recognition and distinction of normal and forced regression in cyclothemic strata a plio pleistocene case study from eastern north island new zealand

seismicity of the galapagos 95 5 w propagating rift an array of ocean bottom seismometers was deployed for 21 days about the 95 5 w galapagos propagating rift tip to obtain seismic evidence for the propagating rift hypothesis and to investigate the dynamic effects of rift propagation seismicity was fairly constant 10 17 events per day with no indication of swarm activity hypocenters were determined for 304 earthquakes the largest of which is estimated to be mb 2 3 with a seismic moment of 2 9 1020 dyn cm the cumulative seismic moment released by all local earthquakes is estimated to be 6 6 1020 dyn cm a b value of 0 77 0 08 was estimated for the region implying a stress regime dominated by a broad tectonic stress field the distribution of earthquakes is consistent with the propagating rift hypothesis most earthquakes occurred south of the propagating rift within the de steiguer deep seismicity within the transform zone is clustered in such a way as to suggest that two or three discrete fault segments may be active at the same time the seismicity observed along the propagating rift is diffuse earthquakes were observed at the propagating rift tip but not in front of it almost all earthquakes occur south of the northern pseudofault and its extension west of the rift tip that is earthquakes occur almost exclusively in lithosphere that has been or is being overtaken by the propagating rift six composite focal mechanisms are consistent with the propagating rift hypothesis first motions from earthquakes located on the propagating rift indicate extension whereas motions associated with the failing rift area indicate compression a compressional regime is also indicated west of the transform zone twelve earthquakes from the transform zone produce a poorly constrained focal mechanism consistent with right lateral strike slip motion along a fault plane that is roughly parallel to the isochrons of the transferred lithosphere

hydroacoustic events located at the intersection of the atlantis 30 n and kane 23 40 n transform faults with the mid atlantic ridge we investigate the characteristics of t phase events located at the ends of two segments of the mid atlantic ridge our motivations for the study were to understand whether t phase locations represent earthquake epicenters and thus whether accurate geological inferences can be made from their spatial patterns and to understand further the relationship between t phase event characteristics and earthquake properties we examine the characteristics of 158 t phase events with respect to both event location water depth and source to receiver distance the propagation paths of the t phases are also modeled to study the effects of encountering seafloor topography we find that existing models for t phase excitation and propagation cannot explain adequately all of our observations the amplitudes received levels of t phases at the hydrophones show no dependence on event water depths in contrast to current excitation models which predict a decrease in event magnitude with increasing water depth the received levels are observed to decrease with increasing source to receiver distance and events from the two study areas exhibit different trends in relative received levels between hydrophones once attenuation is taken into account our acoustic ray trace model is able to reproduce similar trends in relative amplitudes at the hydrophones based on 1 d topography between the event and each hydrophone but the variances in both the observed data and model are high we observe a pattern of short t phase onset times for shallow water events and long onset times for deep water events where onset time is defined as the time interval between the appearance of the t phase envelope above the ambient noise and its first peak this suggests that the onset time may be a function of several variables including efficiency of energy conversion based on local topography efficiency of propagation based on event water depth and hypocentral depth in the crust the results of this study underscore the complexity of t phase excitation and propagation and argue that current models of t phase excitation and propagation need to be improved to explain the observed characteristics of t phase data

transform seismicity at the intersection of the oceanographer fracture zone and the mid atlantic ridge a seismic array consisting of nine hawaii institute of geophysics hig ocean bottom seismometers obss was deployed at the eastern intersection of the oceanographer fracture zone ofz and the mid atlantic ridge mar the 12 day experiment was designed to relate low magnitude earthquakes to the structure and tectonics of the mar ofz intersection an average of 10 locatable events with duration based magnitudes between 1 0 and 2 0 were recorded per day excellent hypocentral locations of 112 events were obtained earthquake locations based on more than eight observations generally show 50 confidence volume constraints within obs location errors the earthquake locations cover a broad swath across the corner of the intersection zone magnitude weighted earthquake location likelihood maps suggest a decline in magnitudes near the intersection bathymetry low composite focal plane solutions suggest source mechanisms which indicate that the region is dominated by extensional tectonics alternative source solutions indicating translational movement are presented but are inconsistent with apparent bathymetric trends the transition from the diverging mar to translational ofz plate margin occurs in the context of reduced magma genesis and crustal thinning due to the influence of the adjacent older lithosphere the region may be described in terms of semirigid plate tectonics accompanying transform valley genesis

mechanisms and depths of atlantic transform earthquakes we have studied 40 earthquakes on seven major transforms along the mid atlantic ridge the majority show the expected transform parallel strike slip motion on steeply dipping fault planes some anomalous mechanisms suggesting either unusually shallow dip or a component of dip slip motion occur near ridge transform intersections the vertical tectonism suggested by transform morphology may thus be restricted to intersections where thermal contrast across the transform is greatest and stress orientations can be oblique to both ridge and transform centroid depths determined by waveform inversion are limited to a shallow zone corresponding approximately to the area above the 400 c isotherm of the thermal structure expected from lithospheric cooling the depth extent of faulting predicted by estimating fault area from seismic moments extends approximately to the predicted 600 c isotherm assuming that the areas above the 400 c or 600 c isotherms fail seismically moment release for six of the transforms accounts for varying fractions of the slip predicted by plate motion suggesting that some aseismic slip occurs if the time period sampled is representative in contrast on the vema transform the inferred seismic slip is greater than predicted unless faulting extends to the 650 c isotherm alternatively the vema may have an unusually long recurrence time or be mechanically anomalous due to its complex history in contrast to transform events centroids for oceanic intraplate earthquakes extend to approximately the 750 c isotherm this result is intriguing if depths reflect lithospheric strength as a function of depth transforms have much higher strain rates than intraplate areas so if both had the same rheology the limiting isotherm for transforms should be higher alternatively if a weaker rheology were appropriate the higher strain rate should cancel the weakening and yield a limiting isotherm similar to that for intra plate regions transforms may thus be either weaker and or have higher temperatures than expected in the latter model the required high temperatures perhaps due to intrusion at depth predict measurable heat flow anomalies the second possibility unusual rheological weakness is reminiscent of the unexplained weakness of the san andreas fault where heat flow data exclude anomalously high temperatures

microearthquake characteristics and crustal velocity structure at 29 n on the mid atlantic ridge the architecture of a slow spreading segment we report the results of a microearthquake and seismic tomography experiment conducted along the southern half of the mid atlantic ridge segment at 29 n and aimed at investigating the relationship of earthquake and seismic structural characteristics to spreading processes the seismic velocity structure is obtained from two dimensional 2 d and three dimensional 3 d tomographic inversions of travel times from shots along an axial refraction line inversion solutions indicate that the velocity structure in the lower crust is heterogeneous with higher velocities and relatively thin crust near the segment end and lower velocities and a thicker layer 3 near the central bathymetrie high the thickness of the lower crust at the segment end is asymmetric across axis with thinner crust beneath the inside corner the indicated variations in crustal thickness are consistent with those inferred from mantle bouguer gravity anomalies the microearthquakes located along axis during the 41 day recording period cluster in three separate along axis regions 1 the southern segment end near 28 55 n 2 the central along axis topographic high at 29 11 n near and north of the broken spur hydrothermal vent field and 3 a region midway between beneath a volcano near 29 02 n the greatest level of microearthquake activity was in a diffuse zone off axis beneath the inside corner of a nontransform offset this pattern of off axis microearthquake activity and the cross axis asymmetry in crustal thickness at the segment end support tectonic models in which normal faulting and consequent crustal thinning occur preferentially at inside corner regions anomalous focal mechanisms for microearthquakes beneath the along axis volcano and the significant seismicity beneath the axial volcanic ridge at the segment center in contrast may be the result of volcanic and hydrothermal processes such as magma movement or thermal stresses generated near cooling plutons a comparison of microearthquake characteristics with residual gravity data and velocity structure leads to the hypothesis that microearthquakes associated with areas of thin crust near the segment end and inside corner are dominantly tectonic in nature whereas microearthquakes associated with volcanic and hydrothermal processes are more likely to occur toward the segment center in areas of greater rates of magma supply and thicker crust along axis well resolved focal depths determined with a 3 d velocity model range from 3 to 6 km beneath the seafloor and do not shoal toward the segment center these observations indicate that the thermal structure of the crust along this slow spreading ridge segment is not in steady state

forearc uplift and extension southern hawke s bay new zealand mid pleistocene to present the maraetotara plateau cape kidnappers area in southern hawke s bay new zealand lies in the forearc region of the obliquely convergent hikurangi margin where the pacific plate is presently subducting beneath the leading edge of the australian plate uplift accompanied by extensional faulting has been the dominant style of deformation in this area since mid pleistocene time principal strain directions and strain ratios determined from mesoscale fault arrays associated with major faults document the regional extension in the cape kidnappers area fault arrays record plane strain with northwest horizontal extension and vertical shortening in the maraetotara plateau area fault arrays record triaxial strain with roughly equal magnitudes of horizontal extension in two principal strain directions either east west and north south or northwest and northeast and vertical shortening the morphology seismicity and nature and distribution of active faults of the southern hawke s bay region suggest that the maraetotara plateau cape kidnappers area is the on land expression of an outer arc high which is developed between the accretionary wedge on the east and the forearc basin on the west the cause of uplift and extension in the maraetotara plateau cape kidnappers area may be either upper plate deformation related to structural evolution of the outer arc high and underlying backstop underplating on the hikurangi subduction zone or a combination of these processes the uplift and horizontal extension occurring in the maraetotara plateau cape kidnappers area contrast sharply with active deformation in adjacent parts of the forearc reflecting a distinct partitioning of strain in this part of the obliquely convergent plate boundary

deep earthquakes beneath central taiwan mantle shearing in an arc continent collision the island of taiwan owes its existence to the collision of an island arc on the philippine sea plate with the eurasian continental shelf most of the earthquakes in the mantle beneath taiwan clearly are related to one of these plates subducting beneath the other but beneath the central part of the island there is a zone of subcrustal seismicity that is not obviously related to any subduction zone we investigated the uncertainties in the locations of these anomalous earthquakes by rereading p and s wave arrival times and computing information density functions for several of these events the seismic zone itself is then defined through a joint probability function incorporating the entire data set individual information density functions show that several events are likely to be located at depths exceeding 60 km with two events exceeding 80 km depth the joint probability function shows a narrow zone of seismicity about 20 km wide along strike plunging to the east combining these results with evidence from surface geology focal mechanisms and subsurface tomography we suggest that these events are caused by an eastward displacement of the eurasian mantle north of 24 n which juxtaposes it against colder subducted crust to the south moreover the heat produced by this type of shearing in the mantle must be insignificant for brittle failure to exist at these depths

rheological evolution of the ocean crust a microstructural view rheological studies of the oceanic lithosphere have used direct observations of rocks and inferences based on thermal mechanical and experimental models modeling studies help to constrain an average crust and mantle rheology and the deep seated processes that control regional scale strength and stress field variations the smoothing of fine scale variations in such models can obscure many of the geological processes that influence strain localization and deformation partitioning in the ocean crust examination of deformation mechanisms and histories in ocean crust rocks provides a complementary approach to modeling examples of structures and deformation histories in diabases from deep sea drilling project ocean drilling program site 504b the hayes and atlantis fracture zones mid atlantic ridge and the troodos ophiolite cyprus are presented in conjunction with a synthesis of microstructural studies of the ocean crust over the last 25 years a survey of brittle quasi plastic and synmagmatic viscous deformation is used to demonstrate the influence of primary compositional and textural characteristics and variable magmatic and hydrothermal histories on deformation mechanisms and strain localization in the ocean crust geological evidence indicates that hydrothermal fluids strongly influence the nature of deformation and that effective stresses may be low due to fluid overpressures melt distribution will strongly influence strain localization at the base of the crust and synkinematic hydration during crystal plastic deformation plays a key role in the relative strengths of polyphase oceanic lithologies a schematic distribution of failure mechanisms in the ocean crust is used to discuss the controls on variations in lateral and vertical strength profiles and their possible relation to spreading rates although microstructural studies of ocean crust are still in their infancy they provide valuable constraints for rheological models and further insights to explain the distribution of seismicity at spreading centers and acoustic signatures

