quantification of tsunamigenic earthquakes by the mt scale abstract a magnitude scale m t previously defined by the author is unique among the many scales in seismology because it is based on the instrumental amplitude of tsunami waves its level being adjusted to agree with moment magnitude despite the simple definition the m t scale reliably measures the seismic moment of an earthquake as well as the overall physical size of the source of the tsunami in reality the seismic moment estimated from m t for 39 events around japan is found to be essentially equivalent to that estimated from seismic waves with an uncertainty factor of two on average the m t scale is useful for a quantitative discrimination of tsunami earthquakes that generate anomalously large tsunamis for their magnitude of 97 tsunamigenic events that occurred around japan during the period 1894 to 1985 at least eight tsunami earthquakes are identified from the m t vs m s relation

bioerosion structures in crepidula mollusca gastropoda as indicators of latitudinal palaeoenvironmental changes example from the marine quaternary of argentina abstract late quaternary marine skeletal concentrations from argentina are rich in molluscs exhibiting a great variety of bioerosion structures the shells of crepidula a characteristic gastropod occurring along more than 2000 km of coastline between the rio de la plata margin and southern patagonia show traces of dwelling predation and anchoring activities made by porifers bryozoans annelids other gastropods and brachiopods caulostrepsis entobia maeandropolydora iramena oichnus finichnus pennatichnus pinaceocladichnus podichnus and renichnus occur on the outer shell surface finichnus and oichnus are the only traces present along the entire area and the full time span considered the most characteristic structures are produced by bryozoans polychaetes and predatory gastropods traces produced by annelids and predatory gastropods occur preferentially in the central shell sector where predators gained access to the soft parts of the prey by contrast encrusting or branching bryozoan colonies are widely distributed as they can attach to any sector regardless of shell features available no strict correlation is evident between ichnodiversity and either time or latitude but ichnodiversity is linked to local oceanographical biotic controls for patagonia with a great majority of ichnotaxa made by bryozoans the general trend of higher bioerosion degree and ichnodiversity at higher latitudes is controlled by sea surface temperature productivity for the modern and the holocene several ichnodiversity peaks match with well constrained conditions substrate salinity thermal fronts by contrast this does not hold for the pleistocene dissimilar conditions probably prevailed especially during the last interglacial colder waters richer in nutrients

half graben versus large offset low angle normal fault importance of keeping cool during normal faulting 1 we study the effect of temperature in the crust on the style of faulting during the extension of a model lithosphere two distinct fault patterns are seen depending on whether the brittle layer thickness hl is greater or less than a critical value hc for a thin layer hl hc a rift develops with multiple faults bounding an evolving set of grabens numerical results show that the value of hc depends on how the thermal structure and therefore the thickness distribution of the lithosphere evolve during extension in the region where the displacement occurs along the fault the brittle lithosphere thins and we show that this promotes development of multiple faults in models with heat transfer only by rock motion and conduction the advective thinning effect can greatly reduce hc with rift velocities greater than 1 cm yr 1 hc is reduced by a factor of 5 to unreasonably small values however inclusion of simulated hydrothermal cooling increases hc this allows large offsets to develop in model lithospheres with brittle layers of order 10 km thick extension of layers thicker than 10 km tends to produce a rift with multiple graben structures finally we compare the result of a simplified analytic model of extension to data we find that the style of faulting seen in a variety of continental rifts is broadly consistent with our model predictions

paleoecology and geochronology of uruguayan coastal marine pleistocene deposits abstract minimum radiocarbon ages of marine pleistocene molluscs from uruguay range from 29 500 600 to 35 500 1900 14c yr b p because knowledge of the marine quaternary stratigraphy of uruguay remains inadequate no attempt is made to correlate between these deposits and recognized lithostratigraphic units analysis of the temperature and salinity ranges of the various molluscs suggests the establishment of a poly euhaline fauna that inhabited waters warmer than present at the same latitude this is supported by a northward retreat in the recent distribution of chrysallida cf c gemmulosa nioche subrostrata and probably anomalocardia brasiliana these marine pleistocene deposits can probably be correlated with the last interglaciation oxygen isotopic substage 5e at that time the fluvial marine front of the rio de la plata was displaced much further northwestward than previously supposed chrysallida cf c gemmulosa and limaria sp are recorded in uruguayan quaternary deposits for the first time

subduction dynamics as revealed by trench migration 1 new estimates of trench migration rates allow us to address the dynamics of trench migration and back arc strain we show that trench migration is primarily controlled by the subducting plate velocity vsub which largely depends on its age at the trench using the hot and weak arc to back arc region as a strain sensor we define neutral arcs characterized by the absence of significant strain meaning places where the forces slab pull bending and anchoring almost balance along the interface between the plates we show that neutral subduction zones satisfy the kinematic relation between trench and subducting plate absolute motions vt 0 5vsub 2 3 in cm a 1 in the hs3 reference frame deformation occurs when the velocity combination deviates from kinematic equilibrium balancing the torque components of the forces acting at the trench indicates that stiff old subducting plates facilitate trench advance by resisting bending

patterns and processes of shell fragmentation in modern and ancient marine environments shell fragments are important components of many recent and fossil marine benthic ecosystems and can provide crucial information on past and present environmental conditions interpreting such fragments requires integrated knowledge in various fields and the information potential is therefore rarely optimally utilized this paper uses the definition of a fragment as being a piece of shell having less than 90 of its original form it then outlines the potential characteristics pathways and fates that shells and their fragments can have fragmentation is a key factor shaping the shelly part of death assemblages but it is difficult to interpret because it can be broadly caused by ecological biostratinomic or diagenetic processes and also depends on shell strength strength in turn reflects multifunctionality during ontogeny and depends on a complex set of skeletal and taphonomic factors therefore no particular shell parameter clearly determines strength but thickness microstructure type and degree of organic matrix have the strongest influence on pre and post mortality strength size measurements are usually less important for shell strength than thickness although ecologically complex size refuges from predation do exist similarly shell shape sculptural features and specific aperture types in gastropods provide various defence strategies rather than increasing strength per se key ecological factors of fragmentation include predation due to crushing peeling along with mistaken predation selfinflicted damage during predation and burrowing and more physical aspects such as impacts by stones modern studies must consider damage by benthic commercial fisheries or dredging by scientific vessels key biostratinomic factors include transportinduced abrasion littoral zone of surf washed beaches bioerosion and dissolution mainly sublittoral environments diagenetically fragmentation during compaction mainly occurs when shells are in direct contact with one another or with coarser grains taphonomic features e g drillholes have only minor influence a key step in interpreting fragments is to categorize breakage into repaired versus unrepaired diagnostic versus nondiagnostic and severe versus localized damage quantifying the above categories can then provide useful information on breakage patterns and underlying processes understanding the many characteristics of shells and their fragments is a significant interpretive tool in maximizing the information gain in palaeoecological and taphofacies analyses d 2003 elsevier science b v all rights reserved

extension of continental crust by anelastic deformation during the 2011 tohoku oki earthquake the role of extensional faulting in the generation of a great tsunami abstract observations of seafloor morphologies and environments made before and after the 2011 tohoku oki earthquake reveal open fissures generated during the earthquake where the fault trace is interpreted on seismic profiles to intersect the seafloor anomalously high heat flow was observed at a landward dipping normal fault in august 2011 five months after the earthquake but by august 2012 heat flow measured at the same station had decreased to close to the background value which suggests that the normal fault ruptured during the 2011 earthquake these seafloor observations and measurements demonstrate deformation that was both extensional and anelastic within the overriding continental plate during the 2011 earthquake seismic profiles as well as seafloor bathymetry data in the tsunami source area further demonstrate that landward dipping normal faults extensional faults collapse the continental framework and detach the seaward frontal crust from the landward crust at far landward from the trench the extensional and anelastic deformation i e normal faulting observed in both seafloor observations and seismic profiles allows the smooth seaward movement of the continental crust seaward extension of the continental crust close to the trench axis in response to normal faulting is a characteristic structure of tsunami source areas as similar landward dipping normal faults have been observed at other convergent plate margins where tsunamigenic earthquakes have occurred we propose that the existence of a normal fault that moves the continental crust close to the trench can be considered one indicator of a source area for a huge tsunami

stratigraphy of holocene sand shell ridges in the bahia blanca estuary argentina abstract the holocene marine transgression has left important testimonies to itself in the bahia blanca estuary with mainly in the inner part of the estuary set almost parallel to the isohypses between 8 and 10 m sand shell ridges being found these structures are composed of a medium to fine sand with a high percentage of shells 47 species of mollusc radiocarbon dating of the shells in the oldest ridge gave a 14 c age of 4615 110 yrs b p the sand shell ridges present a parallel bedding with slight inclination toward the sea the two ridges studied stand discordantly on clay sandy silt containing laminated and flaser structures these sediments change transitionally with depth to clay silt and silty sand the marine sediments are distributed on a calcrete of late pleistocene age the types of sediments making up the ridges and their stratigraphical relationships are similar to those of cheniers however the term chenier is not used here because the sand shell ridges and their associated deposits have been formed by transgressive regressive sedimentary processes during the latest postglacial

palaeobiogeography of the holocene molluscan fauna from northeastern buenos aires province argentina its relation to coastal evolution and sea level changes abstract in northeastern argentina the holocene ca 2500 6000 14c yr b p benthic marine molluscan fauna from the cerro de la gloria member of the las escobas formation represents a higher sea level stand approximately 4 5 m above m s 1 than the present one and belongs to an original shallow water environment most of this fauna 48 belongs to the argentine zoogeographic province while part of it 12 is shared with the magellanian province the rest of the taxa are cosmopolitan 5 or belong to the caribbean antillean and brazilian provinces 35 the latter are considered stenothermic warm water indicators which in the present northeastern marine shelf of argentina are scarcer 6 14 most of them ranging from the antilles to southern brazil or northeastern uruguay it is a thermally anomalous molluscan fauna which is a consequence of local coastal palaeogeography and global mid holocene climatic change on that basis a higher sea water temperature during the mid holocene is inferred probably due to a stronger influence of the brazil current which could have extended farther south and westwards during the hypsithermal the disagreement between the data presented here and the micropalaeontological data is still a problem to be solved likely through critical systematic revisions of the foraminiferids and ostracods not included in this paper

