deep sea biostratigraphy of prograding platform margins neogene bahamas key evidence linked to depositional rhythm abstract new foraminiferal evidence from two boreholes on the paleoshelf and slope of western great bahama bank has wide ranging implications for understanding formation and evolution of carbonate platform margins the new data abundant well preserved planktic foraminifera were obtained by disaggregating samples from intercalated pelagic layers and selected parts of thick hemipelagic limestone earlier efforts to obtain biostratigraphic ages identified six biostratigraphic units in each borehole provided biozonal age alternatives for both holes and resulted in different pliocene biozones between them the new data define six units in one hole and seven in the other bracket the biozones present and their ages indicate different sedimentation rates and show that within the limits of biostratigraphic resolution the biozones are correlative between the holes most importantly the revised ages show that the paleoshelf borehole probably penetrated the late miocene rather than middle miocene the oldest unit is on the paleoshelf and the youngest uppermost pliocene is on the slope between the holes the stratigraphic interval spans the temporal interval from an inferred maximum of 10 2 ma to a minimum of 1 6 ma although the biozones range sequentially from the neogloboquadrina acostaensis n16 zone to the basal part of the globorotalia truncatulinoides truncatulinoides n22 zone globorotalia crassaformis viola subzone absence of key species indicates that deposition was discontinuous numerous periods of erosion and or nondeposition are inferred the largest of which is a condensed section hiatus 1 2 myr above the paleoslope miocene pliocene boundary in addition the late pliocene globorotalia tosaensis tosaensis n21 zone is not recognized on the slope its absence is consistent with a widespread regional unconformity sedimentation rates and depths of series boundaries vary widely in both holes the paleoslope miocene pliocene boundary lies at 540 m below top of the hole the lower upper pliocene boundary is placed at or near 444 m position of the pliocene pleistocene boundary is less certain but is within the top 382 m of the hole its placement anywhere within this interval is a reasonable assessment considering an exceptionally high rate of sedimentation 562 m myr 168 6 m interval based on topmost foraminiferal sample 1 9 1 6 ma as expected the lowest sedimentation rate occurs in the condensed section overlying the miocene pliocene boundary 5 m myr 9 5 m 5 3 4 1 ma the paleoshelf miocene pliocene boundary lies below a hiatal condensed section 295 278 m below top of the hole that has a greater sedimentation rate 89 m myr 17 7 m 5 5 5 3 ma than that at the slope the lower upper pliocene boundary is placed at or near a depth of 236 m and the pliocene pleistocene boundary lies within the top 113 m of the hole sedimentation rates on the shelf range from 15 m myr above the condensed section 22 9 m 5 3 3 8 ma to a late pliocene high of 183 m myr 54 9 m interval based on the point at which the age depth line crosses the 1 9 myr mark between the topmost two fossiliferous samples 2 2 1 9 ma

kinematics of primary contacts between low and relatively high pressure rocks in orogens abstract primary contacts among low and relatively high pressure rocks in convergent orogens have been mainly interpreted as a result of extensional tectonics or reimbrication of high pressure thrust units already resting on low pressure ones and by consequent inversion of their stacking order our work in northern calabria allows us to ascertain the nature and kinematics of several of these contacts and to work out a general model that considers primary pressure break contacts as resulting from the growth of a deep passive roof duplex within a subduction accretion system estimates of pressure differences across these contacts may be obtained for a simple passive roof duplex geometry as a function of the aspect ratio of the structure and of the angle of subduction our results suggest therefore that there may be a further possibility in addition to extension or reimbrication which explains pressure break contacts in orogens

long term acceleration of aseismic slip preceding the mw 9 tohoku oki earthquake constraints from repeating earthquakes a decadal scale deformation transient preceding the 2011 mw 9 tohoku oki japan earthquake was reported from continuous gps data and interpreted as accelerating aseismic slip on the japan trench megathrust given the unprecedented nature of this transient independent confirmation of accelerating slip is required here we show that changes in the recurrence intervals of repeating earthquakes on the japan trench megathrust in the period 1996 to 2011 are consistent with accelerating slip preceding the tohoku oki earthquake all sequences of repeating earthquakes with statistically significant trends in recurrence interval at 95 confidence offshore south central tohoku occurred at an accelerating rate furthermore estimates of the magnitude of slip acceleration from repeating earthquakes are consistent with the completely independent geodetic estimates from a joint inversion of the gps and seismicity data we infer that a substantial portion of the megathrust experienced accelerating slip partly surrounding the eventual rupture zone of the mw 9 earthquake