microearthquake characteristics and crustal vp vs structure at the mid atlantic ridge 35 n we report on the results of a microearthquake experiment conducted at the along axis bathymetric high of the slow spreading ridge segment near 35 n on the mid atlantic ridge mar of a total of 255 microearthquakes recorded during the 43 day experiment 31 were located near the oceanographer transform at the northern end of the segment 79 occurred at the nontransform offset at the southern end of the segment and 145 were at the segment center at the segment center earthquake epicenters lay within the median valley inner floor and formed a 12 km long trend paralleling a steep scarp bounding the western wall of the inner valley focal depths were 3 4 km below the seafloor most 80 of the focal mechanisms for 32 segment center earthquakes are consistent with normal faulting on faults approximately parallel to the axial trend from a joint inversion for hypocenters and p and s wave velocity structures we determined a horizontally averaged v p v s ratio that decreases from 2 9 in the shallowmost 300 m to 1 7 at 2 km depth and we interpret this decrease as indicating a decreasing contribution of thin cracks to fracture porosity with depth the maximum depth of seismicity 4 km is anomalously shallow compared with other mar segments at which microearthquake experiments have been carried out cross axis relief is also anomalously low for this segment s center and on the basis of this and other mar microearthquake experiments there appears to be a correlation between cross axis relief and maximum depth of seismicity from the correlation of cross axis relief and inferred crustal thickness we suggest a relationship between thick crust high crustal temperatures and low cross axis relief in qualitative agreement with thermomechanical models for the depth of the axial valley

stratigraphic simulation of sedimentary basins concepts and calibration a two dimensional stratigraphic simulation program based on semiempirical algorithms has been successfully applied to clastic carbonate and mixed clastic carbonate depositional regimes the program may be used to predict reservoir distribution to constrain interpretations of well and seismic data to test exploration scenarios in frontier basins and to evaluate the fundamental controls on basin stratigraphy applications to seismic and well log data sets from main pass u s gulf coast offshore sarawak malaysia and the baltimore canyon u s east coast demonstrate that the program simulates stratigraphy on a basin wide scale as well as on the scale of individual prospects the main pass section is an offlapping sequence of neogene clastics the model simulates 17 m y of geologic history at a 200 000 yr resolution and reproduces gross basin geometry major sequence boundaries depth and location of nearshore marine sands intervals of sediment bypass into deep water shelf margin positions and paleobathymetry three neogene carbonate buildups of central luconia offshore sarawak malaysia were simulated using a sea level history optimized to reproduce the carbonate growth anatomy the model reproduced observed seismic reflection geometries and transgressive aggradational progradational retrogradational and subaerial exposure phases the program has also simulated jurassic pleistocene basin and stratigraphic sequence geometries and mixed clastic carbonate facies distribution along a 300 km regional dip line of the baltimore canyon trough modeled features of interest include a progradational middle jurassic carbonate margin punctuated by clastic deposition during sea level lowstands and a late jurassic early cretaceous aggradational carbonate margin culminating in the development and subsequent drowning of isolated carbonate buildups simulated bathymetries facies relations and subaerial exposure surfaces for the early cretaceous aggradation margin are consistent with detailed petrographic paleontologic and seismic stratigraphic interpretations

seismicity at the southern east pacific rise from recordings of an ocean bottom seismometer array 1 seismic data recorded by ocean bottom seismometers obss at the southern east pacific rise sepr were analyzed to characterize seismicity at fast spreading mid ocean ridges an automated event detection algorithm based on pattern recognition was developed to identify earthquake generated t phases the waveform cross correlation and least squares method was found to provide more robust estimates of t phase relative travel times than the method of picking t phase maximum amplitude the obss and subsequent equatorial pacific hydrophones recorded earthquakes near an earthquake swarm in 1992 which was located 300 km west of the ridge axis at about 18 s and 116 w the process that generated the 1992 swarm either was reactivated 3 years later during the obs experiment or has been continuously active in the past decade this longterm activity suggests renewable stresses which are consistent with the interpretation that magma injection was the cause of the 1992 swarm in contrast to the swarm characteristics of earthquakes on the west flank of the ridge axis earthquakes on the nazca flank were randomly distributed in time and space this difference in seismicity may result from the difference in the thermal structure of the pacific and nazca plates the few earthquakes located at or near the ridge axis had very small magnitudes reflecting the presence of shallow magma chambers and the thin elastic layer of the sepr this result suggests that acoustic monitoring of earthquakes associated with dike intrusion is difficult at regional scales at the sepr there is no apparent correlation between seismicity and off axis seamounts index terms 3025 marine geology and geophysics marine seismics 0935 3035 marine geology and geophysics midocean ridge processes 4259 oceanography general ocean acoustics 7230 seismology seismicity and seismotectonics 9355 information related to geographic region pacific ocean keywords t phase ocean bottom seismology mid ocean ridges east pacific rise marine seismics seismicity citation shen y seismicity at the southern east pacific rise from recordings of an ocean bottom seismometer array j geophys res 107 b12 2368 doi 10 1029 2001jb001742 2002

investigation of microearthquake activity following an intraplate teleseismic swarm on the west flank of the southern east pacific rise between february 1991 and may 1992 33 intraplate earthquakes having body wave magnitudes between 4 3 and 6 0 were located on the west flank of the southern east pacific rise by the international seismological center seven months after the last teleseismic event we deployed four ocean bottom seismometers at the site of the teleseismic swarm one hundred and ninety two microearthquakes were located using p and s travel times of events recorded by three or more instruments during the 16 day deployment most of the microearthquakes were in a band about 30 km long and 6 km wide between and parallel to seamount chains in addition several events were distributed along a line perpendicular to the main seismicity band and parallel to the ridge axis the focal depths of the microearthquakes range from 1 to 15 km and most are between 5 and 12 km similar to the depth range of the teleseismic events hung and forsyth 1996 the composite p wave polarities indicate that the microearthquakes had a variety of focal mechanisms we developed a new grid search inversion technique that utilizes the p wave polarities and the empirically corrected ratios of p and s wave amplitudes to find the focal mechanisms of individual events within the acceptable travel time and amplitude misfits focal solutions are fairly stable normal faulting is found in the ridge parallel seismicity line the thrust and strike slip faulting in the main seismicity band is distinctly different from the exclusively normal faulting mechanisms of the teleseismic events there is no apparent depth dependence of fault types none of the existing models of the sources of stress ridge push thermoelastic stresses loading by local topographic features caldera collapse and north south extension of the pacific plate provides a satisfactory explanation for both the teleseismic swarm and microearthquakes we propose a new tectonic scenario in this scenario the lithosphere is prestressed by the cooling of the plate magma rising from the deeper mantle induces normal faulting ahead of the dike tips in the lower lithosphere which is already under extensional thermal stress producing the larger teleseismically detected events once the dikes propagate into the lithosphere the region surrounding the dikes behind the tips is compressed by the overpressure of magma depending on the geometry of the dikes the local orientations of the minimum principal stress and the local weaknesses in the lithosphere thrust or strike slip faulting the microearthquakes may occur

hypocenter determination in strongly heterogeneous earth models using the shortest path method the shortest path method moser 1991 for the calculation of seismic ray paths and travel times along them can be applied directly in the hypocenter location method proposed by tarantola and valette 1982 it uses the analogy between seismic rays in the earth and shortest paths in networks to construct first arrival times from one point to all other points of a three dimensional grid simultaneously in a fast robust way in earth models of arbitrary complexity doing this for all stations of a seismic array one can find the hypocenter location by minimizing the difference between the observed and the calculated travel times at the stations over the three dimensional grid the concept of probability density functions allows then for a fully nonlinear examination of the uncertainties in the hypocenter location due to uncertainties in the travel time data numerical errors in the calculated travel times and to a limited extent incomplete knowledge about the earth model the result is a three dimensional contour map of regions of equal confidence for the earthquake location the method becomes especially attractive when more than one event recorded by the same array is studied because the calculation of the travel times which is relatively the most time consuming operation has to be done only once the method is applied on the location of an event that occurred on january 18 1989 in israel

transform fault earthquakes in the north atlantic source mechanisms and depth of faulting we have determined the centroid depths and source mechanisms of 12 large earthquakes on transform faults of the northern mid atlantic ridge from an inversion of long period body waveforms the earthquakes occurred on the gibbs oceanographer hayes kane 15 20 and vema transforms we have also estimated the depth extent of faulting during each earthquake from the centroid depth and the fault width for five of the transforms earthquake centroid depths lie in the range 7 10 km beneath the seafloor and the maximum depth of seismic faulting is 14 20 km on the basis of a comparison with a simple thermal model for transform faults this maximum depth of seismic behavior corresponds to a nominal temperature of 900 100 c in contrast the nominal temperature limiting the maximum depth of faulting during oceanic intraplate earthquakes with strike slip mechanisms is 700 100 c the difference in these limiting temperatures may be attributed to the different strain rates characterizing intraplate and transform fault environments three large earthquakes on the 15 20 transform have shallower centroid depths of 4 5 km and a maximum depth of seismic faulting of 10 km corresponding to a limiting temperature of 600 c the shallower extent of seismic behavior along the 15 20 transform may be related to a recent episode of extension across the transform associated with the northward migration of the triple junction among north american south american and african plates to its present position near the transform the source mechanisms for all events in this study display the strike slip motion expected for transform fault earthquakes slip vector azimuths agree to within 2 3 of the local strike of the zone of active faulting the only anomalies in mechanism were for two earthquakes near the western end of the vema transform which occurred on significantly nonvertical fault planes secondary faulting occurring either precursory to or near the end of the main episode of strike slip rupture was observed for five of the 12 earthquakes for three events the secondary faulting was characterized by reverse motion on fault planes striking oblique to the trend of the transform in all three cases the site of secondary reverse faulting is near a compressional jog in the current trace of the active transform fault zone we find no evidence to support the conclusions of engeln wiens and stein that oceanic transform faults in general are either hotter than expected from simple thermal models or weaker than normal oceanic lithosphere

spatial and temporal distribution of seismicity along the northern mid atlantic ridge 15 35 n 1 a detailed investigation of the relationship between the spatial and temporal patterns of the seismic activity recorded by six autonomous hydrophones and the structure of the northern mid atlantic ridge between 15 and 35 n is presented two years of monitoring yielded a total of 3485 hydroacoustically detected events within the array recorded by four or more hydrophones the seismically active zone extends 20 km to either side of the ridge axis consistent with earlier results from studies of fault morphology along the axis hydrophone recorded activity shows important variations areas with intense and persistent seismic activity stripes stand in sharp contrast to areas that lack seismicity gaps the regions of persistent activity are a new observation at mid ocean ridges in general the patterns of seismically active inactive regions are also recognized in the 28 year teleseismic record implying that these patterns are maintained at timescales between a few years and a few decades we find no simple relationship between individual segment variables e g length or trend of the segment maximum offset of discontinuities or along axis change in mantle bouguer anomaly mba and water depths and number of hydrophone recorded events there does appear to be a correlation between axial thermal structure and seismicity regions of low and high numbers of events would thus correspond to thinner hotter and thicker colder lithosphere respectively seismicity may reflect thermal structure at short timescales decadal or longer while relief and inferred crustal thickness may integrate this structure over longer periods of time order of 1 myr

geometry and facies of tertiary clinothems spitsbergen sandstone clinothems from the battfjellet formation palaeogene on spitsbergen are locally exceptionally well preserved along depositional dip parallel mountainsides the clinothems are more than 1 km wide and more than 100 m thick superposition of several sandstone clinothems separated by mudstones reflects repeated shoreline progradation and transgression deposition took place partly on post transgressional depositional shelves and partly by contributing seaward sloping wedges or clinothems to a ramp progradation shorelines dominated both by mouth bar and shoreface environments have been identified the clinothems are organized into an overall progradational architecture with a geometry having features in common with progradational seismic facies

multibeam bathymetric gravity and magnetic studies over 79 e fracture zone central indian basin a regional scale bathymetric map has been constructed for the 79 e fracture zone fz in the central indian basin between 10 15 s and 14 45 s latitudes and 78 55 e and 79 20 e longitudes using the high resolution capabilities of the hydrosweep multibeam swath bathymetric survey system the bathymetric map clearly reveals the 79 e fz depicting a ridge and trough topography with an elevation difference of over 300 m prominent e w bathymetric lineations and several new features such as seamounts are conspicuous on the map the e w bathymetric lineations bend at the fracture zone toward south resulting in oblique extensional features these features are more prominent in the middle and northern portion of the study area and are less significant in the southern portion the prominence of these features in the north and their subtle presence in the south are probably related to the strong and weak shear coupling at the rti respectively during the various stages of fz evolution gravity and magnetic data were analyzed together with the bathymetric data the observed free air gravity across the 79 e fz is modeled in terms of the lithospheric thickness taking into consideration the thermal evolution of the oceanic lithosphere a lithosphere thickness of about 100 km is inferred from these studies

tectonic and sea level controls on nontropical neogene limestones in new zealand modern shelf carbonates and neogene on land equivalents have formed upon and adjacent to the new zealand sector of the convergent australia pacific plate boundary they are exclusively skeletal hashes dominated by bryozoans bivalves barnacles and foraminifers typical of nontropical foramol carbonates tectonism has influenced carbonate formation by determining terrigenous sediment supply rates and therefore the dilution factor and has affected the structure of the sea floor by forming imbricate thrust ridges and fault bounded platforms that support the carbonates the neogene limestones are more abundant closer to the plate boundary than their modern counterparts reflecting an increase in tectonism with time in the late pliocene sea level movements combined with uplift of the forearc and transform basins to form widespread limestones only during the glacials

