and southern labrador show that only the mealy dykes have a distinctly more radiogenic isotopic signature paradoxically other rnafic suites that intrude older basement rocks north of the grenville front have less enriched nd signatures than do those that intrude younger basement rocks south of the front the argument is made that the subcontinental lithospheric mantle and not crustal contamination played the most influential role in evoiution of the magmas

hydrodynamics of a bathymetrically complex fringing coral reef embayment wave climate in situ observations and wave prediction this paper examines the relationship between offshore wave climate and nearshore waves and currents at hanalei bay hawaii an exposed bay fringed with coral reefs analysis of both offshore in situ data and numerical hindcasts identify the predominance of two wave conditions a mode associated with local trade winds and an episodic pattern associated with distant source long period swells analysis of 10 months of in situ data within the bay show that current velocities are up to an order of magnitude greater during long period swell episodes than during trade wind conditions overall circulation patterns are also fundamentally different the current velocities are highly correlated with incident wave heights during the swell episodes while they are not during the modal trade wind conditions a phase averaged wave model was implemented with the dual purpose of evaluating application to bathymetrically complex fringing reefs and to examine the propagation of waves into the nearshore in an effort to better explain the large difference in observed circulation during the two offshore wave conditions the prediction quality of this model was poorer for the episodic condition than for the lower energy mode however it illustrated how longer period swells are preferentially refracted into the bay and make available far more nearshore wave energy to drive currents compared to waves during modal conditions the highly episodic circulation the nature of which is dependent on complex refraction patterns of episodic long period swell has implications for flushing and sediment dynamics for incised fringing reef lined bays that characterize many high islands at low latitudes around the world

fore arc seafloor unconformities and geology insight from 3 d seismic geomorphology analysis peru new 3d seismic data collected over 4870 km2 in the 3 45 12 30 s peruvian segment of the east pacific subduction system image seafloor erosional surfaces that can be mapped across the forearc basins forearc basins experience various stresses from their base where basal tectonic erosion acts to the seafloor which is influenced by aerial shallow and deep water currents driven by waves or thermohaline oceanic currents previously there has been little interest in stresses on the upper layer and there is a lack of documentation of unconformities and the erosive processes in certain bathymetric domains in forearc basins we address this with the study of examples sourced from 3d seismic reflection surveys of the seafloor offshore peru unconformities occur in two distinctive bathymetric domains associated with the continental shelf and the upper slope of the margin identification and characterization of unconformity surfaces yield estimates of the amount of erosion at the modern seafloor that range from 18 to 100 regional physical oceanography allows us to calibrate potential candidates for these two distinctive domains the first control on erosion is the dynamics of deep to intermediate oceanic currents related to the humboldt peru chile water masses while the second is wave action in the shallower erosional surfaces this study illustrates the unseen landscape of the forearc basins of south america and helps to highlight the importance of erosive surficial processes in subduction landscapes

the dynamics of infragravity wave transformation over a fringing reef a 3 week field study was conducted to investigate the dynamics of low frequency infragravity wave motions over a fringing reef at ningaloo reef western australia short period wave motions 0 04 0 2 hz were observed to dissipate on the reef crest beyond which infragravity wave motions 0 004 0 04 hz gradually dominated toward the lagoon however both the short waves and the infragravity waves were relatively small both 0 3 m on the reef flat owing to the shallow water depth 2 m the results revealed that the surf zone generation of free infragravity wave motions on the steep 1 20 fore reef slope was dominated by breakpoint forcing as opposed to shoaling bound waves which was also supported by detailed numerical simulations of the generation process this is consistent with theory suggesting the efficiency of the breakpoint forcing mechanism should be high in this steep slope regime shoreward propagating infragravity waves traveled across the reef but were damped by bottom friction dissipation however this was at a rate much smaller than experienced by the residual short waves with these rates of frictional dissipation also strongly dependent on the water depth over the reef the infragravity wave heights increased at higher water levels and hence were strongly modulated by the tide due to the strong dissipation of infragravity waves over this wide and shallow reef that is hydraulically rough any seaward propagating infragravity waves that reflected at the shoreline were small leading to the dominance of progressive shoreward propagating infragravity wave motions throughout the reef and lagoon