alpine orogenic p t t deformation history of the catena costiera area and surrounding regions calabrian arc southern italy the nappe edifice of north calabria revised with insights on the tyrrhenian apennine system formation 1 the nappe structured belt of calabria constitutes the eastward termination of the southern branch of the alpine mediterranean belt that delimits the northern edge of the africa plate contrasting hypotheses for the origin and tectonic significance of the north calabrian nappe edifice have been proposed and kinematic data from north calabria have been used to support different interpretations of the alps apennines linkage and the polarity of the tethyan subduction in the apennine region we reconstruct the architecture of the north calabria nappe edifice through a multidisciplinary approach which integrates structural investigations with metamorphic thermobarometry and 40ar 39ar geochronology results from this study indicate that north calabria consists of a tertiary nappe stack resulting from superimposed top to the west extensional shearing late oligocene to middle miocene in age onto a previously structured top to the east compressional belt eocene to oligocene in age this study also documents that the top to the west extensional tectonics was achieved by means of regionally sized extensional detachment fault systems stretching apart and translating as allochthonous fragments the previously accreted units thinning operated by top to the west extensional detachment tectonics also resulted in the direct juxtaposition of non alpine or slightly alpine metamorphosed units upper plate complex onto the previously exhumed deep seated portions of the orogenic wedge metamorphosed under blueschist facies metamorphic conditions lower plate complex these findings support a new tectonic scenario for the orogenic history of north calabria which may be adequately framed within the tertiary apennine tyrrhenian system evolution

tectonic role of ophiolite bearing terranes in the development of the southern apennines orogenic belt new data on ophiolite bearing terranes of the liguride complex together with some information on the terranes of the sicilide complex result in a better understanding of the role and tectonic significance of these units in the construction of the southern apennines orogenic belt the liguride complex is composed of two main tectonic units overlain by a thick turbiditic sequence of late oligocene middle miocene age the uppermost one frido unit is a polydeformed and polymetamorphosed sequence composed of two tectonic subunits of shales and calc schists respectively containing blocks of ophiolite garnet gneiss amphibolites and granitoids this unit is thrust over the un metamorphosed terranes calabro lucano flysch unit consisting of a broken formation with blocks of late jurassic ophiolite and their sedimentary cover cretaceous eocene pelagic sediments and late oligocene volcaniclastic deposits the frido unit underwent hp lt metamorphism p 8 10 kb t 400 500 c resulting in glaucophane and lawsonite assemblages in the ophiolitic rocks and aragonite in the meta limestones and calc schists followed by greenschist fades metamorphism p 4 kb t 300 350 c from a structural point of view units of the liguride complex comprise structures developed at different structural levels indicating progressive non coaxial deformation in response to tectonic transport towards the n ne the ophiolite bearing terranes of the liguride complex can be considered as a remnant of an accretionary complex in which the calabro lucano flysch unit represents the toe of the wedge where frontal accretion processes occur and the frido unit is a deeper portion emplacement of the frido unit is explained as being due to formation of a deep duplex structure during the early stage of continental collision processes the polarity of tectonic transport provides new evidence that the liguride complex represents a suture zone between the apulian and the calabrian blocks the age of collision appears to be not older than late oligocene the allochtonous terranes of the liguride and sicilide complexes therefore represent a complete accretionary wedge which records first subduction of the neotethys ocean beneath the calabrian europe continental margin and later continental collision with the african block