deep tow studies of the vema fracture zone 1 tectonics of a major slow slipping transform fault and its intersection with the mid atlantic ridge the vema transform fault which slips at a rate of 24 mm yr displaces the mid atlantic ridge mar 320 km in a left lateral sense high resolution deep tow studies of the vema ridge transform intersection rti and the eastern 130 km of the active transform fault reveal a complex pattern of dip slip and strike slip faults which evolve in time and space at the intersection both the neovolcanic zone and the west wall of the mar rift valley curve counterclockwise toward the transform fault along trends approximately 30 oblique to the regional north south trend of the spreading axis the curving of extensional structures in the rift valley such as normal faults and the axial zone of dike injection appears to be related to transmission of transform related shear stresses into the spreading center domain intermittent locking of the american and african lithospheric plates across the rti causes shear stresses to penetrate up to 4 km into the mar axial neovolcanic zone where the lithosphere is relatively thin and up to 12 km into the block faulted west wall of the rift valley where the lithosphere is thicker the degree of shear coupling across the rti may vary with time due to changes in the thickness of the lithosphere along the axis 0 10 km the strength of a mantle weld at depth and the presence or absence of an axial magma chamber so that extensional structures at the rti may be either spreading center parallel when coupling is weak or oblique when coupling is strong oblique extension across the rti in addition to other factors may account for some of the down dropping of lithosphere within the deep nodal basin the easternmost 20 km of the active transform fault zone near the rti displays a braided network of three to nine tectonically active grabens and v shaped furrows in a zone 2 4 km wide interpreted to consist of interwoven riedel shears p shears and oblique normal faults clay cake deformation experiments and deep tow observations suggest that p shears and r shears which are 10 20 oblique to the transform slip direction develop during the initial stages of transform faulting near the rti as the newly accreted lithosphere accelerates to full plate velocity some of the r shears propagate along strike and intercept the oblique normal faults resulting in sharply curving scarps at the rti subsequent to this merging of the two fault types some of the r shears develop a significant component of dip slip while other r shears merge with p shears creating a complex anastomosing fault pattern up to 4 km wide a continuous strand within this braided pattern of faults is interpreted to be the principal transform displacement zone near the rti twenty kilometers west of the rti the active transform fault zone narrows to a furrow generally less than 100 m wide with only a few short discontinuous splays this narrow groove cuts through thinly sedimented basalt 20 40 km west of the rti and continues as a narrow furrow less than 100 m wide through up to 1 5 km of layered turbidite fill most of the way to the western rti such a narrow zone of deformation typifies the mature stages of transform faulting where the lithosphere on both sides of the transform fault is relatively old thick and rigid and has completed its acceleration to full plate velocity the transform fault zone is closely associated with a partially buried median ridge and widens to 1 2 km where it transects exposed portions of the ridge the transform parallel median and transverse ridges create the highest topography associated with the transform fault and may be serpentinized ultramafic intrusions capped by displaced crustal blocks of gabbro metagabbro and basalt

flexural uplift of a lithospheric slab near the vema transform central atlantic timing and mechanisms abstract the vema transverse ridge vtr is a prominent long and narrow topographic anomaly that runs for over 300 km along a sea floor spreading flow line south of the vema transform at 11 n in the atlantic it rises abruptly about 140 km from the axis of the mid atlantic ridge mar in 10 myr old crust and runs continuously up to 25 myr old crust it reaches over 3 km above the predicted lithospheric thermal contraction level it is absent in crust younger than 10 myr thus the uplift of the vtr must have ended roughly 10 ma the vtr is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was generated at an 80 km long mar segment based on satellite gravimetry imagery this mar segment was born roughly 50 ma and increased its length at an average rate of 1 6 mm yr multibeam data show that the mar parallel sea floor fabric south of the vtr shifts its orientation by 5 to 10 clockwise in 11 12 myr old crust indicating a change at that time of the orientation of the mar axis and of the position of the euler rotation pole this change caused extension normal to the transform followed between 12 and 10 ma by flexure of the edge of the lithospheric slab uplift of the vtr at a rate of 2 to 4 mm yr and exposure of a lithospheric section vema lithospheric section or vls at the northern edge of the slab parallel to the vema transform ages of pelagic carbonates encrusting ultramafic rocks sampled at the base of the vls at different distances from the mar axis suggest that the entire vtr rose vertically as a single block within the active transform offset a 50 km long portion of the crest of the vtr rose above sea level subsided was truncated at sea level and covered by a carbonate platform subaerial and submarine erosion has gradually removed material from the top of the vtr and has modified its slopes spreading half rate of the crust south of the transform decreased from 17 2 mm yr between 26 and 19 ma to 16 9 mm yr between 19 and 10 ma to 13 6 mm yr from 10 ma to present the slowing down of spreading occurred close in time to the change in ridge transform geometry suggesting that the two events are related a numerical model relates lithospheric flexure to extension normal to the transform suggesting that the extent of the uplift depends on the thickness of the brittle layer consistent with the observed greater uplift of the older lithosphere along the vtr

sea level changes an integrated approach sea level changes an integrated approach in october 1985 sepm sponsored a four day conference entitled sea level changes an integrated approach the purpose of the conference was to provide a forum for an interdisciplinary exchange of ideas on sea level changes and to provide an opportunity for integrating various types of evidence in approaching unresolved issues the conference was successful in bringing together scientists from industry academia and government representing all of the major geosciences disciplines presentations of many new papers plus significant releases of data that were previously held proprietary provided fertile ground for discussion this much cited volume represents the best of the material presented at the conference includes the early vail chart

microearthquakes and faulting in the southern okinawa trough abstract southern okinawa trough represents an early stage of back arc rifting and is characterized by normal faulting and microearthquakes earthquake distribution and deep structure of fault was investigated to clarify active rifting in the southern okinawa trough where two parallel grabens are located a network of ocean bottom seismometers obss that displayed the hypocenters of 105 earthquakes were observed for a period of 4 days in southern graben sg most of the microearthquakes occurred in a cluster about 7 km wide which on a cross section striking n45 e dips 48 to the southwest relocated hypocenters which are recorded by a local seismic network show scattered distribution around the southern graben there are no remarkable surface faults in the southern graben on the other hand the recalculation of hypocenter locations of 1996 earthquakes swarm recorded by a local seismic network suggests that the swarm is associated with normal faulting on the southern side of northern graben ng thus the undeveloped southern graben is located to the south of the developed northern graben southward migration of rifting which may be caused by migration of volcanism could thus be occurring in the southern okinawa trough the extension rate computed for the southern okinawa trough from the fault model of the northern graben is 4 6 cm year which is 59 102 of the extension rate gps measurements this result indicates that the majority of extensional deformation is concentrated within the center of the northern graben in the okinawa trough

variability of lowstand wedges and their distinction from forced regressive wedges in the mesaverde group southeast wyoming the debate on the geometry and origin of shelf sandbodies has evolved from a discussion of how sands and gravels managed to move from shorelines far out onto muddy shelves to an acceptance that many of these sandbodies did not originate on the shelf but were simply resident there after a fall and subsequent rise of sea level in the efforts to reinterpret these sandstones there has been some tendency to underestimate how varied they can be data from eight such shelf sandstone tongues campanian in southeast wyoming are used to illustrate how variable lowstand prograding wedges can be and how these should in addition be distinguished from forced regressive wedges the depositional environments encountered in these two types of wedges are surprisingly different and the lowstand wedges which are frequently tidally influenced show a greater affinity with the overlying transgressive tract than with the preceding wave dominated forced regressive shoreface deposits

late miocene breccia of menorca balearic islands a basis for the interpretation of a neogene ramp deposit abstract neogene tortonian ramp facies associations crop out in the southern sector of menorca balearic island these are made mainly by sigmoidal and oblique clinoform units comprising rhodoliths bryozoans molluscs and foraminifera these units are interpreted as outer ramp deposits a breccia deposit infilling an erosional surface is found at the top of the carbonate ramp sequence the breccia components mainly rhodoliths and oolite clasts may represent erosion of the underlying ramp deposits and inner ramp counterparts which do not crop out on the island the study of components in the breccia deposits confirms the indigenous character of the outer ramp facies associations which suggests that the ramp was steepened distally the breccia deposits correspond laterally to a discontinuity surface locally showing karstic features transgressive sediments including a phosphatic crust are found above the discontinuity surface all together these features and the dolomitisation of the uppermost ramp sediments and breccia deposits suggest that the breccia originated from erosion of the ramp after a major relative sea level fall the breccia and discontinuity surface separate the ramp sequence from an overlying prograding sequence a correlation of this sequence boundary to other areas in the western mediterranean is proposed

brittle deformations in the upper pleistocene deposits of the crotone peninsula calabria southern italy abstract the structural setting of the crotone peninsula calabria southern italy is discussed in this paper structural analyses were carried out both on a regional and an outcrop scale to detect the main structural systems and the stress field which affected the area during the late pleistocene the tectonic features observed in the area are represented by normal faults and extensional joints the regional scale faults are arranged in five main systems striking ene wsw nne ssw e w nw se and nnw sse they are responsible for the dislocation of four marine depositional units cutro terrace correlatable with the isotopic stage 7 s leonardo campolongo isola di capo rizzuto terrace correlatable with the isotopic substage 5e le castella capo rizzuto terrace correlatable with the isotopic substage 5c and capo colonne terrace correlatable with the isotopic substage 5a in several distinct terraces from the studies carried out on a mesostructural scale it is possible to conclude that for the last 200 000 yrs the crotone peninsula was affected by an extensional stress field the comparison between the average uplift rates calculated for the crotone peninsula terraces and those computed for several upper pleistocene deposits located in sicily calabria and campania suggests that the intense tectonics which affected the crotone peninsula can be related to the geodynamic activity of the calabrian arc

horizontal extent of anomalously thin crust near the vema fracture zone from the three dimensional analysis of gravity anomalies recent seismic refraction and reflection experiments have demonstrated that parts of oceanic fracture zones are characterized by very thin crust with total crustal thickness less than half that of normal oceanic crust the horizontal extent of this anomalous crust is not well known to map the lateral distribution of thin crust in a major fracture zone we have constructed free air anomaly bathymetry and sediment isopach maps of a 220 by 280 km region surrounding the western intersection of the vema fracture zone with the mid allantic ridge we isolate those components of the gravity field that are associated with variations in crustal thickness by first subtracting the attraction of the water crust interface the sediment layer and the crust mantle interface assuming constant crustal thickness and density these calculations are performed using a fully three dimensional model the remaining mantle bouguer anomaly is dominated by the effects of the cooling lithosphere variations in density of the crust and upper mantle associated with the cooling of the plate with increasing age of the seafloor are predicted for a thermal model that incorporates advection and the three dimensional conduction of heat the attraction of this three dimensional density structure is subtracted from the mantle bouguer anomaly to generate a map of the residual gravity anomaly which should directly reflect variations in crustal thickness or density the residual gravity anomaly map indicates that there is thin crust along much of the length of the vema fracture zone thin crust is concentrated primarily beneath the walls of the fracture zone rather than beneath the center of the fracture zone valley an interpretation confirmed by seismic refraction data the degree of thinning along the length of the fracture zone is variable ranging from one section with no appreciable thinning to others in which the crustal thickness is less than 3 km the excess depth of the fracture zone compared to surrounding seafloor is not isostatically compensated by local thinning of the crust other major topographic features such as the median valley and the elevated transverse ridge along the south wall of the fracture zone are also not locally compensated by changes in crustal thickness

the stratigraphic architecture of late pliocene 2 8 2 4 ma asymmetrical shelf sequences western wanganui basin new zealand abstract the western wanganui basin in the vicinity of parikino contains a 410 m thick succession of thirteen sequences that accumulated between 2 8 and 2 4 ma traditional facies analysis of the succession indicates accumulation of the sequences in shoreface to mid shelf depositional palaeoenvironments the late pliocene sequences have been correlated with oxygen isotope stages g10 to g92 and each of 41 ka glacial interglacial stage couplets is represented by an individual depositional sequence comprising transgressive tst highstand hst and regressive rst systems tracts most sequence boundaries are unconformable and coincide with transgressive surfaces of erosion tse ravinement surface with ophiomorpha burrows extending down into underlying sandstone one sequence boundary at the base of the wilkies shellbed is a correlative conformity two types of shellbed occur within tsts and are associated with key sequence stratigraphic surfaces onlap and backlap shellbed onlap shellbeds onlap the tse and sequence boundary backlap shellbeds result from stratigraphic condensation at the top of the tst and are overlain by the downlap surface dls in most of the sequences the onlap and backlap shellbeds are superposed forming compound shellbeds rarely a downlap shellbed overlies the dls within the base of hsts hsts are composed typically of massive aggradational mid shelf siltstone that grades up into sandstone which forms thick progradational inner shelf to shoreface lithofacies attributed to regressive systems tracts rst lensoidal toplap shellbeds comprised of reworked nearshore storm emplaced fauna occur within the upper part of rsts all of the parikino sequences have asymmetrical architectures with thin tsts this arises because of the very low rates of non tropical carbonate accumulation during the transgressions that is about half of the 41 ka period of each cycle amounting to a few metres at most of shellbed followed by very high rates of terrigenous sediment accumulation during the latter half of cycles an implication of this work is that marked 40 60 m eustatic sea level changes occurred repeatedly during the interval 2 8 to 2 4 ma this implies substantial and successive continental glaciations from at least 2 8 ma probably in the northern hemisphere

