changing depth distribution of hiatuses during the cenozoic the differential effects of climate change sea level and water mass circulation on deposition erosion of marine sediments can be constrained from the distribution of unconformities in the world s oceans i identified temporal and depth patterns of hiatuses hiatus events from a large and chronologically well constrained stratigraphic database of deep sea sediments the paleogene is characterized by few several million year long hiatuses the most significant cenozoic hiatus event spans most of the paleocene the neogene is characterized by short frequent hiatus events nearly synchronous in shallow and deep water sediments epoch boundaries are characterized by peaks in deep water hiatuses possibly caused by an increased circulation of corrosive bottom water and sediment dissolution the plio pleistocene is characterized by a gradual decrease in the frequency of hiatuses future studies will focus on the regional significance of the hiatus events and their possible causes

abyssal circulation and benthic foraminiferal changes near the paleocene eocene boundary a major change in cenozoic deep sea benthic foraminifera occurred in the atlantic indian and pacific oceans near the paleocene eocene boundary benthic foraminiferal abundance changes began at about 61 5 ma at pacific deep sea drilling project dsdp site 577 a major extinction event followed at 58 57 ma between zones p6a and p6b and a series of first appearances continued until circa 55 5 ma zone p6c these faunal changes occurred during a 6 c warming of pacific bottom water and may indicate that the primary cause was changing temperature other potential causes of the faunal turnover include global changes in surface ocean productivity and changing bottom water source regions comparison of benthic and planktonic 13c records requires no change in the ratio of oceanic phosphorous to carbon during the late paleocene to early eocene which weakens the case for but does not disprove a change in surface ocean productivity at this time interbasinal comparisons of benthic foraminiferal 13c records document that water with high 13c values filled the cape basin during the late paleocene and possibly the early eocene circa 61 57 ma but apparently did not extend into the western basins of the atlantic this pattern suggests a supply of antarctic source water for the cape basin and possible tectonic isolation of the western atlantic basins during at least part of the late paleocene carbon isotope comparisons show that bottom water supply to the cape basin was reduced in the early eocene eolian grain size data suggest that a decrease in zonal wind intensity occurred at the end of the paleocene these late paleocene climatic changes bottom water warming and decreased wind intensity correspond with evidence for an important global tectonic reorganization and extensive subaerial volcanism which may have contributed to climatic warming through increased supply of co2

eocene oligocene global climate and sea level changes st stephens quarry alabama we integrate upper eocene lower oligocene lithostratigraphic magnetostratigraphic biostratigraphic stable isotopic benthic foraminiferal faunal downhole log and sequence stratigraphic studies from the alabama st stephens quarry ssq core hole linking global ice volume sea level and temperature changes through the greenhouse to icehouse transition of the cenozoic we show that the ssq succession is dissected by hiatuses associated with sequence boundaries three previously reported sequence boundaries are well dated here north twistwood creek cocoa 35 4 35 9 ma mint spring red bluff 33 0 ma and bucatunna chickasawhay the mid oligocene fall ca 30 2 ma in addition we document three previously undetected or controversial sequences midpachuta 33 9 35 0 ma shubuta bumpnose lowermost oligocene ca 33 6 ma and byram glendon 30 5 31 7 ma an 0 9 18 o increase in the ssq core hole is correlated to the global earliest oligocene oi1 event using magnetobiostratigraphy this increase is associated with the shubutabumpnose contact an erosional surface and a biofacies shift in the core hole providing a fi rst order correlation between ice growth and a sequence boundary that indicates a sea level fall the 18 o increase is associated with a eustatic fall of 55 m indicating that 0 4 of the increase at oi1 time was due to temperature maximum 18 o values of oi1 occur above the sequence boundary requiring that deposition resumed during the lowest eustatic lowstand a precursor 18 o increase of 0 5 33 8 ma mid chron c13r at ssq correlates with a 0 5 increase in the deep pacifi c ocean the lack of evidence for a sea level change with the precursor suggests that this was primarily a cooling event not an ice volume event eocene oligocene shelf water temperatures of 17 19 c at ssq are similar to modern values for 100 m water depth in this region our study establishes the relationships among ice volume 18 o and sequences a latest eocene cooling event was followed by an earliest oligocene ice volume and cooling event that lowered sea level and formed a sequence boundary during the early stages of eustatic fall

slab pull and indentation tectonics insights from 3d laboratory experiments abstract we investigate using 3d laboratory experiments how the dynamics of indentation process are affected by the evolution at depth of the oceanic and continental subductions lithospheric plates are modelled by sand silicone plates floating on glucose syrup and the density contrast between oceanic and continental lithospheric plates and asthenosphere is reproduced analogue experiments model the convergence between two lithospheric plates a small continent indenting a large continental plate we show that the surface deformation in front of the indenter and above the oceanic subduction zone depends on the behaviour of the slab below the collision zone slab break off following the subduction of the small continent favours the indentation process because it results in an increasing compression in front of the indenter and extension above the neighbouring oceanic subduction both of them being responsible for the appearance of the indenter like geometry of the plate boundary when the slab does not deform significantly at depth in contrast the closure of the oceanic domain in front of the indenter is followed by a longer period of continental subduction during which the tectonic regime within the wide continent remains quite homogeneous comparing the presented analogue experiments with the subductions both part of the arabian indenter within eurasia our results suggest that the different tectonic regime on both sides of the arabia indenter may partly result from the probable occurrence of a detachment at depth under eastern anatolia