structure kinematics and metamorphism in the liguride complex southern apennines italy abstract the liguride complex of south italy is the remains of a cretaceous paleogene accretionary wedge which was emplaced onto the african margin during early miocene time transport directions of thrust sheets and nappes in the internal zone are roughly towards the present day northeast transport directions match well with the corresponding slip vectors derived from relative motion of the calabria sardinia corsica block with respect to africa including adria during late eocene to oligocene time deformation of the internal zone included extensive underthrusting followed by underplating of already lithified sediments ophiolitic rocks and continental basement on to the base of an accretionary wedge down to depths in places exceeding 30 km overthickening by massive underplating caused late paleogene gravity driven extension within the accretionary wedge the contact between the lower ophiolitic nappe and the upper ophiolitic nappe is an extensional fault zone of this age transport directions during the gravity driven deformation do not match plate derived slip vectors it is inferred that the gravitational potential of the wedge dominated the kinematics of the late palaeogene extensional deformation

microfossil biostratigraphy of prograding neogene platform margin carbonates bahamas age constraints and alternatives abstract benthic and planktic foraminifera and calcareous nannofossils were recovered in shallow water carbonate rock cores from two continuous boreholes drilled 7 5 km apart on the west platform margin of the great bahama bank the microfossils define six biostratigraphic units in each hole one unit in each hole represents a correlative condensed section seven foraminiferal biozones are recognized in 11 of the units between the holes middle miocene globorotalia fohsi robusta zone n12 late miocene g acostaensis zone n16 and g humerosa zone n17 early pliocene g margaritae evoluta subzone n19 late pliocene g exilis subzone n21 and tentatively g tosaensis tosaensis zone n21 and early pleistocene g crassaformis viola subzone n22 the twelfth unit is inferred to be of g crassaformis viola subzone n22 age the oldest unit is onshore the youngest is offshore as presently interpreted the nannofossil and foraminiferal zonations are partially correlative although the microfossils unequivocally constrain the series ages of the sediments the incompleteness of the fossil record allows for alternative biozonal age models within the series the miocene and pliocene biozones are common to both holes but the greatest similarities between the holes are the significant mixing of middle and late miocene and late miocene early pliocene faunas the greatly condensed intervals at the miocene pliocene boundary and the early pliocene influx of deep water benthic and pelagic foraminifera of particular importance is the tentative recognition of late pliocene g tosaensis tosaensis zone n21 in one borehole subsequent data not available to this phase of the study indicate that much of the zone is likely missing its absence will lend support to speculations of a regional unconformity in the bahamas the microfossils indicate that 1 several transgressions occurred from the middle miocene to at least the earliest pleistocene 11 5 0 46 ma during which banktop derived sediments accumulating at the margin prograded the platform seaward 2 a condensed interval on the bank top may represent a late miocene lowstand a period of sediment bypassing or a lack of accommodation space 3 the slope received thin layers of pelagic sediments in a condensed interval during the late miocene and early pliocene while the bank top accommodated early pliocene 4 2 3 4 ma deep water indicators prior to a likely period of exposure 2 35 1 89 ma 4 two cycles of banktop sediment production and starvation occurred during the pliocene 5 the pliocene transgression was punctuated by stillstands or low amplitude reversals during which parts of biozones did not accumulate and 6 the sediments containing the most complete microfossil datum record are the thin pelagic strata that mark interruptions in the regular shedding of transgressive deposits from the platform sedimentation rate patterns varied but were generally higher offshore than onshore

deformation partitioning during transpressional emplacement of a mantle extrusion wedge the ronda peridotites western betic cordillera spain abstract composite tectonic emplacement of the world9s largest outcropping body of subcontinental mantle rocks the ronda peridotites occurred during early miocene continental subduction and as a result of oblique plate convergence partitioning of transpressional deformation between coeval orogen parallel wrenching and orogen perpendicular pure thrusting components is recorded by shear zone kinematics and dynamothermal metamorphism in the footwall to the ultramafic rocks left lateral shear characterizing the deeper high pressure eclogitic portions of the continental subduction system propagated through the mantle into the overlying continental crust of the overriding plate whereas top to the foreland frontal thrusting dominated at the leading edge of the hot peridotite body top to the hinterland shear along the upper contact of the peridotites is consistent with the kinematics expected for an extrusion wedge consisting of subcontinental mantle rocks on the other hand coeval strike parallel extension and thinning of the crustal rocks overlying the peridotites confirms that similarly to other alpine mediterranean examples partitioned transpression resulted in the development of a complex deformation pattern with kinematically linked shear zones aiding exhumation the proposed model provides a new coherent scenario for syn convergence peridotite emplacement and exhumation in the western betic cordillera