pleistocene unconformity bounded shelf sequences wanganui basin new zealand correlated with global isotope record abstract eleven successive unconformity bound sequences in the castlecliff section wanganui basin each typically composed of a lower variably conglomeratic 0 1 2 m thick shell bed and an upper 5 20 m thick barren siltstone are identified in the stratotype of the local new zealand middle pleistocene castlecliffian stage based on magnetostratigraphy the section is correlated with the published benthonic oxygen isotope curve for dsdp core 552a from the north atlantic each of the sequences accumulated during one of the odd numbered interglacial isotope stages in the interval 31 to 11 inclusive the unconformities developed during intervening glacial ages when the shelf was exposed through glacio eustatic lowering the correlation provides a geochronologic framework for assessment of the eustatic contribution to the generation of the sequences in a situation where the paleoshorelines and related shelf sequences are preserved and for which an independent approximation of the sea level history is available in the form of an oxygen isotope curve

an ocean bottom seismometer study of shallow seismicity near the mid america trench offshore guatemala five ocean bottom seismometers recorded seismicity near the mid america trench offshore guatemala for 27 days in 1979 the array was emplaced in the lower slope region just above the topographic trench in the area investigated during deep sea drilling project legs 67 and 84 approximately 170 events were recorded by three or more seismometers and almost half were located with statistical hypocentral errors of less than 10 km most epicenters were located immediately landward of the trench axis and many were further confined to a zone northwest of the array in terms of depth most events were located within the subducting cocos plate rather than in the overlying plate or at the plate plate boundary their apparent concentration in the lower crust and upper mantle may suggest that the upper crust does not have the strength to support earthquake generating stresses the data permit construction of a magnitude duration scale calibrated with mb magnitudes for events located by the world wide standard seismograph network wwssn and recorded by our array and by the network recording foreshocks and aftershocks of the 1979 petatlan earthquake most magnitudes ranged between 3 0 and 4 0 mb and the threshold magnitude of locatable events was about 2 8 mb two distinct composite focal mechanisms were determined one appears to indicate high angle reverse faulting in the subducting plate in a plane parallel to trench axis strike the other constructed for some earthquakes in the zone northwest of the array seems to show normal faulting along possible fault planes oriented quasi perpendicular to the trench axis the normal faulting is consistent with the segmentation of the cocos plate that has been proposed from land evidence such segmentation might be evidenced offshore by normal faulting along planes subperpendicular to trench strike alternatively the seismicity zone and associated normal faulting mechanism may be the subsurface expression of the tectonics responsible for the san jose canyon a prominent submarine canyon located farther upslope finally projection of our seismicity sample and of well located wwssn events from 1954 to 1980 onto a plane perpendicular to the trench axis shows a distinct gap between the shallow seismicity located by our array and the deeper wadati benioff zone seismicity located by the wwssn we tentatively ascribe this gap to inadequate sampling but we suggest that it requires further investigation

application of bryozoan zoarial growth form studies in facies analysis of non tropical carbonate deposits in new zealand abstract the fragmental remains of bryozoan colonies are the dominant skeletal contributor in many modern and ancient occurrences of non tropical shelf carbonate deposits the taxonomic complexity of the bryozoa the difficulties of systematic classification except by specialists the wide range of particle sizes and of preservation state in the carbonate deposits and the often well cemented nature of host limestones all limit the sedimentological interpretation of the commonly diverse bryozoan component however following the pioneer work of stach in the 1930 s the potential exists for sedimentologists to obtain useful paleo environmental information about the deposits by recording simply the nature and relative abundances of the various growth shapes of the bryozoan material known as their zoarial colonial growth forms here we propose a simple descriptive terminology covering both cheilostome and cyclostome bryozoans that involves four main growth forms encrusting erect rigid erect flexible and free living and a selection of subcategories based on shape identification of these habitat related growth form types is aided by reference to line drawings specimen photographs and photomicrographs of thin section slices the scheme is illustrated by examining briefly some possible environmental controls on the spatial and or time variations in the nature and distribution of colonial growth forms in several examples of modern and tertiary bryozoan dominated carbonate deposits in new zealand we anticipate that the routine description and quantification of bryozoan colonial growth types in non tropical carbonates generally will facilitate the recognition and interpretation of contrasting sub facies within the global foramol bryomol group of carbonate deposits

structure of the earth oceanic crust and uppermost mantle the past four years have witnessed the introduction of a variety of new instruments and methods for the study of the seismic structure of the oceanic crust and lithosphere the application of these and existing tools has led to the discovery of a number of new phenomena and to a fuller understanding of the genesis and evolution of the oceanic ithosphere borehole seismic instrumentation has become more widely employed ocean bottom seismographs while generally decreasing in number have become significantly more reliable and useful and multichannel seismic systems have been employed in innovative experiments ranging from studies of fracture zones to the regular detection of magma chambers beneath rise axes the techniques available for the analysis of seismic data have become more sophisticated waveforms collected in seismic experiments can now be used directly in constructing and evaluating seismic velocity models and travel time data are regularly inverted directly for structure trial and error modeling has become increasingly unimportant marine seismologists are becoming increasingly involved in understanding the coupling between the ocean and the underlying oceanic lithosphere this has led to a more complete understanding of seafloor noise processes and the partitioning of energy between acoustic and elastic waves

stratal architecture of western mediterranean margins as the result of the stacking of quaternary lowstand deposits below glacio eustatic fluctuation base level abstract through a comparison of high resolution seismic stratigraphy of seven western mediterranean margins similarities and differences in stratigraphic architecture were highlighted and a hierarchy of controlling factors was established the areas are northwestern alboran ebro and northern catalonia in the spanish margin rho ne in the french margin and tuscany latium and calabria in the italian tyrrhenian margin despite their different tectonic physiographic and depositional setting the seismic facies and growth pattern of the middle late pleistocene deposits are comparable the margins are in fact the result of the stacking of lowstand systems tracts of 4th order depositional sequences bounded by polygenetic unconforniities on the shelf and condensed sections on the upper slope this lowstand nature results from the high frequency 100 ka high amplitude 100 m asymmetry falling 65 of the time and stationariness between 10 and 120 m with respect to the present of the glacio eustatic sea level curve if eustasy directly controls the types of deposits making up the margins tectonism conditions their growth pattern as it is the only factor accounting for similarities and differences in the external geometry stratal pattern and distribution of the middle late pleistocene depositional sequences an evolutionary model for the quaternary growth of young passive continental margins is proposed in this model tectonics determines the degree of preservation of lowstand deposits because it conditions the accommodation space left below the erosional profile produced by each glacio eustatic cycle thus some of the sequence stratigraphy principles widely used to set up the model have been modified and adjusted to take into consideration the peculiar nature of quaternary glacio eustasy

accommodation and supply a dual control on stratigraphic sequences abstract it is widely accepted that both eustatic and tectonically controlled regional changes of sea level have contributed to the record of stratigraphic sequences i suggest that environmental change be added as a third autonomous control sedimentologic principles clearly indicate that sequences and their systems tracts are controlled by the interplay of two rates the rate of change in accommodation space available for sedimentation and the rate of sediment supply sea level has direct control on accommodation but its influence on sediment supply is remote and easily overshadowed by environmental factors for instance the record of the most recent sea level rise is a transgressive systems tract where supply is low it is a prograding highstand systems tract in deltas where the supply is high examples of sequence boundaries generated by changes in sediment supply include tectonically driven shifts in sediment input into basins changes in ocean currents pulsating supply from failure of submarine slopes and drowning of carbonate platforms by environmental stress furthermore the stratigraphic sequences in fluviatile continental basins are physically removed from sea level induced changes in accommodation and must have formed by changes in the rate and pattern of supply subaerial exposure of marine sediments at the sequence boundary is a most important criterion for recognizing sea level cycles as opposed to supply cycles other criteria include downstepping of shelf breaks and characteristic patterns in the spacing of time lines within sequences some third order cycles ca 0 5 3 ma duration meet these criteria others do not cycle stacking patterns and the shifting facies belts on cratons indicate that many second and third order cycles lack pronounced exposure unconformities and represent gradual changes superimposed on more rapid shorter oscillations seismic data yield poor images of these gradational changes because they lack resolution seismic models of outcrops show that near the limits of resolution the seismic tool tends to portray rapid changes in facies and bed thickness i e lateral variations in supply as unconformities seismic unconformities do not necessarily correspond to unconformities in outcrop the succession of stratigraphic sequences is best considered a composite record of sea level and environmental change separating the various controls requires carefully planned experiments work on the oxygen isotope curve another proxy for sea level has set an example in this respect

deep tow seismic profiles from the vema transform and ridge transform intersection a recently developed near bottom reflection profiling system has been used to obtain high resolution seismic sections across the vema transform fault at the western ridge transform intersection deep towed 3 5 khz echo sounder data show evidence of fresh lava flows at the seabed within the nodal deep over one kilometre of sediment fills the bottom of the ridge transform intersection deep many strong discontinuous reflectors within the sedimentary section caused by lava layers indicate that the intersection deep has been the site of sustained igneous activity on the eastern side of the deep is a large uplifted fault block created as the newly formed crust moved out of the spreading centre dipping strata within the fault block document back tilting as the new crust is extended by normal faulting profiles across the active transform fault reveal that the zone of deformation throughout the one kilometre thick sediment column is narrow suggesting that the location of the fault has remained stable throughout at least the last million years

barnacle dominated limestone with giant cross beds in a non tropical tide swept pliocene forearc seaway hawke s bay new zealand abstract pliocene non tropical widespread and locally thick up to 100 m limestones occur in hawke s bay eastern north island where they are intimately associated with very thick 5 km terrigenous dominated neogene sequences that formed in a tectonically active convergent margin setting the non tropical character of the limestones is shown unequivocally by 1 the complete dominance of skeletal calcarenites and calcirudites 2 the occurrence of oyster banks as the only in situ organic structures 3 the dominance of barnacles epifaunal molluscs bryozoans echinoderms foraminifers brachiopods and calcareous red algae as skeletal components and 4 the preponderance of calcite over aragonite in the mineralogy of the skeletal grains and cements the abundance of barnacle fragments in the limestones and the related exclusive occurrence of only one major organic association a barnacle epifaunal bivalve bryozoan assemblage is striking and unusual given the extent of the limestones pecten and oyster valves acted as substrates for barnacle attachment and their growth was promoted by strong tidal paleocurrents that swept the depositional setting a long 450 km narrow 30 50 km forearc basin seaway which formed between an actively deforming subduction complex to the east and an uplifting structural ridge to the west synsedimentary deformation promoted limestone formation on the margins of the seaway by creating current swept clastic free submarine ridges that acted as the sites of carbonate production tidal flows dispersed the carbonate constituents and organised them into a wide spectrum of tide influenced cross bedded and horizontal structures most spectacular are occurrences of giant tabular cross beds with sets 10 40 m thick and foreset dips of 7 36 some interpreted as the deposits of major sand bars on carbonate deltas marginal to the mouths of saddles traversing the rising antiforms and others analogous to modern linear sand ridges the small to large scale planar and trough cross beds and the horizontal and lenticular beds that are invariably associated with the giant cross beds and dominate most sections represent mainly the deposits of sand waves and sand sheets at inner to mid shelf depths in the seaway

forced regressions in a sequence stratigraphic framework concepts examples and exploration significance sequence stratigraphic concepts suggest that stratal geometries develop and are largely controlled by changes in relative sea level on the shelf lowstand deposits which form during falls and subsequent stillstands of relative sea level can be recognized by the presence of an unconformity at the base the isolated and basinward position relative to the previous shoreline and the abrupt seaward translation of shallow water and shoreline facies into the basin across an unconformity surface this seaward translation of facies and shoreline regression in response to relative sea level lowering is termed a forced regression a forced regression is independent of variations of sediment flux and is in contrast with normal regressions that occur in response to excess sediment flux relative to space available on the shelf i e accommodation forced regressions commonly are associated with a zone of sedimentary bypass subaerial exposure and possible fluvial erosion between the newly formed and preceding shorelines certain shelf sands previously interpreted as offshore or mid shelf sand bodies thus can be reinterpreted as stranded lowstand shorelines associated with forced regressions this alternative interpretation has economic significance insofar as it suggests different subsurface correlations and reservoir geometries with the potential for development of new play types and enhanced recovery in o der fields examples of forced regression can be observed at a variety of scales and ages several such examples include the modern east coulee fan delta and the lower cretaceous viking formation in alberta canada the quaternary rhone delta and the quaternary hudson valley system