a comparison of reef protected environments in western australia the central west and ningaloo coasts variability in the regional setting and morphology of cuspate forelands on the west coast of western australia is examined in this paper in accordance with this aim principal differences in the geologic and geomorphologic setting of three prominent sites on the west coast were established and their association with historical changes and contemporary oceanographic processes was examined the cuspate forelands investigated are jurien bay winderabandi point and turquoise bay the most significant differences in geologic setting are associated with the structure and location of an extensive offshore reef system morphologically the reef alters from south to north changing from a discontinuous ridge parallel to the shore along the central west coast to a nearly continuous fringing reef at ningaloo the reefs vary in distance from the shore being farthest in the south and closest in the north and they impound a series of inshore basins or lagoons the deeper southern basins are dominated by locally generated wind waves and wind generated currents the shallower northern basins are most markedly affected by tidal currents and wave pumping across the reef flats the large cuspate foreland at jurien on the central west coast has undergone shoreline configuration change in response to changing phases of storminess as well as in response to a change in focus for sediment deposition as a result of offshore reef erosion at winderabandi point on the ningaloo coast relict pleistocene limestone has provided the focus for sedimentation and morphology has been controlled by a balance in refracted wave energy and nearshore currents driven by tidal and wave set up variability at turquoise bay where the lagoonal basin is most shallow and narrow the morphology of the foreland suggests that it may at some stage have been migratory but its present asymmetrical shape is maintained by strong northerly longshore drift and strong currents exiting the lagoon through a nearby gap in the reef crest fundamental differences between the two coastal regions include the structure of the offshore reef processes driving flow of water within the lagoons and the role of storminess in evolution of coastal landforms although many questions regarding storm surge dynamics and landform change remain unanswered this research provides a significant contribution to the understanding of the evolution of morphological systems in low wave energy protected environments copyright 2000 john wiley sons ltd

facies distribution of the lower cambrian cryptic microbial and epibenthic archaeocyathan microbial communities western anti atlas morocco marine microbial communities recorded in the moroccan anti atlas were unaffected across the neoproterozoic cambrian transition a stromatolite dominated consortium was replaced at the beginning of the atdabanian ca 20 myr after the neoproterozoic cambrian boundary by shelly metazoan and thromboid consortia which contain the oldest biostratigraphically significant fossils of the moroccan cambrian the associated collapse of microbial mat stromatolitic growth appears to coincide with a change from pre atdabanian shallow water restricted conditions into atdabanian deeper open sea conditions it is postulated that this environmental change led to an episode of improved water circulation over carbonate platform interiors promoting shelly metazoan immigration into the region the tiout amouslek lithostratigraphic contact in the early atdabanian marks the end of an episodically unstable seafloor as suggested by the abundance of slumping and sliding structures and synsedimentary microfaults and cracks recorded in the underlying tiout member concurrent with the transition is the occurrence of a network of cryptic fissures and cavities that provided habitats for a coelobiontic chemosynthetic heterotrophic microbial community composed of stromatolitic crusts renalcis epiphyton girvanella intergrowths and kundatia thalli in the overlying amouslek formation archaeocyathan thromboid reefs were constrained by substrate stability water depth and subsidence rate four reef geometries are distinguished i patch reefs surrounded by shales ii bioherms in which flank beds intercalate laterally with carbonate and shale inter reef sediments iii biostromes or low relief structures formed as a result of lateral accretion of patch reefs and iv kalyptrate complexes that nucleated because of a marked tendency for aggregation and in which patch reefs and bioherms occur stacked together bounded by clay marl silt seams