late quaternary marginal marine deposits and palaeoenvironments from northeastern buenos aires province argentina a review the late quaternary marginal marine deposits along eastern argentina southwestern atlantic are reviewed according to our present knowledge in the northeastern coastal area of buenos aires province they have been assigned to a series of transgressions and regressions ranging from the late pliocene to the late quaternary the most widely accepted model is frenguelli s 1957 classical chronostratigraphical scheme of belgranense late pleistocene marine sediments at 3 6 m above m s l and ca 26 000 35 000 14c years bp the querandinense pleistocene holocene estuarine sediments below or at present m s l and the most extensive platense mid holocene marine deposits at 4 5 2 m above m s l dated at ca 8000 1340 14c years bp the restricted belgranense deposits recorded in samborombon bay in magdalena at ca 32 000 bp near mar chiquita at ca 24 900 and 30 500 bp and southwards in bahia blanca at ca 26 000 35 500 bp may belong to an interstadial gonzalez et al 1986 the molluscan composition suggests a marine invasion of the area but not a typical interglacial cycle characterized by euhaline and warm water elements however the oxygen isotope record argues against an interstadial during the interval 34 27 ka and the chronological control for these deposits is very poor suggesting that they most probably have been elevated neotectonically the pleistocene holocene querandinense deposits extensively distributed along the bonaerensian coastal plain and continental shelf ca 11 000 14c years bp with very low faunal diversity abundance of freshwater ostracods and absence of the warm water molluscs characteristic of the holocene ridges indicate low salinity and cool water conditions further dating and isotope analysis of these deposits are required for a better understanding of the chronology of climatic events by the end of the pleistocene in this area and to establish whether or not they could correspond to the younger dryas event of the northern hemisphere platense littoral ridges formed between ca 7600 and 2500 bp extend in several subparallel rows from ca 34 s to ca 39 s they formed either as beaches or possibly as sublittoral bars in samborombon bay where they reach 120 km long 10 30 m wide and up to 5 m thick at 4 5 5 m above m s l or in the supratidal and intertidal zones in mar chiquita at 2 4 m above m s l molluscs and ostracods suggest a brackish marine to marine brackish environment of lower salinity than the modern atlantic littoral during the mid holocene the oceanic waters were mixing with large amounts of antarctic ice melting in the south atlantic isla 1990 probably reducing salinity along the bonaerensian littoral the oldest littoral ridges accumulated during the hypsithermal and in the punta indio area their different geometry and alignment greatest age highest diversity and warmer affinity of the molluscan fauna suggest a short interval of reversal of the atmospheric circulation pattern further research is necessary for a better understanding of the chronology of the marine late quaternary deposits coastal evolution and climatic changes in this area

chronology of the central atlantic magmatic province implications for the central atlantic rifting processes and the triassic jurassic biotic crisis abstract the central atlantic magmatic province camp is among the largest igneous provinces on earth emplaced synchronously with or just prior to the triassic jurassic t j boundary ca 200 ma in great part due to the controversial connection between the occurrence of camp and the events of the t j boundary the demand for better constraints on the duration and eruptive chronology of this province has increased more than 100 new 40 ar 39 ar ages have been published in the last 15 years with more than half of these in the last 3 years a careful review and selection of available ages as well as the publication of 16 new ages from the carolinas newark basin usa french guyana and morocco are presented judicious selection yields a total of 58 accepted age determinations for camp magmatism ranging from 202 to 190 ma covering every part of the camp a more complete picture develops with intrusive camp magmatism commencing as early as 202 ma extrusive activity initiated abruptly 200 ma reaching peak volume and intensity around 199 ma on the african margin the main period of camp magmatism is confirmed as brief but is suggested to consist of at least two phases over 1 5 ma with magmatism commencing along the africa north american margins and slightly later along the south american margin two volumetrically minor but distinctive magmatic peaks centered at 195 and 192 ma are mirrored in data from all three continents and highlighted by our statistical approach models describing rifting and thermal input and magma production on these timescales are explored despite significant advances in our understanding of the chronology of camp more data of better quality and broader geographical coverage are needed to completely characterize the evolution of the camp and infer its geodynamic origin in addition lack of a well defined t j boundary age as well as the absence of a relevant basis for comparison between u pb and 40 ar 39 ar data for this time period remain limiting factors to unambiguously linking camp in time with the events of the t j boundary

preservation of mollusca in copano bay texas the long term record abstract the stratigraphic taphonomic and biologic records from two cores in copano bay texas were analyzed to determine 1 whether variations in shell content with depth were caused by variations in carbonate preservation carbonate production or sedimentation rate and 2 the extent to which characteristics of fossil assemblages such as species composition numerical abundance biomass and trophic and habitat structure identified similar or different trends below the top few cm of the sedimentary column variations in carbonate content with depth could be attributed to variations in carbonate production most biological attributes varied similarly with depth and hence time on both long 100 yr and short 10 yr temporal scales these variations could not be explained by any taphonomic process sediment reworking and burial or sedimentation rate despite a vigorous taphonomic milieu to obtain the shell content of copano bay sediments requires the preservation of nearly all carbonate produced preservation of a large fraction of the shell carbonate added requires the preservation of most of the relatively large shelled biota large species and adults which retain important evidence of changes in the community s history in this area the results reemphasize the importance of large individuals and biomass in paleontologic reconstruction and suggest that changes in community productivity which in paleontologic usage must be carbonate productivity are preserved in the fossil record

constraints on the structure and tectonic development of the early mesozoic south georgia rift southeastern united states seismic reflection data processing and interpretation seismic reflection data collected by the consortium for continental reflection profiling cocorp over the buried triassic jurassic south georgia rift reprocessed in order to improve the upper 5 s of reflection time provide a window into the regional deep structure of the rift and delineate variations in basin structure and depositional styles located at the southernmost extent of the north american east coast early mesozoic rift system the structure of the south georgia rift basin has previously been known only from scattered drilling and potential field interpretation leaving the internal and boundary as well as the deep structure unknown the reflection data reveal that the south georgia basin consists of a broad complex terrane of isolated synrift grabens with intervening structural highs in contrast with earlier concepts of a single narrow trough or an amorphous sag basin two main subbasin depocenters are observed in west and west central georgia the largest of which is more than 100 km in width and the deepest at least 7 km at its inferred base analysis of the reflection data indicates that the distinct structural and seismic stratigraphic character of the reflection packages is analogous to other better studied extensional provinces e g north american basin and range the reprocessing of the reflection data provides a view of contrasting styles of faulting and subsidence within the rift the structural style of the subbasins varies drastically along strike changing from 1 a narrow asymmetric graben bordered by a planar and more deeply penetrating normal fault to 2 a broad and roughly symmetric graben bordered by a shallower dipping possibly listric fault along which extension may have been accommodated at depth by a low angle detachment or by a more complex system of high angle faults only limited crustal extension is suggested by the narrower graben and planar fault 1 whereas much greater extension would be expected from the wider graben bordered by a listric low angle fault 2 such a dual style of basin formation is predicted by segmented plate models of variable extension along continental margins and intracratonic rifts and may imply a significant change in crustal rheology along the developing rift integration of the interpretation of the regional structure of the south georgia basin with that of the deeper crustal structure as inferred from the 16 s reflection time cocorp data indicates that the synrift basins developed mainly over the upper plate of the alleghanian late paleozoic suture between north america and west africa the major subbasin border faults dip northward in an antithetic relation to the predominantly northward vergence of the suture zone the border faults thus do not appear to have reactivated antecedant structure although the development of the south georgia rift appears to have approximately coincided with early mesozoic thinning of the crust in the southeastern united states no horizontally layered or laminated lower crust is observed as has been associated with other more fully rifted terranes e g north american basin and range continental shelf around british isles however the moho discontinuity appears to have formed as a dynamic feature superimposed on late paleozoic compressional structure probably associated with early mesozoic rifting these findings thus support the interpretation that the south georgia rift developed as a major early mesozoic depocenter and possible incipient spreading center associated with the initial formation of the central atlantic ocean

sources of tsunami and tsunamigenic earthquakes in subduction zones we classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface typical interplate events earthquakes at the outer rise within the subducting slab or overlying crust intraplate events and tsunami earthquakes that generate considerably larger tsunamis than expected from seismic waves the depth range of a typical interplate earthquake source is 10 40km controlled by temperature and other geological parameters the slip distribution varies both with depth and along strike recent examples show very different temporal change of slip distribution in the aleutians and the japan trench the tsunamigenic coseismic slip of the 1957 aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long this asperity ruptured again in the 1986 andreanof islands and 1996 delarof islands earthquakes by contrast the source of the 1994 sanriku oki earthquake corresponds to the low slip region of the previous interplate event the 1968 tokachi oki earthquake tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes tsunami hazard assessments must include such events similarity in macroseismic data from two southern kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights most moment release of tsunami earthquakes occurs in a narrow region near the trench and the concentrated slip is responsible for the large tsunami numerical modeling of the 1996 peru earthquake confirms this model which has been proposed for other tsunami earthquakes including 1896 sanriku 1946 aleutian and 1992 nicaragua

taphonomic processes affecting late quaternary molluscs along the coastal area of buenos aires province argentina southwestern atlantic molluscan concentrations were abundantly deposited along the coastal area of buenos aires province argentina southwestern atlantic by different sea level episodes during the late quaternary most of them represent holocene ridges highly energetic depositional environments subparallel to the modern shore where the main associations preserved are parautochthonous and others mainly autochthonous associations occurring in tidal flats seaward of the ridges or coastal lagoon deposits accumulated behind them low energetic analysis of three physical taphonomic signatures disarticulation fragmentation and abrasion of the molluscan skeletons of four bivalve species tagelus plebeius mactra isabelleana pitar rostratus brachidontes rodriguezi which exhibit different shell shape and thickness conditions habitats and life habits and represent the most widely distributed and palaeoecologically significant taxa along the area of study support the distinction of both palaeoenvironments characterized by low and high taphonomic grades respectively the taphonomic alteration breakage and abrasion of most shells seems to have been a direct consequence of the amount of reworking of the material especially during periods of exposure along an extensive beach zone more than a consequence of transport from a distant original habitat abrasion signatures are the only probably correlated with after death time span shells showing a near modern aspect and original luster being most probably younger than those with chalky or polished appearance the infaunal species analysed typically living on clays and silts or fine sandy bottoms of the infralittoral or intertidal t plebeius m isabelleana are better preserved less fragmented and abraded than shells of the epibyssate b rodriguezi which lives on hard bottoms of the intertidal and supralittoral areas more exposed to highly unstable conditions with longer episodes of exposure although these nearshore concentrations may represent a time averaging of 1000 yr b p taking into account the ecological requirements of the most constant and dominant taxa together with the taphonomic analysis of shells from parautochthonous and autochthonous associations they still represent valid indicators of past marginal marine conditions in the area but are only reliable to establish long term palaeoenvironmental variations

coseismic slip distribution of the 2011 off the pacific coast of tohoku earthquake m9 0 refined by means of seafloor geodetic data 1 on 11 march 2011 the devastating m9 0 tohoku earthquake occurred on the interface of the subducting pacific plate and was followed by a huge tsunami that killed about 20 000 people several geophysical studies have already suggested that the very shallow portion of the plate interface might have played an important role in producing such a large earthquake and tsunami however the sparsity of seafloor observations leads to insufficient spatial resolution of the fault slip on such a shallow plate interface for this reason the location and degree of the slip has not yet been estimated accurately enough to assess future seismic risks thus we estimated the coseismic slip distribution based on terrestrial gps observations and all available seafloor geodetic data that significantly improve the spatial resolution at the shallow portion of the plate interface the results reveal that an extremely large greater than 50 m slip occurred in a small about 40 km in width and 120 km in length area near the japan trench and generated the huge tsunami the estimated slip distribution and a comparison of it with the coupling coefficient distribution deduced from the analysis of the small repeating earthquakes suggest that the 2011 tohoku earthquake released strain energy that had accumulated over the past 1000 years probably since the jogan earthquake in 869 the accurate assessments of seismic risks on very shallow plate interfaces in subduction zones throughout the world can be obtained by improving the quality and quantity of seafloor geodetic observations