microearthquake seismicity of the mid atlantic ridge at 5 s a view of tectonic extension we report measurements made with an ocean bottom array which was operated for 10 days on the mid atlantic ridge just south of the 5 s transform fault fracture zone a total of 148 locatable earthquakes with magnitudes 0 5 2 8 were recorded seismic activity appears to be concentrated within the western half of the median valley the median valley seismic zone is bounded in along axis direction by the transform fault to the north and the tip of the axial volcanic ridge to the south a few scattered events occurred within the inside corner high on the transform fault and in the western sidewall close to the segment center earthquakes reach a maximum depth of 8 km below the median valley floor and appear to be predominantly in the mantle although a few crustal earthquakes also occurred the presence of earthquakes in the mantle indicates that it is not strongly serpentinized we infer the median valley seismic activity to primarily arise from normal faulting

microseismicity in the middle okinawa trough we conducted a microearthquake survey with ocean bottom seismometers obs in the middle okinawa trough in 1984 during about one month of operation very active microseismicity was observed the microearthquake activity was characterized by the ceaseless occurrence of numerous small earthquakes in the vicinity of obs stations this activity was recorded by almost every obs station but not by land based instruments swarm activities also occurred at many places average activity levels of earthquakes whose durations are longer than 30 s recorded at individual obss are from 4 to 20 events per day which seem higher than those of ordinary mid oceanic ridge fracture zone systems and other back arc basins apparently the number of smaller size earthquakes is fewer than expected this might indicate the rather high attenuation of seismic waves or low q of the crust around obs stations which makes it difficult to detect small size events at a distant range attenuation of seismic waves was large beyond the epicentral distance of about 120 km this seems to indicate low q in the lower crust also a particularly low q area was found at the northern most part of the research area where high heat flow measurements were obtained this suggests that igneous activity reaches to the surface in that vicinity from the bottom of the crust where magmatic activity has been implied in other studies at least two types of swarm activities are indicated for this area some have no dominant events show high b value properties and seem to be volcanic others have dominant events and appear to be tectonic in nature among the swarm activity two kinds of signals closely resembling volcanic tremors were observed from these features we conclude that the crust of this area is highly fractured and subject to intense tectonic activities as well as some kind of igneous processes

pleistocene sea levels from raised coral reefs of haiti the elevations and ages of a sequence of three uplifted pleistocene coral reefs on the northwest peninsula of haiti have been determined with the assignment of a sea level of 6 meters relative to the present day at 130 000 years before present and constant uplift of the reefs the data indicate that sea level stood 10 and 13 meters at 108 000 and 81 000 years before present respectively these results are in substantial agreement with those reported for barbados and new guinea and support the hypothesis of constant uplift for each area sea level data from raised reefs indicate that the interglacial marine oxygen isotope oscillations during oxygen isotope stage 5 are a result of 30 percent ice volume effects and 70 percent temperature effects

origin of rollover rollover is the folding of hanging wall fault blocks by bending or collapse in response to slip on nonplanar commonly listric normal faults the shapes of rollover folds are controlled by a number of variables including 1 the shape of the fault 2 the total fault slip after a bed is deposited 3 the direction of relative particle motion in hanging wall collapse 4 the history of sedimentation rate relative to fault slip rate and 5 compaction the importance and role of each of these variables is illustrated by a two dimensional balanced structural modeling technique that treats continuously curved faults as though composed of a large number of straight fault segments in this modeling an active axial surface oriented parallel to the direction of relative p rticle motion in hanging wall collapse emanates from each fault bend and is the instantaneous locus of folding the quantitative correctness of this theory of rollover is tested by modeling natural structures from the gulf of mexico for which both fault shape and fold shape are known from high quality seismic and well sections the direction of hanging wall collapse commonly is in the antithetic or synthetic normal fault or coulomb shear orientations although sliding along weak bedding planes also is an important collapse mechanism in some regions collapse is in the antithetic shear direction for concave fault bends and in the synthetic shear direction for convex bends these collapse directions can be observed directly in some high quality seismic images as axial surfaces emanating from fault bends the shapes of rollovers within growth strata depend strongly on the sedimentation rate relative to fault slip rate as well as the total slip after a bed is deposited the crests of classic gulf coast rollovers are growth axial surfaces along which are abrupt changes in sedimentation rate within the growth st atigraphic interval these changes are produced by deformation of the sediment water interface along active axial surfaces compaction can substantially modify the relationship between fault shape and rollover shape however under certain common conditions the history of compaction can be neglected if the folding is modeled in the compacted state

linked sequence stratigraphic and structural evolution of propagating normal faults two distinct phases in the structural evolution of normal faults can be identified in the miocene gulf of suez rift 1 an initial growth fold stage when the fault is a buried structure and 2 a subsequent surface faulting stage during the growth fold stage strata thin and become truncated toward the fault zone and are rotated and diverge away from the buried fault into growth synclines in contrast once the fault breaks surface strata form a divergent wedge which is rotated and thickens into the fault the two tectono stratigraphic styles also occur contemporaneously along the length of a single fault segment growth folding characterizes deformation around the ends of fault segments where the fault is blind whereas the center of fault segments are characterized by surface faulting these observations suggest that marked along strike variation in stratal surfaces and facies stacking patterns will occur in depositional sequences in areas of normal faulting

tidal triggering of earthquakes in the northeast pacific ocean summary there have been many searches for evidence of tidal triggering in earthquake catalogues with the exception of volcanically active regions the more rigorous studies in continental settings tend to find no correlation or only a very weak correlation in the oceans the effect of loading by the ocean tides can increase tidal stresses by about an order of magnitude over continental settings in recent years several studies have reported evidence of tidal triggering in oceanic regions and such observations can represent a useful constraint on models of earthquake rupture in this paper i systematically search for a link between ocean tide height and the incidence of earthquakes in the northeast pacific ocean a region of high amplitude open ocean tides the focal mechanisms of most of the earthquakes in these catalogues are unknown but it can be shown that tidal stresses will in most instances promote failure at low tides i investigate three declustered data sets comprising 1 earthquakes from 1980 to 2007 on the juan de fuca plate and in the queen charlotte fault region from land based catalogues 2 earthquakes from 1992 to 2001 on the juan de fuca plate located with the us navy s sound surveillance system sosus hydrophone array and 3 earthquakes from 1980 to 2001 south of alaska and the aleutians located with land based networks i look at the distributions of earthquakes with ocean tide phase height and tidal range and apply schuster and binomial tests and monte carlo simulations to determine if they deviate significantly from random the results show no evidence of triggering during intervals of increased tidal range but all three data sets show a significant increase in earthquake incidence at low tides the signal is particularly strong in the land based catalogue for the juan de fuca plate and queen charlotte fault regions where there is a 15 per cent increase in the rate of seismicity within 15 of the lowest tides the signal is weakest in the sosus data set which may reflect the lower average tidal range at epicentres in this data set or an analysis that is influenced by gaps in the catalogue the triggering signal in the alaska aleutian may be partially obscured by earthquakes in the aleutians where the total tidal stresses can be significantly out of phase with the ocean tide height the increase in the rates of seismicity i observe at low tides is less than observed by local networks on mid ocean ridges similar to the prediction from an analysis of global thrust earthquakes and greater than inferred by extrapolating laboratory simulations of fault failure under tidal loading

microstratigraphy of calcite cements in pliocene cool water limestones new zealand relationship to sea level burial and exhumation events

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

a review of the milankovitch climatic beat template for plio pleistocene sea level changes and sequence stratigraphy the milankovitch theory of climate change predicts that global ice volume and hence sea level changes were controlled by long term quasi periodic variations in the earth s orbital parameters obliquity precession and eccentricity d 18 o records from deep sea cores are a proxy for sea level changes and have an orbitally tuned chronology covering the last 5 ma the sea level signal in d 18 o data from east equatorial pacific core v19 30 is well calibrated with sea level data from coral terraces on huon peninsula new guinea over the last 140 ka and with less certainty back to 340 ka over the last 140 ka the sea level contribution to benthic glacial interglacial isotopic variation is about 1 2 1 3 or 0 011 m 1 and for core v19 30 the glacial age temperature contribution from deep ocean cooling of 1 7oc is 0 4 independent constraints on late pliocene sea level changes interpreted from shallow marine continental margin records indicate that the sea level d 18 o calibration may not have been the same over the last 2 6 ma and the temperature correction is unlikely to have been the same in all glacial periods and all ocean settings nevertheless the astronomically tuned isotopic records from deep sea cores provide an accurate chronology and approximate the magnitudes of sea level changes over the last 2 6 ma against which the facies architecture of stratigraphic sequences can be analysed and the concepts of sequence stratigraphy properly evaluated 1998 elsevier science b v all rights reserved

an introductory perspective on non tropical shelf carbonates abstract this brief account introduces the topic of non tropical shelf carbonate deposits both modern and ancient specific aspects of which are described and discussed in the 22 accompanying papers in this special issue of sedimentary geology non tropical shelves are loosely defined as lying poleward of the present limit of development of hermatypic coral reefs or beyond the mean annual surface water isotherm of about 20 c typically near 30 latitude most geoscientists have traditionally regarded shallow water carbonate formation as a low latitude phenomenon however wherever the rate and extent of terrigenous sediment dilution is small the potential exists for extensive deposits of skeletal carbonate to accumulate on shelves beyond the sub tropics the facies attributes of the modern non tropical carbonates are distinctive and contrast significantly with the structural lithological faunal floral mineralogical and diagenetic character of their tropical counterparts in the rock record non tropical limestones probably occur throughout the phanerozoic but generally remain to be identified and or interpreted as such they are presently described mainly from the ordovician permian and tertiary quaternary periods of major continental glaciations and accentuated latitudinal thermal gradients a uniformitarian approach to the recognition and interpretation of ancient shallow marine limestones requires acceptance of both tropical and non tropical carbonate facies models

sequence stratigraphic context of syndepositional diagenesis in cool water shelf carbonates pliocene limestones new zealand abstract pliocene limestones in eastern north island new zealand are cool water deposits composed of barnacle epifaunal bivalve bryozoan dominated calcitic biota formed in shoal areas about the margins of a tectonically active forearc basin seaway vertical changes in the thickness and depositional facies of the limestones and fundamentally the position and nature of internal bounding discontinuities within them have enabled identification of high order likely 41 ka transgressive deepening upwards and transgressive regressive deepening then shallowing upwards sequences within the formations syndepositional or sea floor or pre compaction diagenesis in the limestones involves a spectrum of destructive to constructive features including skeletal abrasion bioerosion encrustation dissolution and cementation upward changes in the type and extent of these pre compaction diagenetic features within the discontinuity bounded sequences relate closely to the stacking patterns of depositional facies and their associated skeletal and mineralogical composition consequently it has been possible to develop a series of predictive diagenetic models for cool water shelf carbonates deposited upon and about upthrust antiforms in the forearc that integrate their patterns of syndepositional diagenesis into a sequence stratigraphy based framework driven ultimately by relative sea level changes marine constructive diagenesis characterized by bioencrustation turbid cements and interstratified internal sediments occurs most commonly in those facies close to the sequence bounding discontinuities within the lower levels of transgressive systems tracts tsts and the uppermost levels of regressive systems tracts rsts these sediments equate to the highest energy facies upon antiform tops close to the wave abrasion base where sedimentation rates were low in contrast destructive diagenetic processes typically dominate over constructive ones often exclusively so in the upper part of tsts in highstand systems tracts hsts and in the lower portion of rst carbonates where the sediments were not so influenced by active fluid circulation or strong reworking for prolonged periods but were more rapidly buried within accommodation traps below wave abrasion base about the antiforms the link between the facies motifs and synsedimentary diagenesis in these cool water pliocene limestones helps unlock their otherwise often cryptic high order sedimentary architecture such an interpretive opportunity is not afforded by the subsequent post compaction diagenetic features in the limestones because these are no longer closely related to the sedimentation realm

tectonic control of the oil rich large igneous carbonate salt province of the south atlantic rift abstract the early cretaceous south atlantic magmatic province samp which includes the parana etendeka lip produced about 8 million km 3 of tholeiitic basalt and diabase over an area of 4 million km 2 huge pre salt oil reserves discovered in 2007 by petrobras in non marine carbonates are estimated at more than 45 billion barrels here we show the close causal relationship of the southward increasing width of the wedge shaped south atlantic rift with the similarly southward increase in igneous activity in the thicknesses of non marine carbonate and salt and in the size of oil reserves all controlled mainly by south america s early clockwise rotation away from africa about a pole in its northeast large diabase dike swarms transversal to the rift witness to south america s rotation that opened in its wake the southward widening south atlantic rift westward increasing pressure on the equatorial margin by south america s clockwise rotation forced open the benue trough and created pre late aptian folds in the demerara plateau and in brazil s solimoes upper amazonas basin prerift and synrift volcanic activity increases southward culminating in the parana etendeka lip and in the offshore volcanic sdrss that continue southward to the cape basin berriasian valanginian rift sediments deposited from about 145 ma 10 ma before the flood basalts of the parana etendeka lip the largest transversal dike swarm continued in the proto walvis ridge that separated the central south atlantic endorheic rift basin from the sea in the south erosion and leaching of basalts supplied ca mg and sio 2 to the endorheic basin for the deposition of non marine carbonates and authigenic clays basalt flows intercalated with carbonates nearly until salt deposition about 113 ma hypogenic leaching of carbonates by mantle derived co 2 created optimal reservoirs supergiant oil deposits occur where the widest endorheic basin and the volcanic province overlap