neogene subsidence and stratigraphy of the simeulue forearc basin northwest sumatra abstract interpretation of new multi channel seismic data reveals the neogene subsidence history of the simeulue forearc basin located off northwestern sumatra the trench parallel elongated basin extends over 260 km in nw se direction and 100 km in sw ne direction maximum water depth is about 1300 m and the neogene sedimentary fill shows a thickness of up to 5 s two way traveltime we identified three stages of subsidence evolution after the formation of a regional basal neogene unconformity which are characterized by changing accumulation space sedimentation rates source areas and tectonic phases an early mid miocene stage is marked by occurrence of subsidence in several half grabens forming along the western border of the basin during the late miocene pliocene this small scale subsidence changed to a steadily subsiding trench parallel trough the present setup of the forearc region under the influence of strike slip faults due to oblique subduction is active at least since this time as evidenced by wrench faulting at the end of this stage subsidence expanded significantly eastwards drowning a large carbonate platform that evolved in the then shallow southern and eastern parts of the basin presently the central part of the simeulue basin is subject to inversion probably related to reactivation of early mid miocene half grabens

modeling of nonlinear wave propagation over fringing reefs abstract the applicability of existing nonlinear triad spectral models for steep slopes 0 1 0 2 characteristic of reef environments was investigated using both deterministic phase resolving and stochastic phased averaged formulations model performance was tested using laboratory observations of unidirectional wave transformation over steep and smooth bathymetry profiles the models developed for mild slopes were implemented with minimal modifications the inclusion of breaking parametrizations and linear steep slope corrections required by laboratory data the deterministic model produced typically more accurate predictions than the stochastic one but the phase averaged formulation proved fast enough to allow for an inverse modeling search for the optimal breaking parametrization the effects of the additional assumptions of the stochastic approach resulted in a slower than observed evolution of the infragravity band despite the challenge posed by the fast wave evolution and energetic breaking characteristic to the steep reef slopes both formulations performed overall well and should be considered as good provisional candidates for use in numerical investigation of wave current interaction processes on steep reefs

spectral wave dissipation over a barrier reef 1 a 2 week field experiment was conducted to measure surface wave dissipation on a barrier reef at kaneohe bay oahu hawaii wave heights and velocities were measured at several locations on the fore reef and the reef flat which were used to estimate rates of dissipation by wave breaking and bottom friction dissipation on the reef flat was found to be dominated by friction at rates that are significantly larger than those typically observed at sandy beach sites this is attributed to the rough surface generated by the reef organisms which makes the reef highly efficient at dissipating energy by bottom friction results were compared to a spectral wave friction model which showed that the variation in frictional dissipation among the different frequency components could be described using a single hydraulic roughness length scale surveys of the bottom roughness conducted on the reef flat showed that this hydraulic roughness length was comparable to the physical roughness measured at this site on the fore reef dissipation was due to the combined effect of frictional dissipation and wave breaking however in this region the magnitude of dissipation by bottom friction was comparable to wave breaking despite the existence of a well defined surf zone there under typical wave conditions the bulk of the total wave energy incident on kaneohe bay is dissipated by bottom friction not wave breaking as is often assumed for sandy beach sites and other coral reefs

observations of swash under highly dissipative conditions video measurements of swash were made at the low sloping beach of the multiple bar system at terschelling netherlands the majority of the measurements were conducted under highly dissipative conditions with iribarren numbers 0 the ratio of beach slope to the square root of offshore wave steepness less than 0 2 infragravity 0 004 0 05 hz waves dominated the swash with an average ratio of infragravity and total swash height rig r of 0 85 using linear regression we investigated the dependence of swash parameters on environmental conditions such as short wave height period and local beach slope on average rig was about 30 of the offshore wave height h0 the slope in the linear h0 dependence of rig amounted to only 0 18 considerably smaller than that observed on steeper beaches the data set shows evidence for saturation of the higher infragravity frequencies for 0 less than roughly 0 27 in our opinion this saturation caused the constant of proportionality in the linear relationship between rig h0 and 0 to be significantly larger than that observed under higher iribarren number regimes the saturated tails of the swash spectra had an approximate f 3 roll off where f is frequency whereas in general the nonsaturated parts were white this lack of significant peaks casts doubt on the causality between infragravity waves and nearshore bars