late triassic and early jurassic palaeogeography of the world abstract a new version of global and regional palaeogeographic maps is presented for two time intervals these maps depict the plate tectonic configuration palaeoenvironment and lithofacies during the late triassic carnian norian and early jurassic hettangian toarcian time the individual maps illustrate the general conditions present during the maximum marine transgressions of higher frequency cyclicity within the absaroka sequence of sloss during triassic time pangaea began to stretch initiating the rifting and future break up of the supercontinent the continued northward drift of the cimmerian continent corresponded with the progressive closure and consumption of palaeotethys oceanic crust and the opening of the neotethys ocean the most significant late triassic convergent event was the indosinian orogeny occurring as a result of the consolidation of south china and north china blocks also indochina and indonesia were sutured to south china at the same time the qiangtang block approached the eurasian margin the consolidation of the north chinese and amurian blocks left open a large embayment of panthalassa between amuria and laurasia the so called mongol okhotsk ocean active subduction existed along the margin of this ocean dipping cratonwards towards east siberia the last collisional events of the uralian orogeny took place during the triassic and early jurassic time the conclusion of the uralian orogeny was accompanied by uplift of the adjacent areas of eastern europe and western siberia during the early jurassic the palaeotethys ocean was finally closed and the cimmerian continent collided with asia causing the cimmerian orogeny the time around the triassic jurassic boundary marked an important biotic extinction event plate tectonic activity caused palaeogeographic and palaeoclimatic change which may have contributed to the mass extinction from the plate tectonic and palaeogeographic point of view the following events could have influenced the extinction 1 the closure of palaeotethys and assembly of the asian part of pangaea 2 the break up of pangaea in the future central atlantic area and transition from rifting to drifting and 3 the very extensive basaltic volcanism of the central atlantic magmatic province

lithospheric sources of north florida usa tholeiites and implications for the origin of the suwannee terrane abstract sr nd and pb isotopic data for mesozoic tholeiites from six drillholes in northern florida suwannee terrane indicate derivation of the magmas from a source that was slightly incompatible element enriched with respect to bulk earth probably continental lithospheric mantle variably contaminated by more enriched crust initial isotopic compositions of the florida tholeiites show a considerable range 87 sr 86 sr 0 7059 to 0 7097 end 4 6 to 0 1 206 pb 204 pb 18 30 to 18 65 207 pb 204 pb 15 57 to 15 66 and 208 pb 204 pb 38 22 to 38 35 these compositions are distinctly different from those of mesozoic alkali basalts from the subsurface of southwestern florida heatherington a l mueller p a 1991 geochemical evidence for triassic rifting in southwestern florida tectonophysics 188 291 302 it is suggested that the north florida tholeiites were derived from within the continental lithosphere in contrast with mesozoic volcanic rocks from southwestern florida which have been previously suggested to be related to a mantle plume sm nd isotope systematics suggest derivation of the tholeiites from a 1 ga source a similar age to that proposed by pegram pegram w j 1990 development of continental lithospheric mantle as reflected in the chemistry of the mesozoic appalachian tholeiites usa earth and planetary science letters 97 316 331 for the source of tholeiitic basalts within the carolina terrane this supports proterozoic tectonic models which suggest that both the suwannee and carolina terranes had affinities with south american portions of gondwana containing 1 ga crust specifically the sunsas rondonian province nance murphy 1996 mueller p a kozuch m heatherington a l wooden j l offield t w koeppen r p klein t l nutman a p 1996 evidence for mesoproterozoic basement in the carolina terrane and speculations on its origin in nance r d thompson m d eds avalonian and related peri gondwanan terranes of the circum north atlantic gsa special paper 304 pp 207 218

the carnian pluvial event in western europe new data from iberia and correlation with the western neotethys and eastern north america nw africa regions abstract at the beginning of the late triassic a period of increased precipitation occurred over large tracts of central pangea the carnian pluvial event it was a short lived event 1 ma coeval with a marked sea level fall and active rifting new and classical sections of carnian age in the iberian peninsula have been studied and a new correlation scheme is proposed for the iberian domain integrating field data and subsurface electric log data this is the first time that the portuguese and spanish late triassic carnian sediments are correlated the carnian pluvial event is identified in the iberian peninsula as a continental clastic wedge manuel fm and coeval sediments in between marine evaporite formations during a marked sea level drop event its age is considered to be julian early carnian by palynological and paleogeographic considerations a new correlation of the late triassic sediments of the iberian peninsula with coeval sediments of western europe nw africa and eastern north america is proposed and a new paleogeographical map of central eastern pangea for this period is presented in order to demonstrate that the geographical extent of the siliciclastic sediments related to the carnian pluvial event was much larger than accepted before

outer rise earthquakes and seismic coupling outer rise earthquakes can be separated into two basic categories based on focal mechanism compressional and tensional events reflecting compressional and tensional outer rise stress regimes we propose a model that connects the stress regime in the outer rise to the behavior of large subduction earthquakes and explains the focal mechanisms and both the temporal and spatial occurrence of outer rise earthquakes we find that compressional outer rise events tend to occur oceanward of seismic gaps in seismically coupled subduction zones or equivalently just prior to large subduction zone earthquakes the compressional stress regime is probably caused by slip in adjacent subduction zone segments there are two modes of occurrence for tensional outer rise earthquakes depending on whether they occur in coupled or uncoupled subduction zones in coupled subduction zones tensional events tend to occur after large subduction events as tensional stress from slab pull is temporarily transmitted to the outer rise in subduction zones that are inherently uncoupled tensional events can occur at any time therefore in uncoupled subduction zones the outer rise is always in tension and only tensional outer rise events occur there while in coupled subduction zones the outer rise is in a compressional state prior to large subduction earthquakes and a tensional state afterwards due to the large displacements

shallow dynamic overshoot and energetic deep rupture in the 2011 mw 9 0 tohoku oki earthquake strong spatial variation of rupture characteristics in the moment magnitude mw 9 0 tohoku oki megathrust earthquake controlled both the strength of shaking and the size of the tsunami that followed finite source imaging reveals that the rupture consisted of a small initial phase deep rupture for up to 40 seconds extensive shallow rupture at 60 to 70 seconds and continuing deep rupture lasting more than 100 seconds a combination of a shallow dipping fault and a compliant hanging wall may have enabled large shallow slip near the trench normal faulting aftershocks in the area of high slip suggest dynamic overshoot on the fault despite prodigious total slip shallower parts of the rupture weakly radiated at high frequencies whereas deeper parts of the rupture radiated strongly at high frequencies

pangea s complex breakup a new rapidly changing stress field model over the years numerous proposals have been presented describing the relationship between the central atlantic magmatic province camp and the break up of pangea recent proposals have focused on refuting the original proposal of a mantle plume origin for the large igneous province and attendant dikes and instead propose an upper mantle or passive cause for the break up and camp in this paper we examine the fundamental assumption that the multiple dike orientations associated with the break up are geographically or temporally delimited new mapping and compilations of older local maps demonstrate that multiple dike orientations ranging from nw to ne trending were intruded throughout the western carolinas and are not geographically limited further new plagioclase 40 ar 39 ar dates indicate that all three orientations observed were emplaced within a 2 ma period c a 199 197 ma based on this information we propose that a rapidly rotating stress field must be responsible for the emplacement of multiple dike orientations in the carolinas and the break up of pangea and go on to propose several new tectonic models to account for this rotating stress field d 2005 elsevier b v all rights reserved

thermal and mechanical aspects of magma emplacement in giant dike swarms we consider the thermal history and dynamics of magma emplacement in giant feeder dikes associated with continental flood basalts for driving pressure gradients inferred for giant dike swarms thicknesses of 10 m would enable dikes to transport magma laterally over the distances observed in the field up to thousands of kilometers without suffering thermal lock up using time dependent numerical solutions for the thermal evolution of a dike channel under laminar and turbulent flow conditions in the presence of phase transitions we investigate the possibility that the observed dike thicknesses of the order of 100 m result from thermal erosion of the country rocks during dike emplacement this implies that the observed range of dike widths in giant dike swarms may reflect variations in the source volume and not the excess magma pressure it is found that the total volume of intruded magma required to produce an order of magnitude increase in dike width via wall rock melting broadly agrees with the estimated volumes of individual flows in continental flood basalts the presence of chilled margins and apparently low crustal contamination characteristics of some giant dikes may be consistent with turbulent magma flow and extensive melt back during dike emplacement in this case measurements of the anisotropy of magnetic susceptibility most likely indicate magma flow directions during the final stages of dike intrusion shear stresses generated at the dike wall when the dike starts to freeze strongly decrease with increasing dike width which implies that thicker dikes may have less tendency to produce consistent fabric alignment our results suggest that if the dike was propagating downslope off a plume related topographic swell the mechanism responsible for flow termination could possibly have been related to underpressurization and collapse implosion of the shallow magma plumbing system feeding the intrusion radial dikes that erupted at the periphery of the topographic uplift might have increased rather than decreased extensional stresses in the crust within the topographic uplift upon their solidification

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

variation of coastal dynamics during the last 7000 years recorded in beach ridge plains associated with river mouths example from the central brazilian coast abstract the reconstruction of relative sea level changes during the last 7000 years has shown that the central brazilian coast was been subjected to submergence until approximately 5100 yr b p however the emergence after 5100 yr b p was interrupted by two important fluctuations with an amplitude of approximately 2 3 m and a duration of 200 300 yr this evolutionary history which is quite different from that of several other regions in the world played an essential role in the development of the central brazilian coastal plains whether or not they are associated with important river mouths the periods of submergence characterized by erosional phases introduced noticeable changes in the geometry of the coastal deposits the periods of emergence gave rise to sandy terraces covered by beach ridges whose orientation is determined by the longshore drift elsewhere on a low sandy coast the direction of longshore sand transport depends on the orientation of the swell in a coast subjected to various swell patterns the efficient swells are defined as those which determine the resulting longshore transport direction such swells are not necessarily the most prevalent for instance along the central brazilian coast the southern sector waves in spite of their infrequent occurrences are much more powerful than the northern sector waves and therefore generate a longshore sand transport from south to north when fossil beach ridges are present as is the case on the central brazilian coast their geometry reflects the past directions of longshore sand transport this makes it possible to determine the provenance of past efficient swells and to establish the past wind patterns a detailed study of the beach ridge geometry of some sections of the central brazilian coast showed a sequence of reversals of the longshore drift during the last 5100 yr with a duration ranging from 10 to 100 yr these reversals represent changes in the direction of effective waves which determine the longshore transportation of sediments and consequently indicate changes in the wind pattern

factors controlling normal fault offset in an ideal brittle layer we study the physical processes controlling the development and evolution of normal faults by analyzing numerical experiments of extension of an ideal two dimensional elastic plastic brittle layer floating on an inviscid fluid the yield stress of the layer is the sum of the layer cohesion and its frictional stress faults are initiated by a small plastic flaw in the layer we get finite fault offset when we make fault cohesion decrease with strain even in this highly idealized system we vary six physical parameters the initial cohesion of the layer the thickness of the layer the rate of cohesion reduction with plastic strain the friction coefficient the flaw size and the fault width we obtain two main types of faulting behavior 1 multiple major faults with small offset and 2 single major fault that can develop very large offset we show that only two parameters control these different types of faulting patterns 1 the brittle layer thickness for a given cohesion and 2 the rate of cohesion reduction with strain for a large brittle layer thickness 22 km with 44 mpa of cohesion extension always leads to multiple faults distributed over the width of the layer for a smaller brittle layer thickness the fault pattern is dependent on the rate of fault weakening a very slow rate of weakening leads to a very large offset fault and a fast rate of weakening leads to an asymmetric graben and eventually to a very large offset fault when the offset is very large the model produces major features of the pattern of topography and faulting seen in some metamorphic core complexes