variability of fault slip behavior along the san andreas fault in the san juan bautista region an improved understanding of the time history of fault slip at depth is an essential step toward understanding the underlying mechanics of the faulting process using a waveform cross correlation approach we document spatially and temporally varying fault slip along the northernmost creeping section of the san andreas fault near san juan bautista sjb california by systematically examining spatiotemporal behaviors of characteristically repeating earthquakes cres the spatial distribution of pre 1998 sjb earthquake 1984 1998 fault slip rate inferred from the cres reveals a 15 km long low creep or partially locked section located near the 1998 mw 5 1 sjb earthquake rupture a finite fault slip inversion reveals that the rupture of the 1998 sjb earthquake is characterized by the failure of a compact 4 km2 asperity with a maximum slip of about 90 cm and corresponding peak stress drop of up to 50 mpa whereas the mean stress drop is about 15 mpa following the 1998 earthquake the cre activity was significantly increased in a 5 10 km deep zone extending 2 7 km northwest of the main shock which indicates triggering of substantial aseismic slip the postseismic slip inferred from the cre activity primarily propagated to the northwest and released a maximum slip of 9 cm in this 5 10 km depth range the estimated postseismic moment release is 8 6 1016 n m which is equivalent to mw 5 22 the aseismic slip distribution following the 1998 earthquake is not consistent with coseismic stress driven afterslip but represents a triggered long lasting slow earthquake

late neogene planktonic foraminiferal biostratigraphy and climatostratigraphy of the solo river section java indonesia abstract paleoclimate changes are reconstructed from late miocene earliest pleistocene deep water sediments of the solo river section using fluctuations in planktonic foraminiferal faunas water depth and clastic influx intervals with common cold water planktonics globigerina bulloides neogloboquadrina cf pachyderma are thought to reflect periods of increased upwelling triggered by antarctic glaciations correlations with well bodjonegoro 1 java dsdp site 62 1 and lamont core rc 12 66 equatorial pacific using planktonic foraminiferal datum levels and paleoclimate trends suggest non synchroneity of several commonly used datums probably due to differences in species concepts it also allowed dating of the climate changes with regard to the magnetostratigraphic and numerical time scales for correlations between different faunal provinces global climate changes are an extremely useful tool probably superior to any single faunal datum using this with other evidence the following correlations could be obtained between the five climatostratigraphic zones in the solo river section and the mediterranean and new zealand stages from bottom to top 1 zone i samples sr 63 sr 53 cool older than 6 0 m y 5 8 m y magnetostratigraphic epoch 6 and older basal epoch 5 planktonic zone upper n16 n17 mediterranean stages upper tortonian early messinian new zealand stages tongaporutuan early kapitean late miocene 2 zone ii sr 52 sr 35 moderately warm with fluctuations 5 8 m y 5 0 m y epoch lowermost gilbert upper n17 n18 late messinian evaporite phase late kapitean late miocene 3 zone iii sr 34 sr 33 cool 5 0 4 8 m y within lower gilbert n18 basal n19 latest messinian lago mare phase latest kapitean or earliest opoitian latest miocene 4 zone iv sr32 sr 18 warm to very warm 4 8 2 7 m y within lower gilbert within upperg gauss n19 within n21 zanclean piacenzian opoitian pliocene 5 zone v sr17 sr 12 cool 2 7 m y age of top unknown upper gauss lower matuyama within n21 n22 santernian or basal calabrian s l waipipian basal pleistocene late pliocene of workers with different pliocene pleistocene boundary concepts

towards a universal rule on the recurrence interval scaling of repeating earthquakes 1 a recent analysis of recurrence properties of small repeating earthquakes on a creeping oblique thrust fault in eastern taiwan reveals a weak variation in recurrence interval tr with seismic moment mo compared to the scaling of tr with mo from repeating earthquake data near parkfield in california the repeating data from eastern taiwan has recurrence intervals that are 2 times shorter also in northeastern japan tr of repeating quakes are 4 times shorter than those expected from the parkfield scaling law when adjusted to account for differences in the geodetically derived slip rates for the three fault zones however the tr mo scaling is remarkably consistent among the three regions it suggests that the tectonic loading rate is likely the most important factor that controls the repeat time it also suggests that there seems to exist a universal rule on recurrence interval scaling of repeating earthquakes in diverse tectonic settings