structure and seismotectonics of the vema fracture zone atlantic ocean the structure and seismo tectonics of the vema fracture zone and the adjacent regions of its transverse ridge to the south and the mid atlantic ridge spreading centre to the north is discussed based on an analysis of seismic refraction profiles micro earthquake activity and gravity anomalies crustal structures were determined by analysis of wide angle seismic data from explosive and airgun sources recorded on arrays of ocean bottom seismometers micro earthquakes were recorded by six instruments for a period of 14 days in an array to the north of the ridge transform intersection immature crust similar to that reported by fowler 1976 from the famous area lies beneath the spreading centre the lower crust thins and normal mantle appears as the fracture zone is approached the thinnest crust lies beneath the northern end of the topographic deep at the fracture zone intersection this is also the locus of current microearthquake activity at sub bottom depths of 3 4 km which a composite fault plane solution suggests may originate on normal faults that parallel the trend of the local topography on the eastern wall of the median valley within the central region of the fracture zone valley the crust thickens but still retains an anomalous velocity structure seismic layer 3 is absent and crustal velocities are low this is probably the consequence of attenuated crustal accretion of faulting and cracking and of hydrothermal alteration the pattern of residual gravity anomalies after correction for topography for sediment thickness and for mantle temperature variations is consistent with the crustal structure defined by the seismic experiments the gravity measurements indicate that there is a broad thinning of the crust over 20 40 km in the vicinity of the fracture zone on which are superimposed localized regions of low and high densities to the south of the fracture zone the crust thickens and assumes a normal velocity structure including a mature seismic layer 3 the seismic moho is warped upwards under the transverse ridge suggesting a tectonic origin from buckling rather than uplift by serpentinization

plio pleistocene cliff bound wedge shaped warm temperate carbonate deposits from rhodes greece sedimentology and facies abstract the pliocene to pleistocene temperate carbonates of rhodes were deposited in a tectonically active region strongly influenced by a complicated and rapidly changing topography provided by the highly tectonised late cretaceous lindos limestone as basement rock deposition on this basement took place in accommodation loci restricted to micrograbens and their downslope extension to the foot of steep submarine cliffs to basement neptunian dykes and depressions in the basement rock consequently the sediments comprise a high degree of facies variability and are typically thin and laterally discontinuous the integration of several outcrops is necessary for the reconstruction of the stratigraphy and the relative sea level changes the sediments were deposited during a large scale tectonically driven transgressive regressive cycle in water depths changing from zero to several hundreds of metres at the studied lindos pefkos road cutting the kolymbia limestone bound to the foot of lindos limestone cliffs marks the onset of the marine deposition in the late pliocene its fabric is a rudstone consisting of unsorted angular lindos limestone clasts up to boulder size with a matrix dominated by molluscs and coralline algae the overlying plio pleistocene st paul s bay limestone consists of deep water float and rudstones containing the white coral community dominated by the coral lophelia pertusa its matrix shows a complex fabric of up to five sediment zones separated by differing states of lithification in this maximum flooding phase mineralised hardgrounds indicate depositional hiati the subsequent shallowing phase is represented by the cape arkhangelos calcarenite a series of distinctive facies of very patchy distribution they are characterised by the bryozoan brachiopod facies overlain by a facies heavily dominated by the bivalve mytilaster sp mytilaster facies conspicuous for the mytilaster facies is the inverse concave up stacking pattern of shelly material this facies is followed by a serpulid framestone and associated serpulid rudstone neptunian dykes cut through the first two facies and are filled with an intraformational breccia grading into a breccia with abundant mytilaster facies clasts neptunian dyke facies the sedimentology and interpretation of each facies include a description of the ichnology in particular the bioerosion peculiar to each facies the separation of different gravity transport processes in steep submarine environments is rarely described and most of the literature concentrates on siliciclastic dominated coarse grained sandy or gravely sand delta environments for a classification of the transport processes of the examined deposits the following criteria were evaluated sediment body symmetry inclination of the palaeorelief sediment constituents fabric complexity sedimentary structures and availability of fine matrix sedimentary structures and grading can be camouflaged in carbonates due to density differences of bioclasts however bioclast orientations such as bivalve stacking patterns can give information about the sedimentary process we suggest the inverse concave up stacking pattern of bivalve shells to be a texture potentially indicative for debris falls because of the above mentioned criteria we classified the kolymbia limestone as rock fall deposits and the st paul s bay limestone as well as the mytilaster facies of the cape arkhangelos calcarenite as debris fall deposits the bryozoan brachiopod facies could not be classified with certainty because of the lack of sedimentary structures and bioclast orientation however a grain flow or most likely a debris fall transport process seems probable

non tropical carbonate deposits on the modern new zealand shelf abstract extensive ca 50 000 km 2 shallow marine platforms 70 caco 3 despite proximity to a tectonically active plate margin in these regions the terrigenous sediment supply is presently low and growth of epibenthos is fostered by firm substrates rock gravels shells seaweeds and the generally energetic nature and high nutrient levels of open shelf waters rapid transition into adjacent terrigenous dominated facies is characteristic irrespective of water depth the carbonates are coarse grained and fragmental carbonate mud is rare calcite dominates over aragonite high mg calcite widespread off northern new zealand is rare in the south skeletal material is dominated by bryozoans and bivalve molluscs with significant local contributions from foraminifers barnacles calcareous red algae and echinoderms the name bryomol is suggested for this distinctive temperate region skeletal carbonate facies which can be usefully subdivided based on dominant zoarial growth forms of the bryozoan component known to be habitat related bioerosion is an important mechanism of skeletal fragmentation and degradation many grains especially aragontic bivalves are infested by endolithic borers and have low preservation potential ages of skeletal material in the surficial deposits range from more than 20 000 years b p to modern which is consistent with both low rates of carbonate production and sediment accumulation and the wide range in preservation state of grains some data suggest that the skeletal carbonates are dispersed and mixed mainly during infrequent movement of sand ribbons sand waves and sand sheets driven by storm assisted tidal flows tracts of modern palimpsest and relict carbonates can occur in juxtaposition the facies characteristics of the new zealand shelf carbonate deposits contrast significantly with those of the classical bahaman type carbonate model however they are similar to those reported from many other mid to high latitude carbonate shelves and afford good analogues for most onland occurrences of new zealand cenozoic limestones

biocalcarenite and mixed cool water prograding bodies of the mediterranean pliocene and pleistocene architecture depositional setting and forcing factors

late cretaceous heterozoan carbonates palaeoenvironmental setting relationships with rudist carbonates provence south east france abstract during late cretaceous coniacian early santonian times heterozoan rich bioclastic carbonates prevailed in the provence platform area they substituted for rudist rich carbonates which flourished during the late turonian in the same area this article presents a detailed sedimentological study of these heterozoan rich carbonates carried out in the mejean syncline west of marseille at this locality the thickness of the bioclastic series exceeds 500 m the heterozoan carbonates are referred mainly to packstones and grainstones organized in meter thick cross bedded sedimentary strata microfacies analysis has shown that bryozoans red algae and echinoderms are the main skeletal constituents mixed with reworked fragments of rudists and benthic foraminifera detritic elements are mainly represented by quartz and glauconite grains in the study area heterozoan carbonates underwent a complex diagenetic evolution iron precipitation occurred early in environments with high carbonate productivity and a high hydrodynamic level early marine cements developed mainly in the upper portions of shallowing upward parasequences during intervals with decreasing sedimentation rates late burial cements occurred sequentially as syntaxial rim cements dentate calcite crystals and blocky calcite during the mesogenetic burial history microstylolitisation developed in the packstones deposition of the heterozoan carbonates began during the transgressive coniacian cycle following a relative sea level fall recorded in the late turonian bioclastic sedimentation rates during the coniacian vary between 74 and 88 m myr a value similar to that recorded in the holocene australian and new zealand shelves coniacian santonian heterozoan carbonates had a widespread palaeogeographic distribution in south east france and sardinia the availability of hard substrates due to a high hydrodynamic level during the coniacian transgression was a major cause for the development of the heterozoan association as a whole likewise iron rich terrigenous supply and upwelling patterns probably enhanced nutrients on the shelf and favoured in turn the development of suspension feeders and mesotrophic organisms possible open marine relatively deep cool water conditions were responsible for the deposition of the heterozoan association versus the photozoan rudist association during the coniacian this latter association progressively recovered and thrived when shallow marine warm water conditions re established during the santonian comparisons are made with modern analogue environments in which heterozoan dominated cool water carbonate sedimentation occur

onlap offlap and the origin of unconformity bounded depositional sequences christie blick n 1991 onlap ofttap and the origin of unconformity bounded depositional sequences mar geol 97 35 56 unconformity bounded depositional sequences represent the fundamental building blocks of sedimentary successions they are typically characterized by onlap at the base and by offlap at the top and they tend to be markedly asymmetrical with onlap accounting for a larger part of any cycle of sedimentation than offlap offlap cannot be attributed solely to erosional truncation but instead implies that sequence boundaries develop over a finite interval of time depositional sequences are commonly associated with a cyclic arrangement of facies but transgressive regressive cycles are out of phase with respect to sequence boundaries which in down dip locations are both overlain and underlain by progradational deposits and hence form during times of regression of the shoreline these observations are used to develop some ideas about the origin of unconformity bounded sequences with reference to the inter related roles of changes in depositional base level and sediment supply in particular it is shown than onlap and offlap are due to lateral migration of a line of critical bypassing defined so as to incorporate the effects of sediment loading and compaction as well as the rate of change of elevation with respect to sea level downward shifts in onlap may be achieved by either an increase in the rate of eustatic fall or a decrease in the rate of tectonic subsidence and it is premature to assume that eustatic and tectonic controls on sea level may be distinguished solely on the basis of the frequency of depositional cyclicity small shifts in the position of onlap can also be produced by changes in sediment supply and more attention needs to be paid to the influence of sediment supply in the development of minor boundaries unconformities related to eustatic fluctuations are thought to correspond approximately to times of relatively rapid sea level fall inflection points but questions remain about the existence of possible leads and lags of up to cycle and hence about the degree to which sequence boundaries of eustatic origin may vary in age both within a given basin and from one basin to another

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

lowstand terraces on tyrrhenian sea steep continental slopes abstract wave built terraces formed during sea level stillstands at depths lower than present are found along margins with fairly steep morphology and no to ill developed continental shelf for instance the margin of volcanic islands terraces deposits are up to 20 30 m thick and are flat topped with well defined edges they extend up to 1 km in dip section the deposits are composed of intrabasinal sediments mainly bioclastic sand and silt and are probably due to deposition below the storm wave base during large scale energy episodes two areas of the tyrrhenian sea have been analysed one in a volcanic context the pontine archipelago and the other in a highly active structural context the calabrian arch similar wave built terraces were found in both areas with a main terrace placed at a depth of 100 150 m which is thought to be wurmian older terraces are also present emplaced during the preceding lowstands in the late pleistocene a possible present day terrace is found at about 20 m depth in the pontine archipelago a mechanism of subsequent formation below wave base level and removal because of sea level lowering has been hypothesised during long lasting periods of late pleistocene sea level falls terraces are thus thought to preserve only at the minimum lowstand position and to remain relict on the slope during sea level rise and highstand the analysis of the distribution and depths of these terraces allows some neotectonic interpretation for example in the volcanic archipelago whose western sector has probably risen at an average rate of 2 5 mm yr over the last 20 000 years