genesis and environmental significance of upper cretaceous shell concentrations from the cauvery basin southern india abstract upper cretaceous shallow marine calcarenites of the upper campanian maastrichtian kallankurichchi formation from the cauvery basin of southeastern india are rich in shell concentrations dominated by large bivalves with minor admixtures of brachiopods bryozoans and larger foraminifera rarely these concentrations occur as lenses or pavements but usually as beds 10 to 200 cm thick depending on the dominant skeletal elements five types of shell concentrations can be distinguished isognomon pycnodonte ceratostreon inoceramid platyceramus cataceramus and polyspecific concentrations these shell concentrations occur in various preservational states which reflect the gradual mixing and mechanical destruction of former communities analysis of the taphonomic signatures of their components and of the biofabric suggests that storm induced waves and currents reduced sediment input the settling behaviour of taxa and a high rate of production of biogenic hardparts were responsible in varying degrees for the formation of the various shell concentrations accordingly they can be classified as storm wave proximal tempestite and primary biogenic concentrations the shell concentrations formed below fair weather wave base in shallow subtropical low stress environments

pleistocene and holocene interglacial molluscan assemblages from patagonian and bonaerensian littoral argentina sw atlantic palaeobiodiversity and palaeobiogeography abstract our analysis of palaeobiodiversity and palaeobiogeography of quaternary molluscan assemblages provide information on the last coastal scenarios in the sw atlantic we present patterns of species richness areas of endemism and between areas relationships based on records 721 taxa from argentina and northwards in south america surinam brazil uruguay including pleistocene holocene and modern sites 48 localities in argentina our data since the mid late pleistocene mis11 1 ca 400 ka b p to present suggest environmental changes linked to sea level and climatic fluctuations large enough to alter benthic associations compositional geographical variations including a few extinctions the southwards decreasing diversity trend for modern bivalves mostly infaunal taxa and gastropods mostly epifaunal taxa from surinam to tierra del fuego responds mainly to latitudinal sst salinity barriers hydrodynamic forces rivers and shallow currents minima peaks fit with local estuarine and marginal marine sites influenced by the rio de la plata plume highest peaks with fully marine warmer waters influenced by the brazilian current bonaerensian sector a similar trend applies for the holocene pleistocene maxima peaks in patagonia belong to mis11 and mis5 highstands warmer than today multivariate cluster analysis ca minimum spanning trees superimposed to principal coordinates analysis pco and cladistic parsimony analysis of endemicity pae methods showed congruent relationships between localities discriminating marginal from open marine environments but pco combined with pae was most advantageous for our area and dataset 382 gastropods 339 bivalves 29 pleistocene 28 holocene and 35 modern sites allowing objective quick comparisons shifts of areas of endemism during the holocene and pleistocene relate with sst and salinity changes a more homogeneous mid holocene ca 7 5 ka b p scenario of regionally increased salinity and higher ca 2 c sst defined the bonaerensian and northern patagonian sectors facilitating southwards shifts of bivalve and gastropod taxa for patagonia a colder pleistocene scenario south of sao is documented by tegula atra exclusive pleistocene cold water species extinct in the swa probably a signal of post lgm palaeoceanographical changes in the mar argentino

rupture process of the 2004 sumatra andaman earthquake the 26 december 2004 sumatra andaman earthquake initiated slowly with small slip and a slow rupture speed for the first 40 to 60 seconds then the rupture expanded at a speed of about 2 5 kilometers per second toward the north northwest extending 1200 to 1300 kilometers along the andaman trough peak displacements reached 15 meters along a 600 kilometer segment of the plate boundary offshore of northwestern sumatra and the southern nicobar islands slip was less in the northern 400 to 500 kilometers of the aftershock zone and at least some slip in that region may have occurred on a time scale beyond the seismic band

principal horizontal stress orientations prior to the 2011 mw 9 0 tohoku oki japan earthquake in its source area 1 we determined principal horizontal stress orientations as of 1999 at sites in the source area of the 2011 mw 9 0 tohoku oki earthquake on the deep sea terrace of the japan trench the maximum principal horizontal stress orientation at site 1151 of ocean drilling program leg 186 located in an aseismic zone was east southeast parallel with the plate convergence direction and the stress orientation at site 1150 of leg 186 in a seismically active zone was south southeast suggestive of a local stress feature due to the influence of seismic activity aftershocks with normal faulting mechanisms observed in the hanging wall of the plate interface that ruptured during the tohoku oki earthquake indicate a normal faulting postseismic stress regime which clearly differs from the stress state prior to the earthquake consequently the stress state may have changed from a reverse to a normal faulting stress regime during the tohoku oki earthquake sequence

tsunami earthquake generation by the release of gravitational potential energy earthquakes that generate large tsunamis share a number of unusual features they commonly have long source time functions involve large displacements of 10 m or more of the prisms of poorly consolidated sediment that form the accretionary wedge and have many aftershocks with normal faulting mechanisms on the landward side of the trench these features are not easily understood if the only source of the energy involved is the stored elastic strain the observations especially those from the tohoku japan 2011 earthquake instead suggest that the observed behaviour results from the release of gravitational potential energy as well as elastic strain a simple model of this process can account for these and other observations

opening of the sea of japan back arc basin by asthenospheric injection abstract the position of a volcanic front on an arc is governed by dehydration reactions of amphibole chlorite in down dragged hydrated peridotite at the base of the mantle wedge as these reactions are essentially pressure dependent ca 3 5 gpa the paleo position of a volcanic front offers an estimate for the depth of the subducted oceanic lithosphere beneath it this paper evaluates volcanism in the northeast japan arc since the oligocene and discusses the mass transportation in the mantle wedge during the episode of back arc extension in the sea of japan with reference to the migration of the volcanic front the volcanic front during the oligocene ca 30 ma was located along the western coast of the present northeast japan are and migrated about 200 km trenchwards at 23 ma before the opening of the sea of japan back arc basin it follows that the angle of subduction became steeper during the period of 30 23 ma triggered by the injection of the asthenosphere into the mantle wedge the back arc extension of the sea of japan between 21 and 14 ma is a manifestation of the dynamic process of the lithosphere due to the asthenospheric injection the temperature of the injecting asthenospheric mantle was higher than that of the surrounding upper mantle or lithosphere this caused the characteristic volcanism at 23 22 ma 1 high magnesian andesite volcanism took place in the near trench region 2 across arc compositional variation was not observed 3 the volume of volcanic materials increases towards the back arc side of volcanic arc the volcanic front has migrated over a short distance inwards less than 100 km since 15 ma possible as a result of cooling in the mantle wedge because both amphibole and chlorite break down at slightly higher pressures at lower temperatures the upper mantle materials which injected into the mantle wedge and caused back arc spreading are identical to those of the mid oceanic ridge basalt source in sr nd isotopic compositions because volcanic rocks in the back arc region of the northeast japan arc show secular variation in isotopic compositions from enriched to depleted isotopic characteristics during the last 30 ma

southern oscillation signal in south american palaeoclimatic data of the last 7000 years abstract during strong el nino events rainfall anomalies and changes in wind patterns are observed in different regions of south america along the central brazilian coast during the 1983 el nino year the frontal systems were blocked to the south provoking a reversal of the longshore sand transport long duration reversals of longshore transport were also recorded in holocene beach ridge terraces from the rio doce coastal plain this led to the formulation of a model relating these reversals of longshore transport to el nino like conditions el nino like conditions are past average climate situations that generate the same perturbations as the strong el nino events observed during the last decade they are likely to correspond to the long duration low phase of the southern oscillation to confirm this hypothesis we compared the holocene beach ridge record with other palaeoenvironmental records from regions where strong el nino events would have a substancial signal as well 1 water level fluctuations of lake titicaca 2 a pollen and sediment record in an eastern amazonian lake 3 changes of the rio xingu discharge in eastern amazonia and 4 variations of sand supply at the rio piura and rio chira outlets in the sechura desert the occurrences of el nino like conditions were numerous before 3900 3600 yr b p absent between 39003600 and 2800 2500 yr b p and infrequent after 2800 2500 yr b p

can molluscan assemblages give insights into holocene environmental changes other than sea level rise a case study from a macrotidal bay marennes oleron france during the late holocene the rate of sea level rise decreased and climate changes hydrodynamic processes or anthropogenic impacts became predominant parameters governing the sedimentary infill of estuarine environments the aim of this study is to describe the response of past benthic mollusc communities to these forcing factors mollusc skeletal remains were sampled from three 8000 5500 and 2600 year long sedimentary records in the marennes oleron bay atlantic coast france where environmental changes have previously been identified thanks to a combination of very high resolution seismic profiles and sedimentological data molluscan assemblages successfully record environmental changes even at the smallest temporal scale they provide relevant information regarding bathymetry salinity hydrodynamics oxygen content and sediment supply in the 8000 year long record the transition from an upper to a lower intertidal mudflat community provides evidence of a transgressive phase that occurred between 8000 and 7600 years bp in the 5500 year long record an oyster bank developed on the slopes of a tidal channel has been recovered the composition of the fossil community and the careful examination of skeletal remains suggest that the channel was hydraulically inefficient in response to the sediment infilling of its drainage area in the 2600 year long record a soft bottom shallow water subtidal mollusc community similar to that found nowadays in the marennes oleron bay has been recovered from both lower mixed sand mud and upper muddy units separated by a major regional unconformity dated to about 1000 years bp this sharp transition is recorded by a few rare species and was responsible for the local extinction of the bivalve lepton squamosum however the abundance of the most common species and the quantity of fragmented shells remain stable which may indicate that mud drape deposition was related to an increase in mud supply rather than a decrease in water energy the level of detail of the palaeoenvironmental reconstructions varies depending on the ecological requirements of the species stenotypic indicator species provide accurate quantitative information whereas eurytypic species add undesired variability to the assemblages since fossil molluscs are easy to collect and to identify and provide long term smoothed records of environment variations this study suggests that they can be used prior to any other biological proxy as a first step into reconstructions of holocene coastal environments

initiation of salt diapirs with frictional overburdens numerical experiments we have studied the initiation of salt diapirs with frictional overburdens under lateral compression and extension the overburden is assumed to be elasto plastic with mohr coulomb yield criterion and non associated flow rule plastic flow localizes into narrow shear bands which may represent faults the geometry and the evolution of these shear zones are simulated for various rheological parameters the dilational and frictional properties of frictional overburdens are found to be important controls on the types of shear zones formed in lateral compression and extension the friction and dilation angles influence the angle of shear and the width dip spacing and development history of conjugate zones of shear plus dilation fault zones that define wedges and grabens