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

quantitative analysis of quaternary nannoplankton in the subtropical northwestern pacific ocean abstract time progressive changes of nannoflora and biometry of coccoliths for the last 1 3 m y were investigated for a piston core recovered from the west mariana ridge the species compositions were quantitatively investigated under an electron microscope and six assemblages assemblages a through f were identified significant changes in relative abundances of various types of genus gephyrocapsa are the main cause for the stratigraphic change of the flora a dominant occurrence of gephyrocapsa protohuxleyi var a which can be identified only under an electron microscope was observed for the first time in a thin sequence which corresponds to the lowest part of the brunhes epoch 0 75 0 62 ma because of this characteristic occurrence assemblage d was further subdivided into three subassemblages the dominant occurrence of small gephyrocapsa which is well known in the small gephyrocapsa zone of the equatorial pacific ocean was not observed but the almost complete absence of medium to large gephyrocapsa during approximately 200k years of the middle pleistocene was confirmed a stratigraphically restricted occurrence of a large form of reticulogenestra was also confirmed by the biostratigraphic study under a light microscope and the last and first occurrences of the largest form reticulofenestra sp a 6 5 m were identified at 0 81 and 0 98 ma respectively in addition to these four events precise ages for another five well known evolutionary events were also calibrated these ages are generally in good agreement with previously reported ages from other world oceans and diachronism was not evident for most of these evolutionary events

reworked paleogene to early neogene planktic foraminifera implications of an intriguing distribution at a late neogene prograding margin bahamas abstract a rich uniquely distributed assemblage 80 species of reworked paleogene to early neogene 66 15 ma planktic foraminifera is documented within 470 m of late neogene 5 7 1 6 ma borehole limestone from the great bahama bank west margin tests vary in size from 0 13 to 1 0 mm and are unusually well preserved where obtainable from the rock the relict species constitute an estimated 20 to 50 of all taxa paleocene faunas occur in upper miocene host rocks eocene species occur in upper miocene and basal upper pliocene rocks and mixed oligocene and early miocene taxa occur throughout the section including the uppermost pliocene i e the oldest groups disappear successively uphole each host rock interval is separated by major disconformities this record provides a substantial proxy for 1 a probable source feature an epipelagic marine tectonic wall escarpment or outcrop composed of pelagic and hemipelagic sediments 2 a probable tectonically active source area northern cuba or southern bahamas and 3 a variety of events that span 60 myr and have wide ranging implications from local progressive isolation of the oldest faunas to regional changes in water depth indicated by the faunas to global hiatuses that correlate with one or more major sequence boundaries in the atlantic bahamas and gulf of mexico as well as proposed eustatic lowstands seven unidentified foraminiferal biozones correspond to a proposed early eocene composite condensed section on the eustatic curve and may have regional implications the relict faunas also have significant implications for the timing and types of paleogene and early neogene sedimentation or nondeposition sea level fluctuations slumping or changing depth of effective marine erosion and regional late neogene events that occurred over 4 myr age of host rocks the direction of the source area from the secondary emplacement site and the estimated percentages of relict faunas indicate that not all the microfossils in the late neogene borehole sediments and by extrapolation therefore along the entire west margin of the platform were locally derived the amount of a possible attendant sedimentary component is not known