tectonic erosion of the peruvian forearc lima basin by subduction and nazca ridge collision 1 subsidence of lima basin part of the peruvian forearc is controlled by tectonic erosion by the subducting nazca plate multichannel seismic reflection data coupled with age and paleowater depth constraints derived from ocean drilling program odp coring now allow the rates of erosion to be reconstructed through time in trenchward locations the forearc has experienced limited recent relative uplift 700 850 m likely due to preferential basal erosion under the center of lima basin long term subsidence driven by basal tectonic erosion dominates and is fastest closest to the trench since 47 ma eocene up to 148 km of the plate margin have been lost at an average rate of up to 3 1 km myr 1 appoximately 110 km of that total appears to be lost since 11 ma implying much faster average rates of trench retreat 10 km myr 1 since collision of the nazca ridge with the lima basin at 11 ma although there is no clear subsidence event at odp site 679 during the time at which nazca ridge was subducting beneath this part of the forearc 4 11 ma the more trenchward odp sites 682 and 688 show significant deepening after 11 ma indicating that subduction of the ridge accelerates tectonic erosion long term rates of crustal erosion in the region of lima basin are greater than estimates of regional arc magmatic productivity implying that such margins are net sinks of continental crust

reconstructing the stratal geometry of latest eocene to oligocene sequences in new jersey resolving a patchwork distribution into a clear pattern of progradation nine latest eocene to oligocene 34 2 23 9 ma sequences were identified and dated from eight sites situated on the onshore new jersey coastal plain and nearshore region these sequences show a patchy distribution with more complete lower oligocene sections updip and more complete upper oligocene sections downdip we projected these sequences onto a dip profile and reconstructed their original thicknesses and distributions by 2 d flexural backstripping this demonstrated that depocenters migrated offshore during the oligocene indicating that the patchy distribution can be best explained by progradation of generally thin and spacially limited clinoforms over an eocene carbonate ramp during the late eocene to oligocene the new jersey passive margin underwent a major morphologic change reconstructions indicate that the margin was a relatively steeply dipping carbonate ramp 1 500 paleoslope gradient during the eocene and was transformed into a siliciclastic margin characterized by a gentler gradient of 1 1000 and prograding clinoforms by the miocene clinoform progradation probably began during the latest eocene increased sediment supply during the oligocene resulted in the further progradation of sediments across the antecedent carbonate ramp the heights of the clinoforms ranged from 20 m during the latest eocene and earliest oligocene to nearly 50 m during the late oligocene most sediment accumulated within clinoform wedges with little or no sediment being preserved behind the clinoform inflection point q 2000 elsevier science b v all rights reserved

middle to late miocene canyon cutting on the new jersey continental slope biostratigraphic and seismic stratigraphic evidence we have identified and dated a major miocene erosional surface m1 on the new jersey continental slope this surface was penetrated at deep sea drilling project dsdp site 612 which was drilled near the thalweg of a buried v shaped canyon biostratigraphic data at site 612 firmly constrain the age of strata above the buried canyon surface as zones cn7 nn9 and n16 lowermost upper miocene the upper miocene surface at site 612 lies above lowermost oligocene strata because of coalesced unconformities we traced the m1 erosional surface to the cost b 3 well where upper middle miocene strata underlie it biostratigraphic studies of other new jersey continental slope boreholes asp 14 asp 15 suggest that elsewhere the sediments immediately below the m1 surface encompass the globorotalia fohsi robusta zone zone n12 earliest n13 middle middle miocene the best age estimate is that m1 was eroded between 11 5 and 10 0 ma this erosional event apparently correlates with a similar event on the irish and florida continental margins and with oxygen isotope evidence for a glacio eustatic lowering

cometary impacts into ocean their recognition and the threshold constraint for biological extinctions the montagnais impact crater is presently the only site in the ocean where the effect of a meteorite fall on marine organisms has been studied the impact crater is 45 km in diameter and was formed at 50 8 ma by a fall of probably an old cometary nucleus 3 4 km in diameter into shallow 600 m ocean comparison of the impact structure and related deposits with those on land shows several major differences of which the most significant is the absence of an elevated crater rim instead the crater perimeter is bevelled and eroded as a consequence of impact induced bottom currents and turbulent return water flow into the excavating cavity by this process most of the fall out breccia is reworked back into the crater cavity where it accumulates in much larger thickness than in impact craters on land at a microscopic scale the shock metamorphism features are about the same as those for land impacts geochemically impacts of comet nucleii may not leave a recognizable signature at the impact horizon except for a minor increase in iridium thus stratigraphic horizons associated with extinctions and or major changes in biota have to be closely examined for other impact indicators like the presence of tectites glass spherules and quartz grains with shock features occurrence of megatsunami wave deposits extensive erosion on continental margins margin failures and faunal mixing above erosional unconformities are other potential impact indicators there is no single indicator that can provide sufficient proof of an impact event such interpretations have to be based on multiparameter studies of global extent since many of the impact indicators are only of regional extent the lack of extinction of any marine plankton genera or of bottom dwellers at the montagnais impact site allows us to place a lower limit for biological extinctions caused by cometary impacts on those with nucleus 4 km in diameter the calculated frequency for a cometary impact which could result in a 10 extinction of marine genera is about 6 10 7 yr 1 and for the k t boundary type of extinctions about 2 10 8 yr 1 even allowing for a large degree of uncertainty in these estimates it is unlikely that the biological extinction events for the last 250 ma identified by sepkoski 1990 could have been all caused by meteorite impacts

late miocene high and rapid surface uplift and its erosional response in the andes of central chile 33 35 s 1 we address the question of the late cenozoic geomorphological evolution of the central chile andes 33 35 s using uplift markers river incision previous and new ages of volcanic bodies and new fission track ages the uplift markers consist of relicts of high elevated peneplains that evidence 2 km of regional surface uplift lasting 2 ma with variable amount along an e w transect the eastern coastal cordillera was uplifted 1 5 2 1 km at 33 34 s and 2 5 at 33 45 s and 1 5 km at 34 30 s erosional response to uplift was characterized by the retreat of a sharp knickpoint with celerities between 10 and 40 mm a 1 extrapolation using a stream power law shows that uplift began shortly before 4 ma or at 10 5 4 6 ma 7 6 ma central age depending on the morphostructural units involved the first alternative implies simultaneous uplift of the continental margin the second model the most reliable one implies that the uplift affected together the eastern coastal cordillera and the principal cordillera while the rest of the western fore arc subsided this regional uplift can be mostly balanced by crustal thickening resulting from coeval shortening related to the out of sequence thrusting event in the principal cordillera and the uplift of the frontal cordillera simultaneously emplacement of the southern edge of the flat slab subduction zone might have partially contributed to this uplift event