refining gondwana and pangea palaeogeography estimates of phanerozoic non dipole octupole fields summary reliable phanerozoic paleopoles have been selected from the stable parts of the gondwana continents and upon appropriate reconstruction have been combined in an apparent polar wander apw path which can be compared with a previously compiled path for laurussia this comparison once again confirms that pangea a reconstructions for late palaeozoic and early mesozoic times cannot be reconciled with the available paleomagnetic data unless these data are corrected for latitudinal errors caused by non dipole octupole field contributions or by inclination shallowing because the discrepancies persist even when only paleopoles from igneous rocks are used inclination shallowing cannot be the sole cause of the problem there is an apparent decrease in the percentage of octupole field contributions needed as a function of time for mesozoic and younger time gondwana laurussia comparisons require on average lower ratios of octupole dipole fields than for palaeozoic time however the gondwana paleopoles for the palaeozoic include a much greater proportion of results derived from sedimentary rocks than do those for the mesozoic so that this apparently diminishing octupole field contribution may be an artefact we have also examined whether the clustering of coeval gondwana poles improves with optimal g3 contributions but found that while there are improvements they are not systematic and not statistically significant a combined apw path has been constructed for pangea for times since the mid carboniferous which accounts for octupole fields or equivalently inclination shallowing we argue that this global path is an improvement over previous constructions as it represents a self consistent plate tectonic model and does not violate widely accepted pangea a reconstructions

why meter wide dikes at oceanic spreading centers abstract numerical models show that maximum dike width at oceanic spreading centers should scale with axial lithospheric thickness if the pre diking horizontal stress is close to the andersonian normal faulting stress and the stress is fully released in one dike intrusion dikes at slow spreading ridges could be over 5 m wide and maximum dike width should decrease with increasing plate spreading rate however data from ophiolites and tectonic windows into recently active spreading ridges show that mean dike width ranges from 0 5 m to 1 5 m and does not clearly correlate with plate spreading rate dike width is reduced if either the pre diking horizontal stress difference is lower than the faulting stress or the stress is not fully released by a dike partial stress release during a dike intrusion is the more plausible explanation and is also consistent with the fact that dikes intrude in episodes at iceland and afar partial stress release can result from limited magma supply when a crustal magma chamber acts as a closed source during dike intrusions limited magma supply sets the upper limit on the width of dikes and multiple dike intrusions in an episode may be required to fully release the axial lithospheric tectonic stress the observation of dikes that are wider than a few meters such as the recent event in afar indicates that large tectonic stress and large magma supply sometimes exist

megathrust friction determined from mechanical analysis of the forearc in the maule earthquake area the seismogenic potential of a given fault depends essentially on its frictional properties and on the mechanical properties of the medium determining the spatio temporal variations of frictional properties is therefore a key issue in seismotectonics this study aims to characterize the friction on the south america megathrust in the 2010 mw 8 8 maule earthquake area from mechanical analysis of the forearc structure and morphology based on the critical taper theory we first show that the rupture area of the maule earthquake also shown to be locked in the interseismic period coincides with the stable part of the wedge in the surrounding area the wedge is critical a finding consistent with various evidence for active deformation there this is in particular true for the arauco peninsula area which seems to have arrested the maule earthquake s rupture to the south this observation lends support to the view that the seismic rupture is inhibited when propagating beneath a critical area the geometry of the critical portion of the wedge suggests a standard internal friction int 0 7 0 13 and a hydrostatic pore pressure within the wedge the effective friction beneath the critical outer wedge is estimated to be eff b 0 27 0 03 this could be related to intrinsically low friction minerals clay or high pore pressure along the megathrust we next use the limit analysis approach to constrain the variation of the effective friction along the megathrust based on the location and geometry of internal faulting within the forearc a low effective friction is found within the rupture area eff b 0 14 to explain the reactivation of thrust fault such as the santa maria updip of the coseismic rupture or the activation of normal splay faults downdip of the rupture area the low effective friction found there could reflect strong dynamic weakening

slab dip surface tectonics how and when do they change following an acceleration slow down of the overriding plate abstract we present analogue models simulating the subduction of an oceanic lithosphere beneath an overriding plate advancing at variable rates the convergence velocity is imposed by lateral boundary conditions in this experimental set we analyze the geometry of the slab and the deformation of the overriding plate experiments confirm the strong correlation between the absolute velocity of the overriding plate on the one hand the geometry of the subducting plate and the deformation of the overriding plate on the other hand following an instantaneous change in kinematic boundary conditions the subduction system progressively shifts to a new steady state regime models suggest that the adjustment time necessary to shift from the previous to the new equilibrium is independent of the imposed upper plate velocity transient stage lasts 12 5 6 m y for the shallow slab dip 100 150 km depth 29 2 10 m y for the deeper slab dip 300 350 km depth and 2 2 2 m y for the upper plate deformation the analysis of present day subduction zones and their evolution through the last 20 m y suggests an adjustment time of 15 m y for shallow slab dip and 20 m y for deep slab dip in nature since only few subduction zones have shown a constant upper plate velocity over the last 15 m y it suggests that most of them are in a transient stage at present day

relationship between outer forearc subsidence and plate boundary kinematics along the northeast japan convergent margin 1 tectonic erosion along convergent plate boundaries whereby removal of upper plate material along the subduction zone interface drives kilometer scale outer forearc subsidence has been purported to explain the evolution of nearly half the world s subduction margins including part of the history of northeast japan here we evaluate the role of plate boundary dynamics in driving forearc subsidence in northeastern japan a synthesis of newly updated analyses of outer forearc subsidence the timing and kinematics of upper plate deformation and the history of plate convergence along the japan trench demonstrate that the onset of rapid fore arc tectonic subsidence is contemporaneous with upper plate extension during the opening of the sea of japan and with an acceleration in convergence rate at the trench in plio quaternary time relative uplift of the outer forearc is contemporaneous with contraction across the arc and a decrease in plate convergence rate the coincidence of these changes across the forearc arc backarc system appears to require an explanation at the scale of the entire plate boundary similar observations along other western pacific margins suggest that correlations between forearc subsidence and major changes in plate kinematics are the rule rather than the exception we suggest that a significant component of forearc subsidence at the northeast japan margin is not the consequence of basal tectonic erosion but instead reflects dynamic changes in plate boundary geometry driven by temporal variations in plate kinematics if correct this model requires a reconsideration of the mass balance and crustal recycling of continental crust at nonaccretionary margins

the 2011 tohoku oki earthquake displacement reaching the trench axis we detected and measured coseismic displacement caused by the 11 march 2011 tohoku oki earthquake moment magnitude mw 9 0 by using multibeam bathymetric surveys the difference between bathymetric data acquired before and after the earthquake revealed that the displacement extended out to the axis of the japan trench suggesting that the fault rupture reached the trench axis the sea floor on the outermost landward area moved about 50 meters horizontally east southeast and 10 meters upward the large horizontal displacement lifted the sea floor by up to 16 meters on the landward slope in addition to the vertical displacement

study of plate deformation and stress in subduction processes using two dimensional numerical models two dimensional finite element modeling is used to model subduction of an oceanic lithospheric plate beneath continental lithosphere the subduction process is initiated along a preexisting inclined fault and continues until reaching 400 km of total convergence the lithosphere is assumed to be underlain by an in viscid asthenosphere different rheological laws have been considered for the lithosphere including elasticity and elastoplasticity the modeling shows that both the stress system in the plates and the surface topography are strongly dependent on two main parameters the density contrast between lithosphere and asthenosphere l a and the coefficient of friction along the subduction plane varying these two parameters allows explanation of the main characteristics of real subduction zones and results in two major regimes manifested by extension or compression in the arc back arc system extension and back arc rifting corresponds to a positive density contrast and a low coefficient of friction while negative values and or high friction leads to a compressional regime the coexistence of trench arc compression and back arc tension is only possible for a coefficient of friction lower than 0 1 the results of the numerical experiments agree with those of experimental modeling conducted under similar physical assumptions

time averaging taphonomy and their impact on paleocommunity reconstruction death assemblages in texas bays the death assemblages in upper copano bay and upper laguna madre texas were compared with the living communities as sampled over a two year period in order to determine how taphonomy and time averaging affect paleocommunity reconstruction the living and death assemblages were compared using taxonomic composition numerical abundance diversity biomass and trophic and habitat proportions taxonomic composition particularly adult taxonomic composition and the biomass of the death assemblage more accurately reflect the characteristics of the living community than do numerical abundance and other community attributes based upon numerical abundance such as diversity taxonomic composition and biomass are less modified by taphonomy and time averaging because they are temporally more persistent characteristics of the community community and paleocommunity attributes are not equivalent because the paleocommunity is described using time averaged data ecological theory must be supplanted by paleoecological theory that takes into account this temporal component

the 2011 magnitude 9 0 tohoku oki earthquake mosaicking the megathrust from seconds to centuries geophysical observations from the 2011 moment magnitude m w 9 0 tohoku oki japan earthquake allow exploration of a rare large event along a subduction megathrust models for this event indicate that the distribution of coseismic fault slip exceeded 50 meters in places sources of high frequency seismic waves delineate the edges of the deepest portions of coseismic slip and do not simply correlate with the locations of peak slip relative to the m w 8 8 2010 maule chile earthquake the tohoku oki earthquake was deficient in high frequency seismic radiation a difference that we attribute to its relatively shallow depth estimates of total fault slip and surface secular strain accumulation on millennial time scales suggest the need to consider the potential for a future large earthquake just south of this event

melt generation at volcanic continental margins no need for a mantle plume melt generation in a rifting environment is studied using a dynamic 2 d finite element model the lithosphere is extended to large realistic thinning factors assuming a mantle temperature of 1333 c the focussing of deformation results in a distribution of thinning factors along the margin at breakup time consistent with observations the timing of melt production late synrift and the amounts of melt are consistent with observations at volcanic margins the dynamical processes related to lithospheric rifting enhance the produced melt volumes sufficiently to explain the sometimes enigmatic melt volumes found at volcanic margins

taphonomic signatures in molluscan fossil assemblages from the holocene lagoon system in the northern part of the coastal plain rio grande do sul state brazil abstract molluscan shell accumulations shell beds are very common in shallow marine and estuarine environments in south america and also on the continental shelf between argentina and southern brazil the development of these shell beds is restricted to the quaternary and can provide a great opportunity for taphonomic studies which are rare in this geographic location this study investigates the influence of environmental processes on the destruction of biological remains in a subtropical lagoonal setting by describing the taphonomic signatures occurring in the mollusk shells from the tramandai lagoon northeastern rio grande do sul state brazil this lagoon system originated during the regressive phase that followed a transgressive maximum at 6 0 6 5 ka bp the taphonomic variables fragmentation margin modification surface alteration hinge fine scale alteration and corrosion degree and their damage states were recorded for 1000 shells of erodona mactroides the shells were recovered from two outcrops 8 km apart 14c ams dating were performed on the shells of e mactroides from those outcrops which yielded similar ages from 1060 to 1490 cal yrs bp the ages would indicate that the tramandai lagoon occupied a larger area and must have been subjected to very rapid progradation towards its present position several taphonomic variables such as fragmentation surface alteration and corrasion extent and oxidation presented significant differences p

geoarchaeological evidence from peru for a 5000 years b p onset of el nino for the tropical west coast of south america where el nino southern oscillation enso is most pronounced archaeological and associated paleontological deposits in northern peru revealed a major climate change at about 5000 years before the present yr b p the data implied the presence of stable warm tropical water as far south as 10 s during the early mid holocene about 8000 to 5000 yr b p these data suggest that enso did not occur for some millennia preceding 5000 yr b p when global and regional climate was warmer than today