late cretaceous eocene nannofossil and magnetostratigraphic correlations near gubbio italy abstract using a modified sample preparation technique we have been able to establish a detailed lower campanian to upper eocene nannofossil stratigraphy in the bottaccione and contessa highway sections near gubbio appearance and extinction levels of virtually all the commonly used calcareous nannofossil zonal markers have been recognized and can now be closely correlated with the planktonic foraminifera zonation and the magnetic reversal stratigraphy previously established in these sections comparisons with the nannofossil calibrations of the oceanic magnetic anomaly sequence in deep sea drilling project dsdp sites suggest that magmetic subchrons c17n and c25n are missing in the bottaccione section the observed variability of the relative stratigraphic position of most plankton events is confirmed to less than one magnetic subchron absolute abundance paleobiogeographic restriction and differential preservation render some of the traditionally used biostratigraphic events less reliable than others

accumulation of bank top sediment on the western slope of great bahama bank rapid progradation of a carbonate megabank high resolution seismic profiles and submersible observations along the leeward slope of western great bahama bank show large scale export of bank top sediment and rapid progradation of the slope during the holocene a wedge shaped sequence up to 90 m thick is present along most of the slope and consists of predominantly aragonite mud derived from the bank since flooding of the platform 6 8 ka total sediment volume of the slope sequence is 40 80 that of holocene sediment currently retained on the bank maximum rates of vertical accumulation and lateral progradation are 11 15 m ka and 80 110 m ka respectively 10 to 100 times greater than previously known for periplatform muds slope deposition of exported mud during sea level highs appears to have been a major mechanism for the westward progradation of great bahama bank throughout the quaternary this may provide a critical modern analogue for ancient progradational margins

the cretan ophiolite bearing melange greece a remnant of alpine accretionary wedge abstract new data collected in central crete on the ophiolite bearing units result in a better understanding of the role and tectonic significance of these units in the construction of this segment of the hellenides these nappes are composed of three main tectonic units that characterized by different metamorphic facies conditions are represented by an un metamorphosed lower unit a greenschist to hp greenschist facies intermediate unit and a blueschist facies upper unit these chaotic thrust nappes include blocks of oceanic and continental deriving rocks and can be considered as a remnant of an accretionary complex the lower unit represents the toe of the wedge whereas the intermediate and upper units refer to the innermost and deeper subducted portions exhumed and superimposed on top of each other during the early stages of continental collision the structural evolution of the accretionary wedge was controlled by four main contractional deformation events that including distinct groups of structures developed at different crustal levels were driven by sse and ssw directions of tectonic transport our data strongly suggest that the ophiolite bearing tectonic wedge was accreted during the paleogene subduction of a late jurassic cretaceous oceanic realm beneath the continental margin of the pelagonian domain and it was successively involved in the continental collision with the adria block the greenschist to blueschist facies metamorphism and the subsequent exhumation and emplacement of the intermediate and upper units above the frontal portion of the wedge may mainly be due to deep duplexing marking the onset of continental collision we thus suggest that the ophiolite bearing units of crete represent a single suture zone related to the closure of a unique oceanic domain pindos cycladic ocean subducted beneath the internal hellenides platform continental domains thus assuming the significance of a southern oceanic seaway of the largest eastern neotethys developed since the triassic between eurasia and africa

the chronostratigraphic significance of seismic reflections along the bahamas transect abstract continuous cores drilled during the bahamas drilling project bdp and the ocean drilling program odp leg 166 along a transect from the top of great bahama bank to the basin in the straits of florida provide a unique data set to test the assumption in seismic stratigraphy that seismic reflections are time lines and thus have a chronostratigraphic significance seismic reflections that are identified as seismic sequence boundaries ssbs were dated by means of biostratigraphy in the five odp sites and by a combination of biostratigraphy magnetostratigraphy and sr isotope stratigraphy in the two bdp sites the seismic reflection horizons are carried across a variety of facies belts from shallow water carbonates over slope carbonates to drift deposits in the straits of florida within this system 17 ssbs were identified and dated despite the fact that the seismic reflections cross several facies belts their ages remain remarkably constant the average offset in all sites is 0 38 myr in no cases do the seismic reflections cut across time lines the age differences are the combined result of the biostratigraphic sampling frequency the spacing of marker species that required extrapolation of ages and the resolution of the seismic data in addition uncertainties of age determination in the proximal sites where age diagnostic fauna are rare add to the age differences between sites therefore it can be concluded that the seismic reflections which mark the ssbs along the bahamas transect are time lines and can be used as stratigraphic markers this finding implies that depositional surfaces are preferentially imaged by reflected seismic waves and that an impedance contrast exists across these surfaces facies successions across the sequence boundaries indicate that the sequence boundaries coincide with the change of deposition from times of high to low sea level in the carbonate setting of great bahama bank sea level changes produce changes in sediment composition sedimentation rate and diagenesis from the platform top to the basin the combination of these factors generates differences in sonic velocity and thus in impedance that cause the seismic reflection the impedance contrasts decrease from the proximal to the distal sites which is reflected in the seismic data by a decrease of the seismic amplitude in the basinal area