examination of selected microparticles from the sentik glacier core ladakh himalaya india analyse par microscopie electronique et spectrometrie atomique rx dispersive des quatre groupes morphologiques des microparticules de l himalaya aplaties anguleuses biogeniques et anormales

tertiary oxygen isotope synthesis sea level history and continental margin erosion tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale making use of improved chronostratigraphic control and additional oligocene isotope data synchronous changes in both benthic and planktonic 18o values which occurred in the oligocene to miocene 36 5 2 ma are interpreted in part to represent ice growth and decay the inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records this association is consistent with a link between oligocene to miocene erosional events and rapid 15 m m y glacioeustatic lowerings of about 50 m high benthic foraminiferal 18o values suggest the presence of continental ice sheets during much of the oligocene to recent 36 0 ma substantially ice free conditions probably existed throughout the paleocene and eocene 66 36 ma the mechanisms and rates of sea level change apparently were different between the early and late tertiary with glacioeustatic changes restricted to the past 36 m y pre oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes we apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands stimulating slope failure and submarine erosion the different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late tertiary on passive continental margins this speculation needs to be confirmed by examination of data from several passive margins

glacioeustasy during the middle eocene insights from the stratigraphy of the hampshire basin uk the sedimentary strata of the hampshire basin constitute some of the best preserved palaeogene sequences worldwide and include the traditional unit stratotype for the bartonian 36 9 41 4 ma the barton clay formation at alum bay on the isle of wight iow was studied to assess the evidence for middle eocene sea level variation in records of grain size sediment properties faunal assemblage foraminiferal diversity indices and foraminiferal stable isotopes sedimentary cycles of 1 10 myr third order and 0 2 0 5 myr fourth order duration are reported and interpreted to reflect 20 60 metre variations in water depth additionally an integrated magneto bio chemostratigraphical age model for the succession at alum bay is presented and new and published litho and bio stratigraphical markers are used to correlate additional successions based on this age model it appears that during the late lutetian and early bartonian 42 38 ma water depth variation identified within the basin was synchronous sedimentary and fossil evidence supports episodic uplift in the eastern part of the hampshire basin during the bartonian which at present precludes the calculation of eustatic sea level however the amplitude and frequency of water depth variations identified in the barton clay formation and correlations to published sea level curves are consistent with a component of these changes being glacioeustatic during the middle eocene there is also evidence for a large excursion delta 18o 1 per mille sign in the mono specific benthic foraminiferal oxygen isotope record alum bay 39 9 ma which is correlated to the isotope excursion at the middle eocene climatic optimum previously reported in the southern ocean and other localities a contemporaneous water depth increase of 40 m at alum bay may indicate that a component of this global oxygen isotope excursion results from a reduction in continental ice storage

flat subduction dynamics and deformation of the south american plate insights from analog modeling received 14 june 2007 revised 13 january 2008 accepted 12 march 2008 published 21 june 2008 1 we present lithospheric scale analog models investigating how the absolute plates motion and subduction of buoyant oceanic plateaus can affect both the kinematics and the geometry of subduction possibly resulting in the appearance of flat slab segments and how it changes the overriding plate tectonic regime experiments suggest that flat subductions only occur if a large amount of a buoyant slab segment is forced into subduction by kinematic boundary conditions part of the buoyant plateau being incorporated in the steep part of the slab to balance the negative buoyancy of the dense oceanic slab slab flattening is a long term process 10 ma which requires the subduction of hundreds of kilometers of buoyant plateau the overriding plate shortening rate increases if the oceanic plateau is large enough to decrease the slab pull effect slab flattening increases the interplate friction force and results in migration of the shortening zone within the interior of the overriding plate the increase of the overriding plate topography close to the trench results from 1 the buoyancy of the plate subducting at trench and 2 the overriding plate shortening experiments are compared to the south american active margin where two major horizontal slab segments had formed since the pliocene along the south american subduction zone flat slab segments below peru and central chile nw argentina appeared at 7 ma following the beginning of buoyant slab segments subduction in northern ecuador and northern chile the process of slab flattening resulting from the carnegie and iquique ridges subductions respectively seems to be active but not completed the formation of flat slab segments below south america from the pliocene may explain the deceleration of the nazca plate trenchward velocity citation espurt n f funiciello j martinod b guillaume v regard c faccenna and s brusset 2008 flat subduction dynamics and deformation of the south american plate insights

interpretation of the chemical and physical time series retrieved from sentik glacier ladakh himalaya india analyse a partir de mesures de la radioactivite d analyses chimiques chlorures sodium fer silicates phosphates ammonium d 18 o et ph et de donnees physiques des variations cycliques temporelles des glaciers application aux paleoclimats

eocene eustatic sea level changes evidence from western sinai egypt abstract a major horizon of reworked material lies within the thebes formation near the top of the upper part of the lower eocene planktonic foraminiferal zone p9 the thebes formation is well exposed in western sinai and this reworked horizon lag deposit has been found to be widely distributed and coeval with similar deposits in israel and jordan when examined in detail it is found to consist of concentrated phosphatic nodules coprolites phosphatic grains and polished benthic and planktonic foraminifera the thebes formation has generally been described as a deep water basinal deposit however the presence of this lag deposit indicates an important period of shallow water deposition before the return to deeper water conditions in which the upper 20 m of this formation and the beds of the overlying darat formation were laid down a second but less pronounced phase of shallowing both in terms of concentration of lag deposits and its geographic distribution occurred at the top of the middle eocene khaboba formation top of zone p14 the shallowings are attributed to eustatic sea level falls and not to tectonic movements the eocene period in sinai was one of general tectonic stability and the effect of movements associated with the syrian arc folds are not detectable south of hammam faraun the two horizons of reworked material coincide stratigraphically with two global low sea level stands