larger tsunamis from megathrust earthquakes where slab dip is reduced a subset of megathrust earthquakes produce anomalously large tsunamis for their magnitude all of these recorded tsunami earthquakes in the past 50 years had extensional aftershocks in the upper plate these include the two largest and most destructive earthquakes of that period the 2004 sumatra andaman and the 2011 tohoku events evidence from the region of tohoku indicates that normal fault slip in the upper plate during the earthquake may have contributed to the tsunami size here we present a numerical model that shows how a reduction of the dip of a subducting slab on a timescale of millions of years can result in an extensional fault failure above a megathrust earthquake on timescales of seconds to months slab dip reduction bends the upper plate so that the shallow part fails in extension when a megathrust rupture relieves compressional stress this results in a distribution of extensional aftershocks comparable to that seen above the tohoku megathrust volcanic arc migration and uplift data for tohoku and several other tsunami earthquakes is consistent with slab dip reduction the collection of more such data might identify other areas of tsunami hazard related to slab dip reduction tsunami generation by megathrust earthquakes is enhanced by extensional faulting in the upper plate when the subducting slab shallows according to numerical modelling and observations from the sumatra andaman and tohoku earthquake tsunami events

rupture process of the 2011 tohoku oki earthquake and absolute elastic strain release 1 on 11 march 2011 the tohoku oki earthquake in eastern japan and the devastating tsunami that followed it caused severe damage and numerous deaths to clarify the rupture process of the earthquake we inverted teleseismic p wave data applying a novel formulation that takes into account the uncertainty of green s function which has been a major error source in waveform inversion the estimated seismic moment is 5 7 1022 nm mw 9 1 associated with a fault rupture 440 km long and 180 km wide along the plate interface the source process is characterized by asymmetric bilateral rupture propagation but we also found continuous slips up dip from the hypocenter which led to a large maximum slip 50 m long slip duration 90 s and a large stress drop 20 mpa the long slip duration large stress drop extensional normal faulting aftershocks in a previously compressional stress regime and low angle normal slips at approximately the depth of the plate interface suggest that the earthquake released roughly all of the accumulated elastic strain on the plate interface owing to exceptional weakening of the fault the stress accumulated on the plate interface was about 20 mpa near the trench and 0 10 mpa in the down dip source region

updip limit of the seismogenic zone beneath the accretionary prism of southwest japan an effect of diagenetic to low grade metamorphic processes and increasing effective stress off southwest japan the seaward limit of coseismic displacement or updip limit of the seismogenic zone of the 1946 mw 8 3 thrust earthquake reaches to 4 km depth and 40 km landward of the trench this limit coincides with the estimated location of the 150 c isotherm and has been linked to changes in physical properties associated with the smectite to illite clay mineral transition here we show that this limit correlates with a suite of diagenetic to low grade metamorphic processes characterized by 1 declining fluid production and decreasing fluid pressure ratio and 2 active clay carbonate and zeolite cementation and the transition to pressure solution and quartz cementation these diagenetic to low grade metamorphic changes cause the onset of velocity weakening during thrust faulting an increase in effective stress and strengthening of the hanging wall which together combine to produce recordable earthquakes

the influence of inherited structures on dike emplacement during gondwana breakup in southwestern africa abstracta kinematic analysis of cretaceous and pre cretaceous structures was carried out along the west coast of southern africa extending from the cape town area into namibia with the aim of investigating whether inherited structural discontinuities controlled gondwana breakup and the associated opening of the south atlantic in the cretaceous this was augmented by map and satellite image analyses of mainly cretaceous mafic dikes exposed from sw angola to the southern tip of africa the combined results provide consistent evidence of e w to ne sw directed extension during the cretaceous e w extension dominated from southern angola to the namibian south african border and ne sw directed extension occurred in an area extending from west central namibia to the southwestern coast of south africa these two extension directions overlap from west central namibia to southernmost namibia the kinematic analysis of pan african structures provides evidence of ene wsw directed shortening in the western saldania

earthquake supercycles as part of a spectrum of normal fault slip styles like thrust boundaries in subduction zones some normal faults show evidence of temporally transient slip patterns such as temporally clustered earthquakes or periods of elevated creep rate we create rate and state friction numerical models to assess whether typical distributions of frictional parameters can explain transient slip on normal faults we constrain our models with geometry from the wasatch fault zone where 5 10 kyr timescale earthquake clusters are recorded on fault scarps we use numerical models to test whether these earthquake clusters could be the geologic representation of supercycles which we define as long period stress cycles containing multiple seismic cycles field observations laboratory data and analogue experiments suggest that the ratio of velocity weakening to velocity strengthening material varies with the ratio of brittle to ductile material in the shear zone we therefore employ a linear transition of frictional parameters with depth to simulate bulk frictional properties of the shear zone we find that for generalized normal faults rate and state friction predicts a spectrum of slip behaviors including steady aseismic creep aseismic transients clustered earthquakes and regular seismic cycle earthquakes the dominant control on slip behavior is the degree of weakening material in the potentially seismogenic fault patch earthquake clusters occur with higher proportions of velocity weakening material which correspond to more competent less ductile shear zone assemblages we find that rate and state friction predicts a wider spectrum of slip behavior on active normal faults than has been previously recognized potentially due to geologic preservation and identification issues

evolving force balance during incipient subduction nearly half of all active subduction zones initiated during the cenozoic all subduction zones associated with active back arc extension have initiated since the eocene hinting that back arc extension may be intimately associated with an interval several tens of myr following subduction initiation that such a large proportion of subduction zones are young indicates that subduction initiation is a continuous process in which the net resisting force associated with forming a new subduction zone can be overcome during the normal evolution of plates subduction initiation is known to have occurred in a variety of tectonic settings old fracture zones transform faults and extinct spreading centers and through polarity reversal behind active subduction zones although occurring within different tectonic settings four known subduction initiation events izu bonin mariana ibm along a fracture zone tonga kermadec along an extinct subduction boundary new hebrides within a back arc and puysegur fiordland along a spreading center were typified by rapid uplift within the forearc followed by sudden subsidence other constraints corroborate the compressive nature of ibm and tonga kermadec during initiation using an explicit finite element method within a two dimensional domain we explore the evolving force balance during initiation in which elastic flexure viscous flow plastic failure and heat transport are all considered in order to tie theory with observation known tectonic settings of subduction initiation are used as initial and boundary conditions we systematically explore incipient compression of a homogeneous plate a former spreading center and a fracture zone the force balance is typified by a rapid growth in resisting force as the plate begins bending reaching a maximum value dependent on plate thickness but typically ranging from 2 to 3 1012 n m for cases that become self sustaining this is followed by a drop in stress once a shear zone extends through the plate the formation of a throughgoing fault is associated with rapid uplift on the hanging wall and subsidence on the footwall cumulative convergence not the rate of convergence is the dominant control on the force balance viscous tractions influence the force balance only if the viscosity of the asthenosphere is 1020 pa s and then only after plate failure following plate failure buoyancy of the oceanic crust leads to a linear increase with crustal thickness in the work required to initiate subduction the total work done is also influenced by the rate of lithospheric failure a self sustaining subduction zone does not form from a homogeneous plate a ridge placed under compression localizes subduction initiation but the resisting ridge push force is not nearly as large as the force required to bend the subducting plate the large initial bending resistance can be entirely eliminated in ridge models explaining the propensity for new subduction zones to form through polarity reversals a fracture zone fz placed in compression leads to subduction initiation with rapid extension of the overriding plate a fz must be underthrust by the older plate for 100 150 km before a transition from forced to self sustaining states is reached in fz models the change in force during transition is reflected by a shift from forearc uplift to subsidence subduction initiation is followed by trench retreat and back arc extension moderate resisting forces associated with modeled subduction initiation are consistent with the observed youth of pacific subduction zones the models provide an explanation for the compressive state of western pacific margins before and during subduction initiation including ibm and tonga kermadec in the eocene and the association of active back arcs with young subduction zones on the basis of our dynamic models and the relative poles of rotation between pacific and australia during the eocene we predict that the northern segment of the tonga kermadec convergent margin would have initiated earlier with a progressive southern migration of the transition between forced and self sustaining states

on the mechanism of seismic decoupling and back arc spreading at subduction zones to address the problem of the great variability of the mechanical state of subduction zones we investigate the mechanics of back arc spreading and seismic decoupling back arc spreading is assumed to be due to rifting of the upper plate and hence occurs when trench normal tension reaches a critical value seismic decoupling is assumed to occur when the normal stress at the frictional interface is decreased by an amount sufficient to cross the friction stability transition two forces are important in this problem the first is a small component of the slab pull force which remains unbalanced by the subduction resistance and exerts a vertical suction force at the trench the second is a sea anchor force exerted on the slab that resists its lateral motion assumed to occur at the upper plate velocity both forces contribute to the coupling problem only the sea anchor force is responsible for back arc spreading the unbalanced slab pull force is determined from a force balance for subduction the sea anchor force is computed as the hydrodynamic resistance to the facewise translation of an elliptical disc through a viscous fluid the model predicts three regimes seismically coupled compressional arcs with advancing upper plates seismically decoupled extensional arcs with retreating upper plates and strongly extensional arcs which also have back arc spreading this model is applied in detail to the izu bonin marianas system it shows that back arc spreading occurs when the integrated tension in the upper plate exceeds a value of about 1 1013 n m 1 and requires a residual tension of about a third that to drive the back arc spreading once rifting is completed it shows why the plate interface near guam is seismically coupled while the plate boundary everywhere farther north is decoupled when applied globally it successfully predicts the state of seismic coupling and back arc spreading in more than 80 of the world s subduction zones of the remaining half can be seen to contain additional complications not included in the model about 10 of cases remain unexplained but some of these may have incorrectly determined seismic coupling coefficients

concavity orientations of bivalve shells in estuarine and nearshore shelf sediments georgia box core x ray radiographs from estuarine and closely adjacent nearshore shelf sediments of georgia were used to study interrelationships among detrital valve orientations sediment textures and physical and biogenic sedimentary structures the cores represent preserved sediment sequences and thus should be more indicative of the rock record than shell orientations observed on substrate surfaces with little bioturbation physical processes mainly govern shell configurations horizontal concave down valves predominate in large and small scale crossbedded sands laminated sands and thin shell beds inclined concave down valves also are common together with lesser numbers of horizontal concave up valves vertical and inclined concave up valves predominate in thick shell beds shell filled burrows and thin shell beds consisting of shell ornamented diopatra tube fragments horizontal concave up valves also are common in each of these and inclined concave down valves are abundant in thick shell beds with progressively more intense bioturbation as is commonplace in estuarine and nearshore shelf sediments proportions of horizontal and inclined concave down valves decrease as those of vertical and inclined concave up valves increase the latter two becoming dominant the abundance of horizontal concave up valves increases initially but decreases between 90 and 100 percent bioturbation some of these results especially the predominance of vertical and inclined concave up valves differ from those of previous workers small sizes of dominant georgia bivalves probably account for much of the difference because of pronounced local variations large numbers of measurements also are imperative