depth limited design wave heights in very flat regions abstract engineers working on the flat tops of offshore coral reefs or offshore banks require design criteria for depth limited wave heights over horizontal beds particularly in regions leeward of the bank or reef edge surf zone this paper presents convincing evidence of the fact that in engineering practice both in the laboratory and in the field the value of 0 55 is the largest wave height to water depth ratio realizable for stable shallow water oscillatory waves propagating in water of constant depth horizontal bed this value is much lower than that often used in engineering practice namely a value of about 0 8 which is historically based on the theoretical considerations and laboratory experiments associated with solitary waves this result based on a review of laboratory data and the original results of a major field experiment undertaken on the great barrier reef of australia leads to a significant reduction in design wave forces design crest levels and wave induced scour velocities at the bed

coral reefs reduce tsunami impact in model simulations 1 significant buffering of the impact of tsunamis by coral reefs is suggested by limited observations and some anecdotal reports particularly following the 2004 indian ocean tsunami here we simulate tsunami run up on idealized topographies in one and two dimensions using a nonlinear shallow water model and show that a sufficiently wide barrier reef within a meter or two of the surface reduces run up on land on the order of 50 we studied topographies representative of volcanic islands islands with no continental shelf but our conclusions may pertain to other topographies effectiveness depends on the amplitude and wavelength of the incident tsunami as well as the geometry and health of the reef and the offshore distance of the reef reducing the threat to reefs from anthropogenic nutrients sedimentation fishing practices channel building and global warming would help to protect some islands against tsunamis

tectonic features of the southern sumatra western java forearc of indonesia 1 multichannel reflection seismic profiles along the active sunda arc where the indo australian plate subducts under the overriding eurasian margin revealed two accretionary wedges the inner wedge i is of assumed paleogene age and the outer wedge ii is of neogene to recent age the inner wedge i is composed of tectonic flakes stretching from southeast sumatra across the sunda strait to northwest java implying a similar plate tectonic regime in these areas at the time of flake development during upper oligocene today wedge i forms the outer arc high and the backstop for the younger outer wedge ii the missing outer arc high of the southern sunda strait is explained by a combination of neogene transtension due to a clockwise rotation of sumatra with respect to java and by arc parallel strike slip movements the rotation created transtensional pull apart basins along the western sunda strait semangka graben as opposed to transpression and inversion on the eastern sunda strait within the new detected krakatau basin the arc parallel transpressional mentawai strike slip fault zone mfz was correlated from the sumatra forearc basin to the northwest java forearc basin off the sunda strait northward bending branches of the mfz are connected with the right lateral sumatra fault zone sfz along the volcanic arc segment on sumatra it is speculated that the sfz was attached to the cimandiri pelabuhan ratu strike slip fault of java prior to the presumed rotation of sumatra and that since the late lower miocene the main slip movement shifted from the volcanic arc position to the forearc basin area due to increasingly oblique plate convergence

observations of infragravity waves infragravity wave periods of one half to a few minutes energy levels observed for about 1 year in 8 m water depth in the pacific and in 8 and 13 m depths in the atlantic are highly correlated with energy in the swell frequency band 7 to 20 s periods suggesting the infragravity waves were generated locally by the swell the amplification of infragravity wave energy between 13 and 8 m depth separated by 1 km in the cross shore is about 2 indicating that the observed infragravity motions are dominated by free waves not by group forced bound waves which in theory are amplified by an order of magnitude in energy between the two locations however bound waves are more important for the relatively few cases with very energetic swell when the observed amplification between 13 and 8 m depth of infragravity wave energy was sometimes 3 times greater than expected for free waves bispectra are consistent with increased coupling between infragravity waves and groups of swell and sea for high energy incident waves

wave transformation on a coral reef wave transformation of regular waves was measured in a laboratory model of a fringing reef with a steep face and an outer reef top slope gradually decreasing in the landward direction data was obtained for various wave conditions and water levels a nonlinearity parameter fco g1 25ho 0 5t2 5 hc 1 75 based upon one proposed by swart and loubser 1979 is proposed as a suitable parameter for classifying wave transformation regimes on this reef in particular when fco 150 waves plunge on the reef edge and the amount of wave energy reaching a shore or structure is small 16 when fin co 100 waves spill on the reef top but the greater part of their energy is transmitted over the reef top the maximum values of the wave height to water depth ratio on the reef top were found to be consistent with nelson s analyses for laboratory and field data which indicate that the maximum stable wave height to depth ratio h d on a horizontal bottom never exceeds 0 55 for shallow water waves fc 500 the experimental data confirms that the maximum value of h d decreases when fc decreases but that it also increases when the bottom slope increases