sequence stratigraphy eustasy or tectonic imprint sequence stratigraphy is viewed by many geologists as the expression in the stratigraphic record of the history of sea level changes sequence boundaries are its most distinctive elements and are used for global correlations until now correlations between unconformities and postulated lowerings of sea level have been seen primarily in terms of cause and effect relationships the postulated fall providing an explanation for the presence of an unconformity they have not been truly perceived in terms of stratigraphic correlations which would imply the establishment of synchrony between the cause and the effect to decipher the presence of a global sea level signal in the stratigraphic record the synchrony of sea level related unconformities in widely separated basins must be established which can be achieved by comparing the degree of overlap between hiatuses in different stratigraphic sections outcrop sections wells and the deep sea record reveal an apparently worldwide unconformity ies across the lower middle eocene boundary characterized by intensive reworking and or relatively rapid shallowing in some areas e g cyrenaica ne libya california egypt the lower middle eocene intervals in sections on the north atlantic margins have been correlated through magnetobiostratigraphy and the durations of the hiatuses have been precisely determined this record is compared with that in sections in other basins where shallowing and intensive reworking occur this study shows that there are two groups of unconformities and associated hiatuses around the lower middle eocene boundary the older hiatuses in the latest early eocene are generally short even on the shelf 1 m y the younger hiatuses in the early middle eocene are somewhat longer 2 m y the stratigraphic record around the lower middle eocene boundary suggests that there may be two mechanisms acting simultaneously to produce unconformities unconformities of a given age on the shelf may result from a global fall in sea level while their stratigraphically correlative unconformities on the slope and rise may result from tectonic instability unconformities related to the third order cycles in sea level changes in vail s model primarily may reflect tectonoeustasy

high resolution stratigraphy with strontium isotopes the isotopic ratio of strontium 87 to strontium 86 shows no detectable variation in present day ocean water but changes slowly over millions of years the strontium contained in carbonate shells of marine organisms records the ratio of strontium 87 to strontium 86 of the oceans at the time that the shells form sedimentary rocks composed of accumulated fossil carbonate shells can be dated and correlated with the use of high precision measurements of the ratio of strontium 87 to strontium 86 with a resolution that is similar to that of other techniques used in age correlation this method may prove valuable for many geological paleontological paleooceanographic and geochemical problems

the effects of strike slip motion along the cobequid chedabucto southwest grand banks fault system on the cretaceous tertiary evolution of atlantic canada the newfoundland fracture zone the southwest grand banks transform and the cobequid chedabucto fault zone form a linked strike slip fault system from the atlantic ocean to southeastern canada this paper suggests that several large scale geological features in southeastern canada are the result of a small amount of strike slip motion on the system during the mid cretaceous and oligocene regional extension features developed in the releasing bend in the laurentian sub basin during the mid cretaceous but the same area experienced oligocene compression this tectonic model accounts for the distribution of mid cretaceous volcanism fault bounded basins and regional unconformities as well as mid to late cretaceous subsidence of the scotian basin and oligocene uplift of the eastern scotian shelf

upper mantle structure in the south central chilean subduction zone 30 to 36 s 1 regional p and s wave travel time data were used to obtain three dimensional seismic tomography models for vp vs and vp vs above the subducting slab in central chile and argentina in this region there is an abrupt change from a normal subduction geometry south of 33 s to a flat subduction geometry to the north we find low vp low vs and high vp vs ratios in the southern half of our study area directly beneath the modern active volcanic arc which we interpret as localized pockets of melt in the northern half of our study area above where the subducting nazca plate flattens at 100 km depth we find low vp high vs and low vp vs ratios these unusual results point to a lack of melt or hydrated mineralogies such as serpentine both of which are characterized by high vp vs values the only mantle rocks that have low vp vs and high vs are mg rich compositions such as dehydrated serpentinite or orthopyroxenite we suggest that significant portions of the mantle overlying the flat slab consist of orthopyroxenite formed by a transient fluxing of silica rich fluids such fluids may have come from sediments that were subducted during the initiation of flat subduction at this latitude at 10 ma this would imply that the hydration of mantle material above a flat slab can be a transient phenomenon which leaves little residual free water behind but significantly alters the mantle chemistry

magnetostratigraphic correlation of palaeogene sediments in the hampshire and london basins southern uk magnetostratigraphic studies of the late palaeocene and early to middle eocene formations in the hampshire and london basins indicate that the geomagnetic field maintained a predominantly reverse polarity throughout the period of deposition of these sediments eight discrete normal polarity magnetozones have been identified within this dominantly reverse polarity interval three of these lie within the bracklesham group three within the london clay formation one in the oldhaven 9formation9 and one within the lowermost part of the thanet formation assuming that the magnetozone boundaries represent absolute time planes except where they correspond with stratigraphic hiatuses they may be used to provide a time framework for chrono stratigraphic correlation between sections and for investigating the dynamics of early palaeogene sedimentation patterns in this region the results indicate that the majority of the marine transgressive and regressive events which form the basis of king9s 1981 subdivisions of the london clay occurred very rapidly and may be regarded as being effectively 9instantaneous9 in marked contrast the boundaries between the sedimentary cycles of the bracklesham group represented by the wittering earnley selsey and huntingbridge divisions are significantly diachronous in relation to the absolute time planes provided by the magnetozone boundaries it is clear that the bracklesham group transgressive events occurred systematically earlier in the east than in the west as might be expected for westward directed transgressions certain biostratigraphic zonal boundaries within the bracklesham group appear also to have been influenced by the changing environmental conditions which accompanied these transgressive regressive cycles