magmatic rifting of pangaea linked to onset of south american plate motion abstract the causes of the transition from amagmatic to magmatic rifting during continental break up are not always clear and have been often linked to the break through of a plume however stress fields recorded in the crust may offer new insights into the relationship between changes in the stress field and the onset of magmatism in this paper stress fields recorded in the crust of north america are used to test possible causes of the break up of pangaea and the transition from amagmatic to magmatic rifting finite element models reveal that the most likely scenario for the break up involves the initial northwest motion of north america at 230 ma followed by a south southeast motion of south america at 200 ma with the initiation of magmatism and finally a weakened area between north and south america sometime soon after 200 ma which resulted in north america being dominated by northwest motion once again it was also determined that plate boundary structure and orientation play a large part in the recorded stress fields and must be taken into consideration when modeling continental break ups and rifting

palaeozoic to early jurassic history of the northwestern corner of gondwana and implications for the evolution of the iapetus rheic and pacific oceans abstract the palaeozoic to mesozoic igneous and metamorphic basement rocks exposed in the merida andes of venezuela and the santander massif of colombia are generally considered to define allochthonous terranes that accreted to the margin of gondwana during the ordovician and the carboniferous however terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains a general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the merida andes and the santander massif we present new zircon u pb icp ms data from 52 igneous and metamorphic rocks which we combine with whole rock geochemical and pb isotopic data to constrain the tectonic history of the precambrian to mesozoic basement of the merida andes and the santander massif these data show that the basement rocks of these massifs are autochthonous to gondwana and share a similar tectono magmatic history with the gondwanan margin of peru chile and argentina which evolved during the subduction of oceanic lithosphere of the iapetus ocean the oldest palaeozoic arc magmatism is recorded at 500 ma and was followed shortly by barrovian metamorphism peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at 477 ma and the intrusion of synkinematic granitoids until 472 ma subsequent retrogression resulted from localised back arc or intra arc extension at 453 ma when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement continental arc magmatism dwindled after 430 ma and terminated at 415 ma coevally with most of the western margin of gondwana after pangaea amalgamation in the late carboniferous to early permian a magmatic arc developed on its western margin at 294 ma as a result of subduction of oceanic crust of the palaeo pacific ocean intermittent arc magmatism recorded between 294 and 225 ma was followed by the onset of the andean subduction cycle at 213 ma in an extensional regime extension was accompanied by slab roll back which led to the migration of the arc axis into the central cordillera of colombia in the early jurassic

influence of continental roots and asthenosphere on plate mantle coupling the shear tractions that mantle flow exerts on the base of earth s lithosphere contribute to plate driving forces and lithospheric stresses we investigate the sensitivity of these tractions to sub lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity lateral viscosity variations generally do not change the direction of shear tractions but deeply penetrating continental roots increase traction magnitudes by a factor of 2 5 compared to 100 km thick lithosphere a low viscosity asthenosphere decreases traction magnitudes by a smaller amount and is important only if 100 km thick increased plate mantle coupling beneath thick continental lithosphere may increase plate driving forces surface deformation and mantle derived lithospheric stresses in these regions by contrast a low viscosity asthenosphere does not decouple the lithosphere from mantle flow highlighting the geological importance of mantle tractions on the lithosphere

the 1992 nicaragua earthquake a slow tsunami earthquake associated with subducted sediments the 1992 nicaragua earthquake was a tsunami earthquake that is it generated tsunamis disproportionately large for its surface wave magnitude m s the moment magnitude m w determined from long period 250 s surface waves was 7 6 significantly larger than the 20 s m s of 7 this m s m w disparity is also characteristic of tsunami earthquakes the nicaragua earthquake is the first tsunami earthquake to be captured by modern broadband seismic networks allowing us to present here seismograms of sufficiently high quality to make inferences about the rupture mechanisms we conclude that the nicaragua earthquake was a slow thrust earthquake which occurred on the subduction interface between the cocos and north american plates and because of the absence of sediments on the trench floor offshore of nicaragua the slip propagated up dip all the way to the ocean bottom exciting large tsunamis the occurrence of slip on a plate interface filled with soft subducted sediments caused the rupture process to be slower than in ordinary subduction zone thrust earthquakes our results reinforce the idea that tsunami warning systems using long period 100 s waves are necessary to reduce the hazard from this type of earthquake

a jurassic granite from southern georgia u s a silicic extension related magmatism along the southeastern coastal plain abstract ion microprobe shrimp u pb dating of single zircon grains extracted from granite cuttings from the subsurface of southeastern georgia pierce co yields a well defined crystallization age of documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape 159 pm 3 end document ma 2 confirming the presence of mesozoic silicic magmatism in the northern suwannee terrane revision of suwannee terrane subcrop age province boundaries is required by these data the granite is 20 40 m yr younger than subsurface diabases in the same r

loci and maximum size of thrust earthquakes and the mechanics of the shallow region of subduction zones earthquakes do not extend updip along the plate interface at most subduction zones all of the way to the plate boundary at the trench axis or deformation front rather the shallowest part of that interface moves mainly through stable aseismic slip that part of the plate boundary referred to here as the aseismic zone occurs along the base of an accreted wedge of young sediments the probable primary cause for the existence of this aseismic zone is the stable slip properties of the unconsolidated and semiconsolidated sediments in that zone subducted sediment is progressively dewatered and underplated to the base of the overriding plate through this process more consolidated rocks eventually come into contact at depth across both sides of the plate boundary from the point where sufficiently hard rock is found across the plate interface that interface will change its slip behavior to unstable stick slip sliding which is characteristic of consolidated material under most conditions this type of motion is accommodated seismically as episodic slip in large earthquakes the location of this transition to seismic behavior referred to as the seismic front marks the deep end of the aseismic zone and the top of the seismogenic zone i e that part of the plate interface that moves primarily in thrust earthquakes several convergent plate margins are discussed to illustrate the seismic front and the aseismic zone we find that the seismic front defined by smaller earthquakes that occur during the interval between large events is nearly the same as that for large events as inferred from the locations of their aftershocks the location of the seismic front is important for making estimates of the maximum possible size of thrust earthquakes along the plate boundary because it delimits the trenchward limit of the potential rupture area of these events the size of large thrust earthquakes is proportional to rupture area and to at least the cube of the downdip width w of that area thus by defining the location of the seismic front for convergent margins it is possible to make better estimates of w and hence to deduce the maximum size of future interplate earthquakes the average repeat time of such events is also related to w and so better estimates of average repeat time are possible from improved knowledge of w the hypothesis that the location of the seismic front is related to the maximum depth of subduction of unconsolidated sediment has implications for forearc mechanics we use a mechanical analysis laboratory modeling and multichannel seismic information to develop a simple model explaining the growth of forearcs the outer arc high or trench slope break that occurs arcward of most accretionary wedges represents an abrupt change in the critical taper of the accretionary wedge we argue that that change is caused by a large arcward increase in the strength of the material within the overriding plate that stronger material is called the backstop our laboratory modeling experiments indicate that a backstop with a trenchward dipping upper surface results in the development of an overlying structure that includes all of the primary morphologic features observed in modern forearcs in our modeling an accretionary wedge develops trenchward of the backstop an outer arc high develops above the trenchward toe of the backstop and farther arcward a passive forearc basin forms above the stronger material of the backstop this model is consistent with the observation that earthquakes do not extend updip along the plate interface all the way to the trench axis we hypothesize that plate motion is accommodated seismically along the base of the consolidated backstop and mostly or entirely aseismically trenchward of the toe of the backstop along the base of the accretionary wedge at several margins the seismic front is approximately coincident with the outer arc high supporting this interpretation

the geological history of northwestern south america from pangaea to the early collision of the caribbean large igneous province 290 75ma abstract northwestern south america preserves a record of the assembly of western pangaea its disassembly and initiation of the far western tethys wilson cycle subsequent pacific margin magmatism and ocean plateau continent interaction since the late cretaceous numerous models have been presented for various time slices although they are based on either spatially restricted datasets or dates that are inaccurate estimates of the time of crystallisation here we review a very large quantity of geochronological geochemical thermochronological sedimentological and palaeomagnetic data that collectively provide tight constraints for geological models these data have been collected over a trench pacific parallel distance of 1500 km colombia and ecuador and reveal important temporal trends in rifting and subduction the temporal framework for our model constraints are obtained from robust concordant zircon u pb ages of magmatic rocks during 290 75 ma the late cretaceous thermal history of the margin

stability of dike intrusion along preexisting fractures we investigate the ability of magma to propagate along preexisting fractures oblique to the least compressive stress relaxation of the preexisting shear stress to zero over the portion of the fracture dilated by magma the dike results in slip for some distance along the closed portion of the fracture ahead of the dike tip and a stress concentration near the dike tip this could lead to the production of new tensile cracks oblique to the parent dike that could capture the flow if the shear stress resolved on the fracture plane is perpendicular to the fracture front mode i ii the front may deviate along its entire length if the shear stress is parallel to the fracture front mode i iii the front may splay into segments for mode i ii dikes the maximum tensile stress occurs at the dike tip and is parallel to the dike if the effective tensile stress exceeds the rock tensile strength then the intruding magma rather than dilating the existing fracture is expected to propagate into a self generated crack analogous to the wing cracks observed to form at the tips of pure mode ii fractures for mode i iii dikes the maximum tensile stress lies within the plane of the dike and is oriented at some angle that depends upon the far field boundary conditions even if the magma pressure exceeds the ambient normal stress it appears to be very difficult for dikes to intrude into preexisting fractures unless one or more of the following conditions is satisfied 1 the fracture is nearly perpendicular to the least compressive stress 2 the resolved shear stress on the fracture is small compared to the excess magma pressure i e the ratio of shear to opening of the dike walls is small 3 the effective ambient dike normal stress is small compared to the rock tensile strength this indicates that it may be quite difficult for dikes emerging from midcrustal to lower crustal depths to follow faults

modeling seismic cycles of great megathrust earthquakes across the scales with focus at postseismic phase subduction is substantially multi scale process where the stresses are built by long term tectonic motions modified by sudden jerky deformations during earthquakes and then restored by following multiple relaxation processes here we develop a cross scale thermomechanical model aimed to simulate the subduction process from 1 minute to million years time scale the model employs elasticity nonlinear transient viscous rheology and rate and state friction it generates spontaneous earthquake sequences and by using an adaptive time step algorithm recreates the deformation process as observed naturally during the seismic cycle and multiple seismic cycles the model predicts that viscosity in the mantle wedge drops by more than three orders of magnitude during the great earthquake with a magnitude above 9 as a result the surface velocities just an hour or day after the earthquake are controlled by viscoelastic relaxation in the several hundred km of mantle landward of the trench and not by the afterslip localized at the fault as is currently believed our model replicates centuries long seismic cycles exhibited by the greatest earthquakes and is consistent with the postseismic surface displacements recorded after the great tohoku earthquake we demonstrate that there is no contradiction between extremely low mechanical coupling at the subduction megathrust in south chile inferred from long term geodynamic models and appearance of the largest earthquakes like the great chile 1960 earthquake