seismic stratigraphy and cenozoic evolution of the lombok forearc basin eastern sunda arc abstract on the basis of single and multi channel seismic reflection data the stratigraphic framework and geologic history of the lombok forearc basin is discussed the lombok basin is built on the southeast sunda shelf continental margin and probably underlain by basement of thinned rifted continental crust it represents a broad ridged residual forearc basin which is still in an early stage of evolution five seismo stratigraphic sequences reflecting the cenozoic history and development of the forearc basin south of the islands of bali and lombok were identified the lowermost sequence 1 probably is composed of synrift deposits of paleogene age which predate the initiation of the present convergent margin sequences 2 and 3 reflect two major phases of tectonic evolution of the accretionary prism between the late oligocene and mid miocene sequences 4 and 5 represent slope front fill deposits and reflect both volcanic activity as well as tectonic uplift of the magmatic arc from mid miocene times onwards by the late miocene increased convergence between the subducting indian and overlying asian plates resulted in a stronger mechanical coupling between both plates this was expressed in the southern forearc basin by folding of the oldest basin fill deposits the present tectonic activity in the lombok basin is governed by the late pliocene collision of the accretionary prism with the scott marginal and roo rise oceanic plateaus this resulted in uplift of both the outer arc ridge and the southern part of the forearc basement

backstop geometry and accretionary mechanics of the sunda margin the convergent sunda margin off indonesia displays all geological features characteristic of an accretion dominated subduction zone a combined interpretation of prestack depth migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high resolution bathymetric data and for the first time allows the exact mapping of backstop regimes initially the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin increasing material strength of the outer high due to lithification formed a dynamic backstop which controls accretion today an out of sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area the existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc which is observed in regimes of frontal as well as of oblique subduction mass balance calculations which account for porosity changes and metamorphism indicate a subduction history dominated by accretionary processes since the late eocene accretion is associated with the low values of basal friction inferred for the sunda margin structural investigations of conjugate fault planes indicate a very weak basal detachment effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments whereas overpressuring within the frontal accretionary prism is negligible

structural evolution of backthrusting in the mentawai fault zone offshore sumatran forearc 1 we present the interpretation of newly acquired high quality industry standard deep seismic reflection and swath bathymetry data to provide insight into the structural style and evolution of the mentawai fault zone mfz the mfz lies along the boundary between the accretionary wedge and the proposed continental backstop this zone exhibits arcuate ridges on the seafloor convex toward the east beneath these ridges the structures developed as landward vergent imbricated backthrusts in the inner part of the accretionary wedge and higher angle backthrusts that deformed the forearc basin sediments in the forearc high anticlines were developed due to the seaward vergent forearc high thrusts originating in the accretionary wedge the imbricated backthrusts may have initiated during the early middle miocene contemporaneously with the slide and back rotation of forearc high thrusts in the late miocene the higher angle backthrusts were initiated continuous contraction induced the frontal higher angle backthrusts and formed a fold thrust belt toward the east during the pliocene the folds and thrusts were disturbed by diapirs and mud volcanoes backthrusting and fold thrust belts developed in the mfz may explain the compressional features observed at the boundary between the accretionary wedge and continental backstop along the southern sumatra margin the backthrusts along the mfz are waning in activity and hence the risk of a large earthquake and associated tsunami at the present time should be small