magnetostratigraphic dating of shallow water carbonates from san salvador bahamas magnetostratigraphic results are reported here from a sequence of late neogene quaternary shallow water carbonate sediments from a continuous core drilled on the island of san salvador bahamas on the basis of the remanent magnetism of 136 samples from a 91 m measured section of core the polarity sequence can be correlated with the magnetic polarity time scale from the gilbert chron early pliocene through the late brunhes chron late pleistocene holocene magnetic polarities were determined on the basis of relative up down direction in the unoriented core extraction studies of the magnetic particles reveal the presence of single domain crystals of magnetite resembling those produced by the magnetotactic bacteria and algae the sequence of reversals provides a minimum of six new major chronostratigraphic markers for the pliocene pleistocene of the bahamas it confirms and refines the local timing of both the lithologic change from skeletal to nonskeletal sediments and the disappearance of coral and molluscan species from the bahamas as upper late pliocene between 2 6 and 2 7 ma that the primary magnetic remanence is preserved in shallow water carbonates including replacement dolomites suggests that this technique could be used to date similar tertiary and possibly even older carbonate sequences the establishment of a reliable magnetostratigraphy provides refined dating of shallow water carbonates and regional faunal appearances or disappearances sediment accumulation rates subsidence and depositional events

sense of overthrust shear in the alpine nappes of calabria southern italy abstract southern italy consists tectonically of ophiolite and basement nappes thrust over the apenninic sedimentary nappes whilst all more recent authors agree that the sediments of the apenninic nappes were deposited on apulian basement i e on african continental crust and that the ophiolites were associated with the oceanic basement of the mesozoic tethys the provenance of the basement nappes is still debated new data based on microstructural criteria have shown that the main shear sense of the ophiolite nappes and of the overlying basement nappes in northern calabria is from west to east in today s co ordinate system the basement nappes might not therefore be of austroalpine african provenance but could be of european origin

sea level events in the south pacific linked with the messinian salinity crisis cores drilled through the carbonate cap of niue a mid oceanic carbonate platform in the south pacific contain an unusually well resolved stratigraphic section of the latest miocene at 35 to 140 m depth below sea level intensely leached and chalky intervals associated with pronounced 18 o and 13 c depletions mark the position of dissolution unconformities representing eight episodes of eustatic falls that outstripped the local rate of subsidence we demonstrate on the basis of sr isotope chronostratigraphy that these eustatic changes are correlative with the messinian salinity crisis events in the mediterranean sea level changes of about 10 m amplitude commenced at 6 14 ma and culminated in a large sea level fall that reached at least 30 m at 5 26 ma corresponding to the miocene pliocene boundary evaluation of the sea level history reported here and the contemporaneous mediterranean events supports the model of coupling between tectonic and eustatic factors that caused the onset and demise of the salinity crisis the available evidence suggests that during messinian time glacio eustatic falls and rises are likely to have modulated the connection between the atlantic and the mediterranean and influenced the evaporite deposition and may have contributed significantly to the final phase of isolation and desiccation of the mediterranean and its subsequent refilling during the zan clean deluge