new zealand and australian cenozoic sedimentary cycles and global sea level changes a global sea level history determined by vail et al consists of 23 sea level cycles between the base of the cenozoic and the pliocene pleistocene boundary 65 to 1 8 ma we have examined the relation between this global sea level history and the new zealand and australian continental margin shallow marine sedimentary record sedimentary cycles in the australian cenozoic marginal marine sequences are bounded by unconformities of various durations the tectonic stability and aridity of the western and southwestern margin of australia during the cenozoic produced a sedimentary record dominated by hiatuses this contrasts with the relatively complete sequences of the tectonically more active southeastern marginal basins the four major sedimentary cycles in the australian cenozoic paleocene to early eocene middle to late eocene latest oligocene to late middle miocene and latest miocene to quaternary correlate with the supercycles ta tb tc td and te of vail et al in new zealand most of the cenozoic stages represent classic sedimentary cycles bounded by unconformities or correlative conformities formed as a result of large rapid eustatic sea level changes the marine tertiary sequence in new zealand consists of 23 stages of the 18 stage boundaries between the end of the paleocene 53 ma and the end of the miocene 5 ma 16 appear to correlate with the boundaries of eustatic sea level cycles eustatic sea level lowstands are well recorded as unconformities in new zealand as a result of its unique tectonic setting during the cenozoic for most of the early cenozoic until the middle oligocene 30 ma new zealand was generally subsiding after the middle oligocene this region began to be uplifted as the pacific australian plate boundary migrated onto new zealand climaxing in the latest cenozoic late pliocene quaternary thus exposing a nearly complete sequence of marine cenozoic strata well suited for the study of sedimentary cycles unconformities which form as a result of eustatic sea level changes represent a very useful correlation tool especially on continental margins with a sufficiently high terrigenous sediment supply and can be used to supplement paleontologic correlations their ultimate usefulness for correlation depends on the rapidity of the sea level changes or the speed at which sedimentary facies change in response to sea level changes if sea level changes rapidly 10 m 1 000 years stratigraphic resolution will be high 105 years but resolution decreases as the speed of sea level change decreases unconformities may well prove to be a vital tool linking the classic land based sections around the world with deep sea sections

seismic investigations of the o higgins seamount group and juan fernandez ridge aseismic ridge emplacement and lithosphere hydration the o higgins seamount group is a cluster of volcanic domes located 120 km west of the central chilean trench on the crest of the juan fernandez ridge this aseismic hot spot track is subducting under south america triggering a belt of intraslab earthquake hypocenters extending about 700 km inland the juan fernandez ridge marks the southern boundary of a shallow subduction segment subduction of oceanic basement relief has been suggested as a cause for the flat slab segments characterizing the andean trench system the juan fernandez ridge however shows only moderate crustal thickening inadequate to cause significant buoyancy in 2001 wide angle seismic data were collected along two perpendicular profiles crossing the o higgins group we present tomographic images of the volcanic edifices and adjacent outer rise trench environment which indicate a magmatic origin of the seamounts dominated by extrusive processes high resolution bathymetric data yield a detailed image of a network of syngenetic structures reactivated in the outer rise setting a pervasive fault pattern restricted to the hot spot modified lithosphere coincides with anomalous low upper mantle velocities gained from a tomographic inversion of seismic mantle phases reduced uppermost mantle velocities are solely found underneath the juan fernandez ridge and may indicate mineral alterations enhanced buoyancy due to crustal and upper mantle hydration may contribute an additional mechanism for shallow subduction which prevails to the north after the southward migration of the juan fernandez ridge

active detachment faulting above the peruvian flat slab the cordillera blanca detachment fault in the peruvian andes is to our knowledge the first active detachment to be documented above a modern flat slab crustal detachment has unroofed the ca 8 ma cordillera blanca batholith now the backbone of the highest mountain range in peru large magnitude slip along the fault was thermally enhanced by emplacement of the batholith the penultimate magmatic event prior to flattening of the nazca slab however extensional models based on arc magmatism and crustal thickening alone do not adequately explain the scale or structural asymmetry of a series of young deep seated west dipping normal faults across peru here we show that the onset of detachment faulting coincided with subduction of the aseismic nazca ridge and consequent flattening of the nazca slab we propose that slab buoyancy from ridge subduction triggered extensional collapse of the prethickened continental crust and that this buoyancy drove footwall uplift that exceeds basin subsidence the west dipping asymmetry of late cenozoic extensional faults in peru may be controlled by a preexisting crustal anisotropy older thrusts and or formation of riedel like shears kinematically linked to the flat nazca slab

deformation partitioning in flat subduction setting case of the andean foreland of western argentina 28 s 33 s 1 the andean foreland of western argentina 28 s 33 s corresponds to back arc deformations associated with the ongoing flat subduction of the nazca plate beneath the south american lithosphere in this paper short and long term tectonic regimes for this andean region are investigated and compared to nuvel 1 and the new gps derived velocity field for the andes mountains from this comparison it appears that 1 the slight convergence obliquity at the plate boundary is accommodated by the subduction zone itself precluding any deformation partitioning at lithospheric scale 2 clockwise rotation of 1 and p axes within the andean foreland of western argentina suggests that plio quaternary deformation partitioning may occur in the upper plate at crustal scale and 3 comparison of seismic rates of shallow deformations determined from moderate to large earthquakes localized between 22 s and 36 s shows that the amount of shortening is about the same 2 4 cm yr 1 between the forearcs of the 30 dipping slab segments and the back arc of the flat segment all together those observations are in close agreement with the idea that interplate coupling in flat subduction settings may play a key role in the present day transfer of plate boundary forces consequently the andean back arc of western argentina might be regarded as an obliquely converging foreland where plio quaternary deformations are partitioned between strike slip and thrust motions that are localized in the east verging thin skinned argentine precordillera and west verging thick skinned pampean ranges respectively