observations of surf beat forcing and dissipation 1 we used a simple energy balance equation and estimates of the cross shore energy flux carried by progressive surf beat to calculate the rate of net surf beat forcing or dissipation on a beach near duck north carolina far inside the surf zone surf beat dissipation exceeded forcing outside the surf zone surf beat forcing exceeded dissipation when incident waves were large surf beat dissipation inside the surf zone and forcing just outside the surf zone were both very strong the surf beat energy dissipated in the surf zone in a single beat period was of the same order as the total amount of surf beat energy stored in the surf zone during storms shoreward propagation of surf beat maintained surf beat energy in the surf zone net surf beat dissipation in the surf zone scaled as predicted by a simple bottom stress parameterization the inferred dissipation factor for surf beat was 0 08 within the range of wave dissipation factors usually observed in the field and 27 80 times larger than drag coefficients appropriate for the mean longshore current the observed rapid forcing rapid dissipation and shoreward propagation of surf beat are not simulated by existing models of surf beat dynamics

episodic circulation and exchange in a wave driven coral reef and lagoon system we examined the role of wave driven circulation relative to wind and buoyancy forcing in a coral reef lagoon system circulation measurements in paopao bay moorea french polynesia during austral summer show the importance of waves in driving flows over the reef crest through the lagoon and out the reef pass tides were comparatively weak due to proximity to amphidromic points and exhibited an unusual spring neap cycle where the major lunar tide modulated the major solar tide and the overall tidal phase stayed approximately constant wind had only a secondary effect compared to surface waves a simple fluid mass balance indicated rapid flushing of the shallow back reef and export through the reef pass and a reef capture zone width of 2 3 km the reef pass circulation dynamics exhibited two layer baroclinic exchange flow when waves were small which was suppressed during large wave events the unusually weak tidal forcing provided an opportunity to more closely investigate wave driven circulation dynamics as expected theoretically there was a wave driven setup of the free surface across the shallow lagoon which drove a highly frictional flow evident by a large drag coefficient cp 0 1 diverging from extant theory the observed setup varied strongly with significant wave height and period overall the circulation and exchange between this coral reef system and the adjacent open ocean were largely determined by episodic remote forcing events and differed significantly from periodic tidal exchange mechanisms

bar generating cross shore flow mechanisms on a beach random waves normally incident on a dissipative beach induce a variety of cross shore flows such as asymmetric oscillatory flow wave grouping induced long wave flow breaking induced turbulent flow and momentum decay induced undertow these flows are identified analyzed and hindcasted in a set of laboratory experiments with the aim of revealing the role of each of the flow mechanisms in the two dimensional case of bar generation on a beach

structural evolution and strike slip tectonics off north western sumatra abstract based on new multi channel seismic data swath bathymetry and sediment echosounder data we present a model for the interaction between strike slip faulting and forearc basin evolution off north western sumatra between 2 n and 7 n we examined seismic sequences and sea floor morphology of the simeulue and aceh forearc basins and the adjacent outer arc high we found that strike slip faulting has controlled the forearc basin evolution since the late miocene the mentawai fault zone extends up to the north of simeulue island and was most probably connected farther northwards to the sumatran fault zone until the end of the miocene since then this northern branch jumped westwards initiating the west andaman fault in the aceh area the connection to the mentawai fault zone is a left hand step over in this transpressional setting the tuba ridge developed we found a right lateral strike slip fault running from the conjunction of the west andaman fault and the tuba ridge in ssw direction crossing the outer arc high as a result extrusion formed a marginal basin north of simeulue island which is tilted eastwards by uplift along a thrust fault in the west the shift of strike slip movement in the aceh segment is accompanied by a relocation of the depocenter of the aceh basin to the northwest forming one major neogene unconformity the simeulue basin bears two major neogene unconformities documenting that differences in subsidence evolution along the northern sumatran margin are linked to both forearc evolution related to subduction processes and to deformation along major strike slip faults