structural signature of tectonic processes in the calabrian arc southern italy evidence from the oceanic derived diamante terranova unit the diamante terranova unit is an oceanic derived element within the thrust and nappe structure of the northern sector of the calabrian arc its metamorphic and structural signature reveals a long history of se dipping subduction and collision related deformation of an oceanic lithospheric section underlying the narrow diamante terranova basin the deformation history of the diamante terranova unit may be related to three main tectonic phases the first one is characterized by a structural association which includes features that developed during a progressive deformation related to the evolution of a deep seated shear zone kinematic indicators characterizing subduction related structures clearly show a top to the northwest sense of overthrust shear the second tectonic phase that can be recognized in the area is characterized by structures that may be related to the onset of shear deformation induced by a postcollisional northeastward motion of the overriding plate the third tectonic phase is recorded by brittle features related to the latest tectonic events affecting the calabrian arc during the opening of the tyrrhenian sea shear sense indicators and available radiometric and stratigraphic information together with plate kinematic data allowed us to infer the time evolution of this sector of the calabrian arc and to assess that the diamante terranova unit cannot be considered as an eoalpine element since its deformation related to subduction collision processes occurred prior to the end of the cretaceous

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

sr isotopic variation in shallow water carbonate sequences stratigraphic chronostratigraphic and eustatic implications of the record at enewetak atoll sr isotope data from two boreholes within the lagoon at enewetak atoll have been used to evaluate the use of such data to correlate date and monitor sea level change in shallow water carbonate sequences correlative stratigraphic intervals of relatively invariant 87sr separated by abrupt transitions to lower 87sr with increasing depth are recognized in both boreholes conversion of 87sr values to age via calibration with the seawater 87sr trend with age indicates that correlative and synchronous deposition of atoll sediments occurred at 0 4 1 2 and 2 1 ma in contrast a 5 m y hiatus is recognized in one borehole but not the other sr isotope stratigraphy sis is a powerful stratigraphic and chronostratigraphic tool in shallow water carbonate sequences only when significant secular variation of 87sr occurs and retention of depositional 87sr values is demonstrated the latter is best demonstrated when 87sr data are integrated with 18o 13c sr content data and petrographic observations several diagenetically altered intervals have greater 87sr values low 13c values and low sr ca ratios relative to adjacent intervals a combination that is consistent with open system meteoric diagenesis calcite cements from these intervals have early pleistocene 1 2 ma 87sr values despite their occurrence well within the late pliocene 2 1 ma sequence thus local sedimentological and diagenetic processes have produced intralagoon variability in the sis of the two boreholes complicating subsurface stratigraphic correlations the occurrence of anomalously young calcite cement relative to adjacent limestone is a direct response of the interaction of sea level change and meteoric phreatic diagenesis whereby overlying metastable carbonates with greater 87sr values are dissolved during periods of atoll emergence and sea level lowstand liberating sr and soil gas co2 to the pore fluid which is then incorporated into downflow meteoric phreatic diagenetic calcite the stratigraphic relation between the sites of carbonate dissolution and diagenetic calcite precipitation suggest an early pleistocene 1 2 ma sea level fall of between 34 and 64 m moreover the stratigraphic position and inferred age of paleophreatic lens calcite when backtracked for subsidence suggest that early pleistocene 1 2 ma sea level was 72 81 m range of 60 100 m below present sea level the enewetak record of early pleistocene sea level lowstand elevation agrees reasonably well with proxy sea level records inferred from sequence stratigraphy and deep sea foraminiferal 18o data however the enewetak record of the amplitude of early pleistocene sea level change is more consistent with the deep sea foraminiferal 18o proxy than the sequence stratigraphy proxy

passive roof duplex geometry in the frontal structures of the kirthar and sulaiman mountain belts pakistan abstract exploration for hydrocarbons over the past few years has greatly improved our understanding of the geometry of frontal mountain belt structures in this study we introduce and discuss the concept of the passive roof duplex using as the main example the kirthar and sulaiman ranges in the baluchistan province of pakistan structures similar to those described here have been recognized previously in other mountain belts and they appear to exist as a common feature in many more frontal regions of mountain belts our example of a passive roof duplex which we describe from pakistan is compared briefly with similar structures reported by others the passive roof duplex is here defined as a duplex whose roof thrust has backthrust sense passive roof thrust and whose roof sequence those rocks lying above the roof thrust remains relatively stationary during foreland directed piggy back style propagation of horses within the duplex