eocene oligocene sea level changes on the new jersey coastal plain linked to the deep sea record we use magnetostratigraphy and sr isotope stratigraphy to improve stratigraphic control for the eocene to oligocene of the new jersey coastal plain acgs4 borehole magnetostratigraphy in many cases is complicated in outcrop sections of shallow water

response of the tectonically erosive south peruvian forearc to subduction of the nazca ridge analysis of three dimensional analogue experiments 1 the collision zone of the obliquely subducting nazca ridge and the erosive peruvian forearc is characterized by enhanced tectonic erosion normal faulting forearc uplift and a minor indentation we apply two three dimensional sandbox experiments monitored by an optical particle imaging velocimetry system to decipher the spatial and temporal evolution of forearc deformation during oblique convergence the rigid model ridge causes a wave of uplift followed by subsidence to shift along the forearc by varying the strength of the analogue material we demonstrate that the forearc response to ridge subduction in particular the amount and rate of uplift and wedge indentation is controlled by the mechanical strength of the forearc the model using the high strength material yields results more compatible with the geological record of the peruvian forearc first the modeled amount of uplift scales to 1 2 km in nature which agrees with 1 0 km of uplift recorded by marine terraces second the calibrated model uplift rate of 0 9 km myr is similar to the natural rate of 0 7 km myr third the development of normal faults above the model ridge is in accordance with the style of faulting above the nazca ridge finally oversteepening of the uplifted wedge and local slope failure as observed in the experiment has also been identified off southern peru a comparison between the erosive peruvian and accretive margins emphasizes that the style of ridge induced deformation depends on the forearc strength and crustal structure which in turn are controlled by the long term evolution of the margin s mass transfer regime

upper eocene to oligocene isotope 87sr 86sr δ18o δ13c standard section deep sea drilling project site 522 we improved upper eocene to oligocene deep sea chronostratigraphic control by integrating isotope 87sr 86sr 18o 13c stratigraphy and magnetostratigraphy most previous attempts to establish the timing of isotope fluctuations have relied upon biostratigraphic age estimates which have uncertainties of 0 5 to over 4 0 m y deep sea drilling project dsdp site 522 contains the best available upper eocene to oligocene magnetostratigraphic record which allows first order correlations of isotope records 87sr 86sr 18o 13c to the geomagnetic polarity time scale gpts empirical calibrations between the 87sr 86sr of foraminifera and magnetochronology at site 522 allow more precise correlation of unknown samples with the gpts for example shallow water and high latitude sections may be tied into the deep sea record sr isotope stratigraphic resolution for the latest eocene to oligocene is approximately 2 m y

paleomagnetic tracking of mountain building in the peruvian andes since 10 ma 1 we report paleomagnetic data from 73 sites 628 samples of upper oligocene to pliocene rocks from central and northern peru the data indicate that the subandean zone has not experienced vertical axis rotation since the upper oligocene whereas the coast and the western cordillera record a coherent pattern of counterclockwise rotations emplaced in the last 10 ma this pattern can best be explained by two competing hypotheses 1 a propagation in rotations from the bolivian orocline toward the north or 2 a more punctual and widespread event linked to subduction of the nazca ridge that caused the rotations on the basis of the time space relationship of paleomagnetic rotations deformation magmatism and nazca south america plate convergence the latter appears more likely moreover because the rotations north of the abancay deflection 15 5 s are synchronous with deformation and exhumation they indicate that major mountain building in the peruvian andes has occurred since 10 ma finally we suggest that a succession of aseismic ridge subduction events played an important role in shaping the modern topography of the andean chain

three dimensional density model of the nazca plate and the andean continental margin 1 we forward modeled the bouguer anomaly in a region encompassing the pacific ocean 85 w and the andean margin 60 w between northern peru 5 s and patagonia 45 s the three dimensional density model that reproduces the gravity field is a continental scale representation of density structure to 410 km depth that characterizes the mantle and crust of the oceanic nazca plate subducted slab and continental margin with a minimum number of bodies we predefined the density of each body after studying the dependency of density on composition of crustal and mantle materials and pressure temperature conditions appropriate for the andean setting a database of independent geophysical information constrains the geometry of the top of the subducted slab locally the moho of the oceanic and continental crusts and indirectly the lithosphere asthenosphere boundary underneath the continental plate other boundaries notably the intracrustal density discontinuity separating upper from lower crust below the continent were not constrained and their geometry is the result of fitting the observed and calculated bouguer anomaly during forward modeling this contribution presents the model to the andean geoscientific community and contains some tools like a sensitivity analysis that helps potential users of the model to interpret its results we describe and discuss some of these results in order to illustrate the application of the model to the study of a wide range of phenomena e g modification of oceanic plate structure by hot spots shape of the subducted slab thermal structure of the continental lithosphere compensation mechanism and formation of orogenic relieve causes of andean segmentation

paleogene magnetic stratigraphy in umbrian pelagic carbonate rocks the contessa sections gubbio the umbrian sequence of pelagic carbonate rocks provides an opportunity for precise correlation between paleogene biostratigraphy and geomagnetic polarity history the red to pink paleocene to middle eocene scaglia rossa limestone the varicolored upper eocene scaglia variegata limestone and the gray green oligocene scaglia cinerea marlstone form a 250 m thick continuous exposure in the contessa valley near gubbio italy magnetostratigraphic lithostratigraphic and biostratigraphic investigations in three sections covering the entire paleogene have confirmed and dated the geomagnetic reversal sequence for most of this period the ferromagnetic mineral in the scaglia cinerea is magnetite and af demagnetization to peak fields of 20 mt is sufficient to define the characteristic remanent magnetization the scaglia variegata and pink scaglia rossa samples contain an additional hematite component which is very pronounced in dark red paleocene scaglia rossa samples changes of magnetic mineralogy take place in these limestones during heating especially above 500 c however thermal demagnetization is effective in isolating the characteristic remanence vectors which form almost antipodal clusters of directions representing normal and reversed polarities the directions are rotated counterclockwise partly due to post oligocene tectonism paleontological zonations of planktic foraminifera and calcareous nannofossils were hampered by poor preservation but the major epoch and stage 29 as in the gubbio bottaccione section paleocene eocene just above anomaly 25 eocene oligocene between anomalies 13 and 15 oligocene miocene just below anomaly 6c these correlations require slight modifications to previous conclusions on paleogene sea floor spreading rates