seismic stratigraphy and cenozoic evolution of west sumatra forearc basin late tertiary neogene sediments deposited in the sunda forearc reveal the stratigraphic and structural evolution of this active plate margin a dense grid of multichannel seismic reflection data and information from 17 exploratory wells were used to establish a detailed seismic stratigraphic framework of the forearc region from 0 to 6 n several important tectonic cycles are recognized paleogene orogeny neogene subsidence and late tertiary tectonism superimposed on these regional tectonic events are three major transgressive regressive cycles of sedimentation related to changes in sea level and provenance paleogene and older metasedimentary and metamorphic rocks comprise basement beneath the landward inner margin of the forearc basin both basement rocks and lower tertiary sedimentary rocks were deformed and eroded approximately 25 to 30 ma the continental shelf was exposed to subaerial erosion and basin deposits were restricted offshore coincident with a worldwide lowstand of sea level in the oligocene the paleogene orogeny probably occurred prior to the erosional event that cut the prominent angular unconformity on the shelf the neogene history of forearc basin development is characterized by subsidence and nearly continuous sedimentation a basal transgression began in latest oligocene time and culminated in the middle miocene alternating sequences of limestone and shale comprise two second order cycles of sedimentation that are superimposed on an overall transgressive event a major regressive sequence followed in the pliocene owing to an influx of siliciclastic clay silt and sand derived from sumatra sedimentation rates were high and large volumes of terrigenous material were deposited in deltaic systems on the shelf the shelf slope break prograded basinward nearly 10 km 6 mi through lateral accretion and aggradation during a relative highstand or stillstand of sea level

the java margin revisited evidence for subduction erosion off java the eastern sunda margin off indonesia from central java to sumba island remains a little investigated subduction zone contrary to its well studied northwestern segment whereas large portions of the sunda margin are considered a classical accretionary zone subduction characteristics along the central java sector indicate erosive processes as the dominant mode of mass transfer the tectonic framework of the central java margin with a convergence rate of 6 7 cm yr insignificant sediment input and a pronounced seafloor roughness where the oceanic roo rise is subducting underneath java facilitates subduction erosion evidence for erosion comes from newly acquired geophysical data off central java local erosive processes in the wake of seamount subduction are documented by a high resolution bathymetric survey and result in an irregular trend of the deformation front sculpted by seamount collision scars subduction of oceanic basement relief leads to large scale uplift of the forearc as recorded on a reflection seismic profile and to a dismemberment of the previous outer forearc high giving way to isolated topographic elevations the broad retreat of the java trench and deformation front above the leading edge of the roo rise has exposed an area of approximately 25 000 km2 of deeper seafloor formerly covered by the previous frontal prism frontal erosion coincides with a steepening of the lower slope angle in the central java sector compared to the neighbouring segments in global compilations the key geological parameters of the central java margin lie in the erosive regime reflecting the interplay of basement relief subduction negligible sediment supply and a high convergence rate on the evolution of the margin

field observations of surf beat 1 progressive edge waves nineteen biaxial electromagnetic current meters have been used to determine the longshore and on offshore structure of currents at surf beat periods 1 4 min the sensors formed two linear arrays a long shore array within the surf zone and an on offshore array stretching from the shoreline to well beyond the breaker line analysis of the longshore current components yields a clear picture of progressive low mode edge waves with frequency wave number dispersion relations which are in remarkably good agreement with predictions some separation of edge wave modes is found with mode zero energy dominating in the frequency band 0 006 and 0 011 hz and mode one between 0 015 and 0 025 hz on offshore currents present a rather different picture which while not inconsistent with the longshore currents suggests that other sources of energy are also important to the on offshore currents these include standing edge waves probably formed by reflections at nearby scripps canyon and motions which are nonresonantly forced by incoming wave groups

wave run up on a high energy dissipative beach 1 because of highly dissipative conditions and strong alongshore gradients in foreshore beach morphology wave run up data collected along the central oregon coast during february 1996 stand in contrast to run up data currently available in the literature during a single data run lasting approximately 90 min the significant vertical run up elevation varied by a factor of 2 along the 1 6 km study site ranging from 26 to 61 of the offshore significant wave height and was found to be linearly dependent on the local foreshore beach slope that varied by a factor of 5 run up motions on this high energy dissipative beach were dominated by infragravity low frequency energy with peak periods of approximately 230 s incident band energy levels were 2 5 to 3 orders of magnitude lower than the low frequency spectral peaks and typically 96 of the run up variance was in the infragravity band a broad region of the run up spectra exhibited an f 4 roll off typical of saturation extending to frequencies lower than observed in previous studies the run up spectra were dependent on beach slope with spectra for steeper foreshore slopes shifted toward higher frequencies than spectra for shallower foreshore slopes at infragravity frequencies run up motions were coherent over alongshore length scales in excess of 1 km significantly greater than decorrelation length scales on moderate to reflective beaches