sequence stratigraphy of sixth order 41 k y pliocene pleistocene cyclothems wanganui basin new zealand a case for the regressive systems tract this study is based on a late pliocene and early pleistocene approximately 2 6 1 7 ma succession about 1 km thick of 20 sixth order 41 k y duration cyclothems of shelf origin exposed in the rangitikei river valley in the eastern part of wanganui basin the cyclothems correlate with 18 o isotope stages 100 58 and each 41 k y glacialinterglacial stage couplet is represented by an individual depositional sequence comprising transgressive highstand and regressive systems tracts unlike most examples inferred from the stratigraphic record these systems tracts were deposited during phases of known sea level cycles indicated by the contemporary oxygen isotope ice volume curve because of the high rate of subsidence of wanganui basin glacioeustatic sea level falls during most cycles were not of sufficient magnitude to expose the outer shelf thus the rangitikei section provides an exceptional example of regressive strata deposited landward of the contemporary shelf break simple one dimensional modeling shows that moderate to high rates of basin subsidence 1 2 mm yr and low rates of sedimentation 0 2 mm yr during transgressions combined to produce an accommodation surplus at the relative highstand this surplus accommodation was infilled during the late highstand and ensuing fall partly by aggradational highstand systems tract shelf siltstone and chiefly by strongly progradational shoreface sediments of the regressive systems tract rangitikei regressive systems tracts are distinguished from forced regressive systems tracts sensu hunt and tucker 1992 by their different stratal geometry by definition forced regressive systems tracts display an erosional contact with the underlying highstand systems tracts and typically occur as a series of downstepped disjunct shoreline wedges stranded on the shelf and or slope in contrast regressive systems tracts exhibit a gradational lower contact above which parasequences are stacked in a strongly progradational pattern terminated by the superjacent sequence boundary cyclothems display two types of motif termed rangitikei dt depositional transgression and rangitikei nt nondepositional transgression which include the following architectural elements in ascending stratigraphic order 1 a basal sequence boundary that is coincident with either the transgressive surface of erosion which displays small scale up to 50 cm erosional relief and may be penetrated by the ichnofossil ophiomorpha or its deeper water correlative conformity 2 either a thick 5 30 m transgressive systems tract comprising a deepening upward nearshore to inner shelf mixed carbonate siliciclastic lithofacies succession depositional transgression or a thin 2 m transgressive systems tract comprising condensed fossiliferous facies deposited on the sediment starved offshore shelf nondepositional transgression 3 a sharp downlap surface separating condensed fossiliferous facies of the transgressive systems tract from terrigenous siltstone of the superjacent highstand systems tract 4 a highstand systems tract comprising a 10 20 m thick interval of aggradational shelf siltstone and 5 a thick up to 45 m progradational inner shelf to shoreface lithofacies assemblage ascribed to the regressive systems tract condensed shell beds are associated with intrasequence and sequence bounding discontinuities and together with the sedimentological and stratal characteristics of the sequences are important indicators of stratigraphic architecture four types of shell bed are associated with surfaces formed by four different types of stratal termination onlap backlap downlap and flooding surface shell beds cf kidwell 1991 are associated respectively with the transgressive surface of erosion apparent truncation at the top of the transgressive systems tract the downlap surface and local marine flooding surfaces a fifth shell bed type termed a compound shell bed forms in offshore environments where the downlap surface converges with the sequence boundary and elements of both the downlap and the backlap shell beds become mixed or superposed the shell beds mark zones of stratal attenuation and can be used as surrogates for seismic discontinuities when applying sequence stratigraphic concepts at outcrop scale

oscillatory spreading explanation of anomalously old uplifted crust near oceanic transforms rocks significantly older than their theoretically predicted age have been recovered in the vicinity of some atlantic transform zones rocks of palaeocene 55 58 myr and possibly even of cretaceous age are found near the vema transform on tectonically uplifted crust which should not be older than 30 myr we present here a model which might explain the presence of uplifted anomalously old crust near the vema transform zone tz the proposed model involves axial rift propagation and reorientation and migration of the transform as a result of a change in the regional stress field during transform migration a crustal wedge formerly part of the african plate is transferred to the south american plate the ensuing oscillatory spreading can explain the anomalous age of the uplifted crustal block compression resulting from rift propagation and transform migration provides a mechanism for crustal uplift

an approach to reduce exploration risk using spectral decomposition prestack inversion and seismic facies classification petroleum companies have invested heavily in the exploration of frontier offshore basins in brazil uncertainties associated with geologic complexity lithostratigraphy fluid content and seismic resolution in these basins are the most common challenges many methods have been used to circumvent these issues such as assessing seismic attributes seismic inversion spectral decomposition and the integration of these methodologies to accomplish an optimal approach because of the complex geology of these areas it is very difficult to identify potential reservoirs within one specific geologic formation hence applying spectral decomposition to frontier basins of high geologic complexity can help overcome the challenges because in the frequency domain it is possible to better define variability and lateral discontinuities the correct choice of the spectral decomposition optimization approach is critical for accurate target zone representation as well as selection of the appropriate transform prestack inversion spectral decomposition and attribute seismic facies classification were applied to evaluate the extent of a reservoir and analyze whether an appraisal well has been sited at a good location the results showed that the new appraisal well was planned at a good location but that its facies were not as good as those of the first discovery well as confirmed after drilling

microearthquakes beneath the median valley of the mid atlantic ridge near 23 n hypocenters and focal mechanisms we report hypocenters and focal mechanisms of microearthquakes located by an ocean bottom seismic network deployed in the median valley of the mid atlantic ridge near 23 n during a 3 week period in early 1982 the network consisted of seven ocean bottom hydrophones and three three component ocean bottom seismometers the instrument coordinates were acoustically determined to within 25 m at the 1 standard deviation level of confidence the hypocentral parameters of the 26 largest microearthquakes are reported 18 of these events have epicenters arid focal depths which are resolvable to within 1 km formal error at the 95 confidence level microearthquakes occur beneath the inner floor of the median valley and have focal depths generally between 5 and 8 km beneath the seafloor composite fault plane solutions for two spatially related groups of microearthquakes beneath the inner floor indicate normal faulting along fault planes that dip at angles of 30 or more these solutions are similar to the mechanisms of nearby large earthquakes microearthquakes also occur beneath the steep eastern inner rift mountains the rift mountain earthquakes have nominal focal depths of 5 7 km and epicenters as distant as 10 15 km from the center of the median valley but these hypocenters have larger uncertainties because of the possible effects of large topographic relief and associated lateral heterogeneity in velocity structure we interpret the depth distribution and source mechanisms of these microearthquakes to indicate that this segment of ridge axis is undergoing brittle failure under extension to a depth of at least 7 8 km we infer that the entire crustal column has been cooled to temperatures within the brittle field of behavior and that significant time has elapsed since the most recent episode of volcanic or shallow magmatic activity along this ridge segment

unsupervised seismic facies classification applied to a presalt carbonate reservoir santos basin offshore brazil mapping of seismic and lithological facies is a very complex process especially in regions with low seismic resolution caused by extensive salt layers even when only an exploratory view of the distribution of the reservoir facies is required the aim of this study was to apply multi attribute analysis using an unsupervised classification algorithm to map the carbonate facies of an exploratory presalt area located in the outer high region of the santos basin the interval of interest is the barra velha formation deposited during the aptian which represents an intercalation of travertines stromatolites grainstones and spherulitic packstones mudstones and authigenic shales which were deposited under hypersaline lacustrine conditions during the sag phase a set of seismic attributes calculated from a poststack seismic amplitude volume was used to characterize geological and structural features of the study area we applied k means clustering in an approach for unsupervised seismic facies classification our results show that at least three seismic facies can be differentiated representing associations of buildup lithologies aggradational or progradational carbonate platforms and debris facies we quantitatively evaluated the seismic facies against petrophysical properties porosity and permeability from available well logs seismic patterns associated with the lithologies helped identify new exploration targets

seismic constraints on the crustal structure within the vema fracture zone results from recent studies of the in well surveyed areas such as the famous area in kane and oceanographer fracture zones suggest that the central north atlantic ramberg et al 1977 significant variations in crustal thickness are transform faults are found every 50 to 100 km associated with parts of these fracture zones making them one of the most common tectonic with total crustal thicknesses in places less than features in the ocean basins morphologically half the thickness of normal oceanic crust the fracture zones on slowly spreading ridges display extent of this anomalous crust and its origin are many similar characteristics deep closed con still very poorly constrained by available geo tour depressions are usually present at ridge physical data here we report results from a 75 axis transform intersections sleep and biehler km long refraction line shot in late 1977 over 1970 the axis of the transform is typically three hawaii institute of geophysics ocean bottom defined by a deep linear trough bounded by escarpments rising eimom uuyuu withe ve valley near its western intersection with the mid transform valley floor a diverse suite of atlantic ridge 10o53 n 43o39 w we show that the apparent crustal thickness beneath this por tion of the vema transform valley is about 5 km not substantially different from crustal thick nesses typically associated with oceanic crust however the vema fracture zone crust is clearly anomalous and similar in many respects to crust reported from the kane and oceanographer fracture zones it is characterized by low compressional wave velocities a relatively uniform velocity gradient throughout most of the crust and the absence of a typical layer 3 refractor this anomalous crust is interpreted as intensely fractured hydrothermally altered and serpenti nized crustal and upper mantle rocks overlain by locally thick accumulations of basaltic rubble from the adjacent fracture zone walls the large variation in moho depths found in these large offset slowly slipping transforms is attributed to differences in the depth and extent of serpen tnnization of ultramafics lying beneath a thin and highly fractured basaltic and gabbroic layer one important consequence of the relatively low seismic velocities and densities inferred for the crust in these fracture zones is that transform valleys are not in local isostatic equilibrium with the surrounding seafloor

in situ study of the eastern ridge transform intersection of the vema fracture zone abstract fourteen dives of the submersible nautile have been carried out at the eastern intersection of the vema fracture zone with the mid atlantic ridge direct observations allow us to locate precisely the surficial expression of the transform fault motion in the southern trough of the fracture zone valley the present day displacement zone corresponding to an extremely narrow less than 300 m transform fault zone is confined to a belt of disturbed terrain situated at the foot of the southern wall and characterized by transform parallel 30 100 m high ridges of basaltic fragments and pelagic sediments active splays of the transform fault zone lie generally between these ridges creating steep v shaped furrows despite a rather sinuous trend one of the fault strands appears to be laterally continuous and other minor active splays branch out from it or run parallel to it the surface manifestations of the tectonic activity along the present day displacement zone include r riedel shears low angle shears p shears en echelon structures open fissures small pull apart structures and funnel alignments in the sedimentary cover the transform fault motion reactivates large normal faults on the adjacent flanks of the southern wall and the median ridge the transform parallel topography defines a 10 km wide zone with dip slip motion the ridge transform intersection area is entirely floored by fresh basalts contrasting with other intersections of major fracture zones such as the kane and oceanographer fracture zones recent volcanic activity is not restricted to the central volcanic ridge but is widespread across the whole nodal basin area the present day displacement zone cross cuts the neovolcanic zone and the disconnected northernmost extension of the volcanic ridge is attached to the median ridge and so belongs to the african plate the extensive volcanism is suggestive of a magmatic production phase in the intersection area extremely fresh pillowed lava forms have been observed at the rti inside corner high indicating possible off axis volcanism

post glacial sea level control and sequence stratigraphy of carbonate terrigenous sediments wanganui shelf new zealand abstract shallow marine mixed carbonate terrigenous sediments have been accumulating in water depths of 20 110 m on wanganui shelf central western new zealand during the latest quaternary post glacial sea level rise since about 18 000 years bp cores show an upward change from coarse mostly terrigenous dominated sands and gravels to shell rich sediments and then to terrigenous dominated muddy very fine sands and sandy muds these post glacial carbonate terrigenous sediments are a modern analogue for similar episodes of cyclothemic shelf deposition that characterise onland plio pleistocene sections in central western new zealand like their ancient counterparts the latest quaternary sediments can be interpreted in a sequence stratigraphic context a transgressive systems tract 4 15 m thick overlies a transgressive erosion surface evident on seismic reflection records and is represented by slightly shelly coarse sands and gravels at the base of several cores and as palimpsest surficial deposits on the modern shallow shelf out to a depth of about 50 m the overlying shelly deposits both shell rich and matrix rich are equivalent to mid cycle or backlap shellbeds described elsewhere they comprise a diverse in situ or locally transported bivalve molluscan and bryozoan biota typical of temperate latitude cool water carbonate shelf facies these condensed section shellbeds are up to at least 1 5 m thick gradationally overlie the transgressive sediments in cores and occur as modern surficial shelf facies between about 25 and 100 m water depth cores are locally capped by up to a few metres of terrigenous sands or muds that represent highstand systems tract deposits fed onto the shelf from opposing onshore and offshore directions in the nearshore region north island derived felsic fine sands are beginning to prograde seawards westwards across the shellbeds while in the offshore zone south island derived micaceous very fine sands and muds are encroaching upon and covering the shellbeds from the west and south

mantle anisotropy beneath northwest pacific subduction zones to assess the location strength and orientation of seismic anisotropy in the southern kuril japan and izu bonin subduction zones we analyzed shear wave splitting in local s phases and teleseismic sks phases fast directions from these phases are roughly parallel to the direction of absolute pacific plate motion wnw in izu bonin roughly parallel to the strike of the trench nne in central honshu and roughly n in the southern kuril back arc in the vicinity of sakhalin island assuming that the orientation and strength of anisotropy does not vary with depth modeling of splitting times from local s and teleseismic phases recorded at sakhalin island requires that the maximum depth of anisotropy lies between 480 km and 950 km in contrast splitting times for local s phases from events in izu bonin rule out anisotropy in the transition zone all the data are consistent with a model in which the lower transition zone 520 660 km and lower mantle are largely isotropic and in which anisotropy occurs intermittently in the upper transition zone 410 520 km possibly due to preferred orientation of spinel assuming that splitting in the upper mantle is produced by preferred orientation of olivine the observed fast directions indicate that the geometry of back arc strain varies systematically between subduction zones the relationship of fast directions and plate motions suggests that the subducting slab exerts significant control on back arc flow but that flow correlated with upper plate deformation must also exist