global implications of lower to middle eocene sequence boundaries on the new jersey coastal plain the icehouse cometh wedocumentninelower middleeocenesequencesonthenewjerseycoastalplainand comparethemwithglobal d 18 oandhaqetal records earlyeocenehiatusesdonotmatch d 18 ochanges anditisunlikelythattheyaretheresultofglacioeustasy consistentwithan ice free early eocene early middle eocene 49 43 ma evidence for a link between sequencesand d 18 oisequivocal andthepresenceoflargeicesheetsisuncertain beginning in the late middle eocene 43 42 ma concomitant increases in planktonic and benthic d 18 o records coincide with the timing of hiatuses on the new jersey coastal plain and a change from carbonate dominated to siliciclastic dominated sedimentation these represent the development of the antarctic ice cap and the beginning of the icehouse world of the 14 sequences predicted by haq et al for this interval 9 are resolvable on the new jersey margin and the other 5 appear to be combined with others we conclude that although ice volume changes controlled sequences since at least 42 ma mechanisms for sea level change prior to then are still not fully understood

the eocene oligocene transition changes in sea level temperature or both abstract the eocene oligocene transition eot 34 ma reflects the onset of major antarctic glaciation the primary geochemical signature of the eot is two 300 kyr spaced shifts in increasing deep sea oxygen isotope values possibly reflecting both global cooling and or increasing ice volume a way to assess the respective contribution of continental ice is to quantify concomitant glacio eustatic sea level change this is usually expressed in relatively shallow marine depositional settings one potentially suitable region is in the vicentinian alps ne italy where marginal marine deposits document sea level changes during the eot by correlating stable isotope bio and magnetostratigraphic information between three distant regions we are able to relate the shallow marine sections to the pacific oxygen isotope record from ocean drilling program odp site 1218 of coxall et al 2005 microfacies sedimentological and biotic analysis suggests that associated with the first isotope shift eot 1 sea level fell 20 m and with the ultimate shift the oligocene isotope event 1 oi 1 sea level fell some 50 60 m distribution patterns of temperature sensitive dinoflagellates from a coeval central italian section reveal that the early stages of the eot were accompanied by sea surface cooling whereas no sustained cooling is noted in association with the oi 1 this suggests that the initial eot shift s reflect a mixed signal of ice volume and temperature whereas the oi 1 primarily reflects expansion of the antarctic cryosphere

unlocking the ice house oligocene miocene oxygen isotopes eustasy and margin erosion oxygen isotope records and glaciomarine sediments indicate at least an intermittent presence of large continental ice sheets on antarctica since the earliest oligocene circa 35 ma the growth and decay of ice sheets during the oligocene to modern ice house world caused glacioeustatic sea level changes the early eocene was an ice free greenhouse world but it is not clear if ice sheets existed during the middle to late eocene doubt house world benthic foraminiferal 18o records place limits on the history of glaciation suggesting the presence of ice sheets at least intermittently since the earliest oligocene the best indicator of ice growth is a coeval increase in global benthic and western equatorial planktonic 18o records although planktonic isotope records from the western equatorial regions are limited subtropical planktonic foraminifera may also record such ice volume changes it is difficult to apply these established principles to the cenozoic 18o record because of the lack of adequate data and problems in stratigraphic correlations that obscure isotope events we improved oligocene to miocene correlations of 18o records and erected eight oxygen isotope zones oi1 oi2 mi1 mi6 benthic foraminiferal 18o increases which are associated with the bases of zones oil circa 35 8 ma oi2 circa 32 5 ma and mil circa 23 5 ma can be linked with 18o increases in subtropical planktonic foraminifera and with intervals of glacial sedimentation on or near antarctica our new correlations of middle miocene benthic and western equatorial planktonic 18o records show remarkable agreement in timing and amplitude we interpret benthic planktonic covariance to reflect substantial ice volume increases near the bases of zones mi2 circa 16 1 ma mi3 circa 13 6 ma and possibly mi5 circa 11 3 ma possible glacioeustatic lowerings are associated with the 18o increases which culminated with the bases of zone mi4 circa 12 6 ma and mi6 circa 9 6 ma although low latitude planktonic 18o records are required to test this these inferred glacioeustatic lowerings can be linked to seismic and rock disconformities for example we link 12 oligocene early late miocene inferred glacioeustatic lowerings with 12 of the sequence boundaries inferred eustatic lowerings of haq et al 1987

can slab melting be caused by flat subduction slab melting has been suggested as a likely source of adakitic arc magmas i e andesitic and dacitic magmas strongly depleted in y and heavy rare earth elements existing numerical and petrologic models however restrict partial melting to very young 5 ma oceanic crust typically at 60 80 km depth paradoxically most of the known pliocene quaternary adakite occurrences are related to subduction of 10 45 ma lithosphere which should not be able to melt under normal subduction zone thermal gradients we propose an unusual mode of subduction known as flat subduction occurring in 10 of the world s convergent margins that can produce the temperature and pressure conditions necessary for fusion of moderately old oceanic crust of the 10 known flat subduction regions worldwide eight are linked to present or recent 6 ma occurrences of adakitic magmas observations from chile ecuador and costa rica suggest a three stage evolution 1 steep subduction produces a narrow calc alkaline arc typically 300 km from the trench above the asthenospheric wedge 2 once flat subduction begins the lower plate travels several hundred kilometers at nearly the same depth thus remaining in a pressure temperature window allowing slab melting over this broad distance and 3 once flat subduction continues for several million years the asthenospheric wedge disappears and a volcanic gap results as in modern day central chile or peru the proposed hypothesis which reconciles thermal models with geochemical observations has broad implications for the study of arc magmatism and for the thermal evolution of convergent margins