wave runup during extreme storm conditions 1 video measurements of wave runup were collected during extreme storm conditions characterized by energetic long swells peak period of 16 4 s and offshore height up to 6 4 m impinging on steep foreshore beach slopes 0 05 0 08 these conditions induced highly dissipative and saturated conditions over the low sloping surf zone while the swash zone was associated with moderately reflective conditions iribarren parameters up to 0 87 our data support previous observations on highly dissipative beaches showing that runup elevation estimated from the variance of the energy spectrum can be scaled using offshore wave height alone the data is consistent with the hypothesis of runup saturation at low frequencies down to 0 035 hz and a hyperbolic tangent fit provides the best statistical predictor of runup elevations

seismic stratigraphy and subsidence evolution of the northwest sumatra fore arc basin abstract new seismic reflection data collected with the indonesian rv baruna jaya iii under a french indonesian cooperative program allowed to study the geodynamic evolution and the seismic stratigraphy off northern sumatra within the sumatra fore arc domain the direction of subduction of the indian oceanic crust below the sunda arc is normal in front of java and oblique along sumatra as a result parts of the sumatra margin are moving northwards along two large strike slip faults parallel to the subduction trench the sumatra fault in sumatra and the mentawai fault at sea which separates the accretionary prism from the fore arc basin we describe the relationships between the seismic stratigraphy and the sedimentary history and we propose a tectonic evolution of the fore arc domain after widespread uplift and erosion during the paleogene the fore arc basin becomes subsident during the miocene as evidenced by shelf deposits organized in two transgressive regressive sequences 2a 2b produced by tectonics and eustatism this is interpreted as the result of a limited e w extension linked to the incipient play of the sumatra and mentawai faults in pliocene quaternary times the fore arc basin is segmented into several sub basins aceh simeulue and nias basins by compressional zones or by strike slip faults the subsidence rate increased strongly producing sequence 3 with two or four subunits for the pliocene and sequence 4 for the quaternary during this period local variations of the sediments thickness and of the number of sequences indicate that tectonics prevails over eustatism in the fore arc basin the sumatra margin is one of the best examples in the world to study the relationships between sedimentary and tectonic processes in an oblique subduction zone

drivers of residual estuarine circulation in tidally energetic estuaries straight and irrotational channels with parabolic cross section the generation of residual circulation in a tidally energetic estuary with constant longitudinal salinity gradient and parabolic cross section is examined by means of a two dimensional cross sectional numerical model neglecting river runoff and stokes drift it is shown how the longitudinal and lateral residual circulation can be decomposed into contributions from various processes such as tidal straining circulation gravitational circulation advectively driven circulation and horizontal mixing circulation the sensitivity of the residual circulation and its components from various processes to changes in forcing is investigated by varying the simpson number nondimensional longitudinal buoyancy gradient and the unsteadiness parameter nondimensional tidal frequency as well as the bed roughness and the width of the estuary for relatively weak salinity gradient forcing the tidal straining circulation dominates the residual exchange circulation in support of classical estuarine circulation up estuary flow near the bed and down estuary flow near the surface the strength of the longitudinal estuarine circulation clearly increases with increased salinity gradient forcing however when the simpson number exceeds 0 15 the relative contributions of both gravitational circulation and advectively driven circulation to estuarine circulation increase substantially lateral residual circulation is relatively weak for small simpson numbers and becomes flood oriented divergent flow near the bed and convergent flow near the surface for larger simpson numbers because of increasing contributions from gravitational and advectively driven circulation increasing the unsteadiness number leads to decreased longitudinal and lateral residual circulation although changes in bed roughness result in relatively small changes in residual circulation results are sensitive to the width of the estuary mainly because of changes in residual exchange circulation driven by tidal straining