tectonic setting and stratigraphic architecture of an early cretaceous lacustrine carbonate platform sugar loaf high santos basin brazil abstract the sugar loaf high is an extensive basement structure within the santos basin offshore brazil in which all of the elements of the pre salt hydrocarbon play are present following a clastic dominated early synrift phase the later synrift and sag phases are characterized by an extensive non marine carbonate platform over the basement high the study area within this new 3d seismic dataset contains carbonates stratigraphically equivalent to the reservoirs within the lula and sapinhoa fields analysis of the 3d seismic data shows that the pre salt carbonates onlap the basement of the sugar loaf high and form aggradational geometries on a platform that has a proximal to distal width of about 100 km and an interpreted relief of some 900 m the carbonate platform has an aggradational and mounded basinward facing gullied margin with slope deposits progradational units are imaged on the platform top that may be either carbonate or clastic in composition the platform is preserved beneath evaporites that form the regional seal within the santos basin platforms of this size are previously unknown from lacustrine settings outside the pre salt stratigraphy of the santos and campos basins brazil

recent faulting and microearthquakes at the intersection of the vema fracture zone and the mid atlantic ridge new air gun reflection profiles 3 5 khz reflection profiles and microearthquake data recorded by an array of ocean bottom seismographs form the basis for this study of the transition from a spreading center to a major transform fault disturbances of the thick normally flat lying turbidite deposits provide indications of recent vertical motions at the western intersection of the fracture zone with the median valley there is a depression in the sediments that represents the southerly extension of the median valley into the fracture zone valley the depression is terminated abruptly on the south by the active transform fault which acts as a locus for vertical as well as horizontal displacement flat lying undisturbed sediments terminate abruptly at the fault the western boundary of the depression is much broader and is characterized by a series of slumplike steps to the west there is little or no evidence for uplift or tilting of sediments which might indicate vertical recovery of the crust as it spreads away from the depression this suggests that uplift and recovery out of the depression is episodic in nature and has been inactive over the last million years along the western boundary to the east there is clear evidence of uplift and tilting of sedimentary layers a basement ridge emerging from the sediments is currently being uplifted and rotated in a manner analogous to processes responsible for the creation and cancellation of median valley relief the transition between the spreading center and the transform fault appears to take place within 1 2 km the width of the transform fault just east of the depression is less than a kilometer microearthquakes were located and displayed by new methods that directly account for nonlinearities associated with small arrays microearthquakes located by three or more ocean bottom seismometers show that the greatest seismic activity occurs along the eastern walls of the median valley at the basement ridge in the eastern portion of the depression and in the crestal mountains very little activity is associated with the western edge of the transform depression and the trace of the transform fault

presalt stratigraphy and depositional systems in the kwanza basin offshore angola the lower cretaceous presalt section in the kwanza basin contains an excellent petroleum system that includes synrift strata barremian overlain by a sag interval aptian and capped by the loeme salt the upper synrift is generally limestone with widespread mollusk packstones and grainstones coquinas deposited in a fresh to moderately saline alkaline lake the sag interval is characterized by carbonate platforms and silica rich isolated buildups formed in highly evaporated highly alkaline lakes shrubby dendritic microbially influenced boundstones and intraclast spherulite grainstones accumulated in shallow water on platform tops microbial cherts were deposited as organic buildups on large isolated structural highs basinward west of platforms and they apparently formed at low temperatures in very alkaline lake water shrubby boundstones and microbial cherts have vuggy pores that are primary and result in high permeability wackestones and packstones with calcitic grains mainly spherulites in dolomite or argillaceous dolomite were deposited in slightly deeper low energy sag environments in addition clays especially stevensite precipitated out of the silica rich highly alkaline lake waters during sag deposition calcite precipitated on the shallow lake floor with morphologies that ranged from spherulites to shrubs and included a continuum of intermediate forms spherulites probably precipitated just below the sediment water interface spherulites and shrubby calcites are commonly recrystallized spherulites floating in stevensite probably formed in deeper lacustrine environments organic rich mudstones were deposited in even deeper lacustrine environments in synrift and sag intervals and they are likely the source of most hydrocarbons in this system these interpretations are supported by seismic core petrographic and stable isotope data

earthquakes in the orozco transform zone seismicity source mechanisms and tectonics as part of the rivera ocean seismic experiment a network of ocean bottom seismometers and hydrophones was deployed in order to determine the seismic characteristics of the orozco transform fault in the central eastern pacific we present hypocentral locations and source mechanisms for 70 earthquakes recorded by this network all epicenters are within the transform region of the orozco fracture zone and clearly delineate the active plate boundary about half of the epicenters define a narrow line of activity parallel to the spreading direction and situated along a deep topographic trough that forms the northern boundary of the transform zone region 1 most focal depths for these events are very shallow within 4 km of the seafloor several well determined focal depths however are as great as 7 km no shallowing of seismic activity is observed as the rise transform intersection is approached to the contrary the deepest events are within 10 km of the intersection first motion polarities for most of the earthquakes in region 1 are compatible with right lateral strike slip faulting along a nearly vertical plane striking parallel to the spreading direction another zone of activity is observed in the central part of the transform region 2 the apparent horizontal and vertical distribution of activity in this region is more scattered than in the first and the first motion radiation patterns of these events do not appear to be compatible with any known fault mechanism pronounced lateral variations in crustal velocity structure are indicated for the transform region from refraction data and measurements of wave propagation directions the effect of this lateral heterogeneity on hypocenters and fault plane solutions is evaluated by tracing rays through a three dimensional velocity grid while findings for events in region 1 are not significantly affected in region 2 epicentral mislocations of up to 10 km and azimuthal deflections of up to 45 may result from assuming a laterally homogeneous velocity structure when corrected for the effects of lateral heterogeneity the epicenters and fault plane solutions for earthquakes in region 2 are compatible with predominantly normal faulting along a topographic trough trending nw se the focal depths however are poorly constrained these results suggest an en echelon spreading center or leaky transform regime in the central transform region

composite sedimentary record of falling stages of pleistocene glacio eustatic cycles in a shelf setting crotone basin south italy abstract a thick pleistocene shelf and nearshore cyclical succession was deposited in the s mauro sub basin of the crotone basin southern italy the regressive units of the cycles are mostly represented by coastal siliciclastic and bioclastic prograding wedges showing a clinoform geometry these are separated by blanket like deposits of high lateral persistence recording major transgressive episodes the aim of this paper is 1 to describe facies patterns and depositional setting of two prograding wedges particularly focussing on their polycyclic internal architecture 2 to analyze these units within a sequence stratigraphic framework and 3 to speculate on the possible origin of the small scale cyclicity the two wedges analyzed in this paper consist of a number of shingles individual shingles consist of two physically connected units 1 a relatively thin package of sigmoid clinoforms grading into 2 a volumetrically dominant package of oblique tangential clinoforms with toplap terminations the shingles are bounded by seaward dipping surfaces with sigmoid clinoform geometry which are ravinement surfaces updip passing into conformable flooding surfaces downdip the wedges are thus organized into high frequency small scale sequences each comprising transgressive highstand and falling stage systems tracts as a whole individual prograding wedges are interpreted as forced regressive units as the shoreline was subject to an overall shift basinwards and downwards along a low angle trajectory in spite of the repeated minor relative sea level rises tectonic subsidence and particularly the syndepositional growth of gentle synclines are thought to have been the key factors allowing the preservation of these forced regressive units progradation of the wedges took place in a high energy wave climate characterized by high frequency of storms and very efficient alongshore redistribution of sediments recurrent storm driven offshore currents led to intense reworking of sediments on the topset platform and gravity spreading on the foreset slope of the prograding wedges well oxygenated conditions over the shelf due to intensified storm activity during glacial periods may have enhanced the rate of production of skeletal foramol type carbonates it can reasonably be assumed that progradation took place from a line source and that the sand bodies are to be regarded as coastal prograding bodies in spite of active syndepositional tectonics the cycles can be correlated to pleistocene high amplitude sea level oscillations the older of the two wedges can be correlated through bio magnetostratigraphy to the major climatic transition which occurred from the marine oxygen isotope stage 25 to 24 22 rio et al 1996 the younger probably developed during the sea level fall that ended with substage 18 2 as suggested by sequence and bio stratigraphic data the prograding wedges are thus interpreted to record long lived sea level falls of fourth order cycles due to the particular depositional setting we are inclined to exclude authigenic mechanisms in the origin of small scale cyclicity although the concomitance and interaction of different controlling factors may be taken into account it is tempting to ascribe this cyclicity to minor eustatic changes punctuating long lived erratic falling stages possibly accompanied by climate driven fluctuations of sediment supply shelf perched and shelf edge prograding units consisting of foramol type carbonates are apparently a common falling stage to lowstand depositional feature in the mediterranean area during the late pliocene and pleistocene

quaternary sedimentary history of adana cilicia and iskenderun basins northeast mediterranean sea detailed single channel airgun and 3 5 khz profiles from the northeastern mediterranean sea show that the continental shelf is formed by superimposed deltaic successions depositional sequences separated by major erosional unconformities each depositional sequence is composed of a sigmoid prograding package overlain by an oblique prograding package representing respectively delta progradation during high and low sea levels of interglacial and subsequent glacial stages during the glacio eustatic lowstands of sea level deltas prograded seaward the present day shelf break denotes the topset to forest transition at maximum progradation during the last glacial period during post glacial transgressions the deltas initially lost their dynamic equilibrium with the environment and rapidly retreated landward leading to the deposition of a thin veneer of sediments originating from reworking of formerly coastal sediments at the highstand position the deltas were re established in the ancestral adana bay and foreset progradation started chronology suggests that the cilicia and iskenderum basins are subsiding at rates of 0 38 m 1000 yr 1 and 0 33 m 1000 yr 1 respectively seismic reflection profiling shows that delta architecture in the adana cilicia and iskenderun basins is mainly controlled by glacio eustatic sea level fluctuations and continuous basin subsidence

global cyclostratigraphy a model of carbonate growth patterns abstract cyclostratigraphy is the study of cyclic depositional patterns produced by climatic and tectonic processes a global scale quantitative cyclostratigraphic model is described which simulates carbonate growth patterns controlled by tectonic and climatic processes the model uses seven factors simulating the effects of physical and chemical environments on the deposition rates of carbonate accumulations these factors are sea level change the rate of basement subsidence food supply influence of nutrients available sunlight temperature salinity and dissolved oxygen the factors are considered as functions of climatic and tectonic processes the model also integrates milankovitch induced short term climatic changes with the long term tectonic evolution of basins to examine the potential carbonate accumulation patterns the two dimensional computer model results provided here show that 1 carbonate growth patterns in different climates and under different tectonic processes can be modelled quantitatively 2 carbonate production increases towards the equator decreasing latitude as the temperature and nutrition supply increase in tropical belts and production changes because of expansion or contraction of the tropical belt in different climatic periods 3 when matched with the turbidity the model describes different carbonate accumulation patterns in different climatic patterns 4 at either abnormally high or low salinity carbonate accumulation rates decline sharply and the salinity becomes normal away from the strand line and 5 cyclic sea level changes cause a cyclic change of carbonate accumulation a case study is presented from the upper pennsylvanian of texas the simulation results indicate that carbonate growth patterns observed from field well or seismic data are accurately modelled by the quantitative procedure given here

postrifting seismicity and a model for the 1993 diking event on the coaxial segment juan de fuca ridge microearthquake data from two deployments of ocean bottom seismographs made shortly after the june july 1993 rifting episode on the coaxial segment of the juan de fuca ridge constrain the propagation path of the magmatic dike and provide insight into the effect of the dike injection on the axial stress field the first deployment consisting of five instruments in a small array aperture of 2 km at the eruption flow site 46 30 n was made 3 months after the episode the second deployment consisting of two larger arrays apertures of 9 km of five instruments each one at the eruption site and one at a site of vigorous venting of bacterial flocculent material floe site 46 18 n was made 11 months after the eruption epicenters from nine posteruption earthquakes near the floe site at the south end of the segment are confined to a narrow zone on the rift axis epicenters from 61 earthquakes observed near the flow site are located throughout the rift valley and include an event cluster just south of the observed lava flows we combine these posteruption earthquakes with t wave data recorded by sosus hydrophones during the rifting episode to develop a model for the diking event in this model magma propagated at depth along the axial rift zone until reaching the eruption site near 46 31 n during the next few days the dike swelled as magma accumulated behind the arrested tip elevating local stress levels and generating a swarm of seismic activity about 3 days after the magma reached the north end of the segment the top of the dike penetrated the seafloor surface resulting in eruptions along the rift zone at the flow site