neogene deformation of the flat subduction segment of the argentine chilean andes magnetostratigraphic constraints from las juntas la rioja province argentina substantial neogene deformation occurred in the part of the andes mountains and the sierras pampeanas that overlie a subhorizontal segment of the subducting nazca plate in northern argentina the history of the volcanic arc suggests that the nazca plate attained the present low angle inclination between about 17 and 10 ma with the region underlain by the flat nazca plate widening eastward thereafter we compare the history of deformation provided by magnetostratigraphy of miocene strata in foreland and intermontane basins to the history of the subducting plate at las juntas in the precordillera thin skinned thrust belt 4 780 m of neogene nonmarine strata record the initiation and termination of sedimentation in a segment of a foreland basin magnetostratigraphic data complemented by fission track dating of an interbedded tuff indicate that the strata span from 18 to 9 ma the earliest strata correlate to the end of a time of volcanic activity in the magmatic arc and the beginning of reverse faulting in the frontal cordillera subsequently the deformation front migrated eastward generating the precordillera thrust belt and finally the sierras pampeanas basement uplifts the rate of strata accumulation varied systematically as new uplifts arose with a maximum rate exceeding 1 0 mm a between 15 and 12 ma a correlation between the deformational history and the geometric history of the subducted slab suggests that thrusting migrated eastward as the angle of subduction shallowed although large scale factors affecting the plate margin beyond the flat slab segment may have partly induced the compressional deformation this correlation suggests that processes related to flat subduction are of importance in deforming the continental plate

the mid oligocene marshall paraconformity new zealand coincidence with global eustatic sea level fall or rise the marshall paraconformity is a regional mid oligocene unconformity that occurs widely in shelf sequences from the eastern new zealand passive continental margin the paraconformity represents a break in sedimentation of at least 3 m y and coincided with a regional sea level highstand in contrast to the prediction of global lowstand at 29 m y b p similar paraconformities are widespread on other southern continents deep sea hiatuses of similar age occurred in the adjacent southwest pacific ocean basins they may or may not have a common origin with the hiatus associated with the marshall paraconformity possible causes for the paraconformity and its associated glauconitic and bioclastic sediments are sea level fluctuations including those related to local geoidal effects or erosive bottom waters stemming from the development of antarctic glaciation or from splitting of the southern continents

integrated late eocene oligocene stratigraphy of the alabama coastal plain correlation of hiatuses and stratal surfaces to glacioeustatic lowerings we integrated strontium and oxygen isotopic biostratigraphic and magnetostratigraphic studies of two upper eocene oligocene boreholes drilled near bay minette and st stephens quarry ssq alabama continuous coring provided fresh unweathered material for magnetostratigraphic studies minimizing problems reported from nearby outcrops difficulties with each technique were encountered because of diagenesis absence of marker fossils and the presence of unconformities however by integrating results from isotopic stratigraphy biostratigraphy and magnetostratigraphy we correlated these relatively shallow water deposits to the geomagnetic polarity time scale gpts at the ssq borehole the upper eocene to lower oligocene section is apparently complete within our stratigraphic resolution 0 2 0 5 m y allowing us to estimate the ages of several stratal surfaces late eocene sr isotope age estimates are as expected at the ssq borehole but oligocene ages are 1 m y older than expected due to diagenesis at the bay minette borehole a latest eocene earliest oligocene and a late early oligocene hiatus were detected we correlate these two hiatuses and stratal surfaces at ssq with global 18o increases inferred to represent glacioeustatic lowerings and with evidence for hiatuses on other continental margins 1 a distinct disconformity at the base of the chickasawhay limestone at both boreholes and a hiatus at bay minette correlates with a global 18o increase we revise the age of this surface equivalent to the tb 1 1 sequence boundary making it 2 m y older than previously reported and 2 a surface at the top of the shubuta member lowermost oligocene has been interpreted both as a condensed section and a disconformity this surface at ssq and a hiatus at bay minette correlate with a sharp global 18o increase and with hiatuses on the new jersey and irish margins the timing of the hiatuses and stratal surfaces correlates with the inflection of the 18o increases and not with the maximum values supporting models that indicate that unconformities form during the maximum rates of sea level fall

northwest african continental margin history of sediment accumulation landslide deposits and hiatuses as revealed by drilling the madeira abyssal plain 1 odp drill sites in the madeira abyssal plain reveal sequences of organic rich turbidites derived from the northwest african margin in which each turbidite has a volume of tens to hundreds of cubic kilometers the frequency of turbidite emplacement has been combined with core and seismic data to show the volume of redeposited sediment the basin began to fill about 22 ma with numerous small turbidites up to 100 per million years each with volumes of a few cubic kilometers the total volume of turbidites deposited increased between 16 and 11 ma as did their individual volumes and then declined to 7 ma at 7 ma there was a dramatic increase in the amount of turbidite input to 768 km3 myr and a rise in the average volume of each unit to 59 km3 these high values have been maintained to the present day the variations in the amount of redeposited sediment most likely reflect the rates of sedimentation on the northwest african margin since high sedimentation leads to oversteepening of the slopes and eventual mass wasting the dramatic changes at about 7 ma may be due to a large increase in upwelling off northwest africa caused by circulation changes associated with increased glaciation of the poles up to 20 of sediment may be remobilized by landslides with each event leaving a hiatus each of these hiatuses extends over an average area of 4800 km2 and represents removal of sediment layers several tens of meters thick and of several hundred thousand years duration