characterisation of triassic rifting in peru and implications for the early disassembly of western pangaea abstract triassic igneous and sedimentary rocks exposed within the basement of the andes were deposited in a series of rifts and may record the early disassembly of western pangaea these rocks are sporadically exposed along almost the entire length of western south america although reliable geochronological and isotopic data are sparse we combine new geochronological zircon u pb isotopic hf nd and geochemical data with stratigraphic observations to constrain the age and tectonic setting of the mitu rift of southern peru the peruvian mitu rift is compared with other triassic rifts in colombia and ecuador palanda rift 240 225 ma bolivia mitu rift triassic bolivia chile and argentina e g cuyo basin 246 230 ma and conclusions are reached regarding the relationship between triassic extension along the western margin of pangaea and the eventual formation of the proto caribbean and central atlantic oceans the mitu rift peru was active during 245 240 to 220 ma and was synchronous with rifting along the pacific margin of colombia and ecuador along the chilean margin and western argentina and probably rifting within bolivia rifting north of the huancabamba deflection was accompanied by subduction and led to seafloor spreading whereas rifting along the peruvian and chilean margins mainly occurred in the absence of subduction and terminated prior to the formation of extensive transitional crust extension within peru and chile probably occurred via a combination of transtension steepening and detachment of an arrested slab we propose that plate tectonic forces initiated the early break up of pangaea by attenuating its margins and enhancing mantle upwelling prolonged extension may have propagated along pre existing weak zones that extended into the continental interior captured melts derived from the upwelled mantle forming a lip e g central atlantic magmatic province became hot and weak and eventually lead to the formation of a juvenile ocean e g central atlantic

splay fault slip in a subduction margin a new model of evolution in subduction zones major thrusts called splay faults are thought to slip coseismically during large earthquakes affecting the main plate interface we propose an analytical condition for the activation of a splay fault based on force balance calculations and suggest thrusting along the splay fault is generally conditioned by the growth of the accretionary wedge or by the erosion of the hanging wall in theory normal slip on the splay fault may occur when the decollement has a very low friction coefficient seaward such a low friction also implies an unstable extensional state within the outer wedge finite element elasto plastic calculations with a geometry based on the nankai kumano section were performed and confirm that this analytical condition is a valid approximation furthermore localized extension at a shallow level in the splay hanging wall is observed in models for a wide range of friction coefficients from 0 to the value of internal friction coefficient of the rock here equals to 0 4 the timing of slip established for the splay fault branch drilled on nankai kumano transect suggests a phase of concurrent splay and accretionary wedge growth 2 ma to 1 5 ma followed by a locking of the splay 1 3 ma active extension is observed in the hanging wall this evolution can be explained by the activation of a deeper and weaker decollement followed by an interruption of accretion activation of a splay as a normal fault as hypothesized in the case of the tohoku 2011 earthquake can be achieved only if the friction coefficient on the decollement drops to near zero we conclude that the tectonic stress state largely determines long term variations of tightly related splay fault and outer decollement activity and thus influences where and how coseismic rupture ends but that occurrence of normal slip on a splay fault requires coseismic friction reduction

displacement above the hypocenter of the 2011 tohoku oki earthquake the moment magnitude m w 9 0 2011 tohoku oki mega thrust earthquake occurred off the coast of northeastern japan combining global positioning system gps and acoustic data we detected very large sea floor movements associated with this event directly above the focal region an area with more than 20 meters of horizontal displacement that is four times larger than those detected on land stretches several tens of kilometers long along the trench the largest amount reaches about 24 meters toward east southeast just above the hypocenter furthermore nearly 3 meters of vertical uplift occurred contrary to observed terrestrial subsidence

shallow subduction zone earthquakes and their tsunamigenic potential summary we have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7 0 or greater as well as some other interesting events in the period range 1 20 s by computing moment rate functions of teleseismic p waves after comparing the source spectral characteristics of tsunami earthquakes earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude with regular events we identified a subclass of this group slow tsunami earthquakes this subclass consists of the 1992 nicaragua the 1994 java and the february 1996 peru earthquakes we found that these events have an anomalously low energy release in the 1 20 s frequency band with respect to their moment magnitude although their spectral drop ov is comparable to those of the other earthquakes from an investigation of the centroid and body wave locations it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up dip until the stable sliding friction regime of unconsolidated sediments stops the propagation sedimentstarved trenches e g near jalisco can produce very shallow slip because the fault material supports unstable sliding the slow tsunami earthquakes also ruptured up dip however their centroid is located unusually close to the trench axis the subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment ocean surveys show that in these regions the ocean floor close to the trench is highly faulted we suggest that the horst and graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate making shallow earthquake nucleation and up dip propagation to the ocean floor possible the rupture partly propagates in sediments making the earthquake source process slow two factors have to be considered in the high tsunami generating potential of these events first the slip propagates to shallow depths in low rigidity material causing great deformation and displacement of a large volume of water second the measured seismic moment may not represent the true earthquake displacement because the elastic constants of the source region are not taken into account in the standard cmt determination

taphonomic comparison between two bivalves mactra and brachidontes from late quaternary deposits in northern argentina which intrinsic and extrinsic factors prevail under different palaeoenvironmental conditions quaternary records of unique skeletal deposits studied at two coastal areas of the bonaerensean littoral bahia samborombon in the north at puente de pascua and canal 15 area and bahia blanca southwards are thick 4 5 m and exceptionally rich in molluscan assemblages ca 75 90 bivalve shells indicating different high sea level episodes through taphonomical aspects of target bivalves mactra brachidontes which differ in shell structure life habit and habitat abundantly present in the marine late quaternary from both areas the aims were 1 to establish quantitative grades of taphonomic alteration of mactra from four lithostratigraphical units at bahia samborombon and identify which signature s better discriminates palaeoenvironments 2 test whether mactra shows different alteration than does brachidontes in response to differences in intrinsic features and ecological requirements 3 compare isotaphonomic assemblages complementing independent geological and palaeoenvironmental interpretations attributes articulation fragmentation abrasion colour encrustation bioerosion were classified for about 5400 shells from 11 samples bar plots ternary taphograms and q mode cluster analysis discriminate beach beach ridges and tidal flat environments and two taphofacies late pleistocene and holocene high energy units and low energy holocene units only abrasion and colour allow discrimination between high and low energy conditions rules cannot be applied to the processes of deposition of shells at least for the late quaternary marine deposits along the buenos aires province coastal area despite the harder mactra or more fragile brachidontes shell architecture in softer mactra muddy substrates infaunal or harder brachidontes varied sandy rocky bottoms epibyssate and deeper mactra infralittoral or shallower waters brachidontes intertidal to supratidal the taphonomic grades of alteration are similar a product mainly of long after death exposure along the taphonomically active zone in high to moderate wave energy conditions

juxtaposed mesozoic diabase dike sets from the carolinas a preliminary assessment aeromagnetic anomaly maps of the north carolina piedmont provide confirming evidence of juxtaposed mesozoic diabase dike sets dikes in north carolina may be divided into four domains on the basis of dike orientations most dikes are olivine normative but there are apparent systematic differences in chemical compositions that are reflected in different orientations and ages two orientations dominate northwest and north south the northwest trending set dominates in the south carolina piedmont and the north south set extends through much of the central virginia piedmont and under the coastal plain in south carolina the north south dikes are part of a set that converges near charleston and georgetown south carolina and diverges northward into virginia preliminary information suggests that the north south dikes are younger than the more common northwest trending dikes current models proposed for stress regimes associated with the breakup of pangea can accommodate the northwest set but not the north south convergent set this convergent set may be evidence for crustal deformation above a mantle plume centered near charleston and georgetown in middle jurassic time it is possible that presumed mafic plutons under the coastal plain near charleston regional seismicity patterns and orientations of the diabase dikes are all related to an intersection of zones of crustal weakness inherited from mid mesozoic time the western margin of northwest trending dikes may be related to tectonic features inherited from pre mesozoic time

time averaging overcompleteness and the geological record time averaging makes events that happened at different times appear to be synchronous in the geological record it can generate overcompleteness by concentrating objects e g fossils clasts from a longer time interval into a unit that formed during a shorter interval time averaging results both from extrinsic agents e g vertical mixing condensation analytical pooling of data and the intrinsic properties of the fossilized objects preservational potential and abundance variation in intrinsic and extrinsic factors results in disharmonious time aver aging different taxa preserved together may be time averaged over different time scales because extrinsic and intrinsic factors vary in time and space the geological record is characterized by vertical and lateral variations in time averaging the importance of time averaging depends on the time aver aging threshold which is determined by the time scale of the process of interest significant time aver aging occurs when the scale of temporal mixi

taphonomy on the continental shelf and slope two year trends gulf of mexico and bahamas abstract the shelf and slope experimental taphonomy initiative was established to measure taphonomic rates in a range of continental shelf and slope environments of deposition eods over a multiyear period we deployed experiments on the forereef slope off lee stocking island bahamas and on the continental shelf and slope of the gulf of mexico for 2 yr in 18 distinctive eods at depths from 15 to 530 m overall most shells deployed at most sites had relatively minor changes in shell condition most eods generated relatively similar taphonomic signatures a few sites did produce taphonomic signatures clearly distinguishable from the central group and these sites were characterized by one or more of the following high rates of oxidation of reduced compounds presence in the photic zone and significant burial and exhumation events thus unique taphonomic signatures are created by unique combinations of environmental conditions that include variables associated with regional gradients such as depth and light and variables associated with edaphic processes such as the seepage of brine or petroleum or the resuspension and redeposition of sediment most sites however showed similar taphonomic signatures despite the variety of eod characteristics present suggesting that insufficient time had elapsed over 2 yr to generate a more diverse array of taphonomic signatures discoloration and dissolution were by far the dominant processes over the 2 yr deployment period periostracum breakdown loss of shell weight and chipping and breakage was less noticeable eods were chosen based on the expectation that the process of burial and the influence of depth and sediment type should play the greatest roles in determining between eod differences in taphonomic signature eod specific edaphic factors often overrode the influence of geographic scale environmental gradients taphonomic alteration was greater on hardgrounds and in brine exposed sites than on terrigenous muds dissolution was less effective at sites where burial was greatest discoloration occurred most rapidly at shallower sites and on hardgrounds water depth was less influential in determining taphonomic signature than burial state or sediment type the limited influence of water depth is likely due to the presence of shallow sites that for one reason or another were protected from certain taphonomic processes and deeper sites that were characterized by unusually strong taphonomic signals

thermal evolution of the central atlantic passive margins continental break up above a mesozoic super plume a near synchronous early jurassic 200 ma magmatic event along the north american and west african passive continental margins can be explained by the upwelling of a large scale mantle plume super plume beneath the west african craton lateral deflection of the outflow from the plume to the ne by the ambient sub lithospheric mantle flow could account for tholeiitic basaltic magmatism extending over a distance of nearly 5000 km from brazil to southern spain continental break up is considered to occur above the stagnation streamline along which cooler material eroded from the base of the continental lithosphere by the plume is returned to the convecting mantle the dynamic model is a scaled version of that recently proposed for the hawaiian plume and satisfactorily explains the apparent lack of involvement of thermally anomalous mantle in the generation of central atlantic oceanic crust

coseismic and postseismic stress rotations due to great subduction zone earthquakes 1 the three largest recent great subduction zone earthquakes 2011 m9 0 tohoku japan 2010 m8 8 maule chile and 2004 m9 2 sumatra andaman exhibit similar coseismic rotations of the principal stress axes prior to each mainshock the maximum compressive stress axis was shallowly plunging while immediately after the mainshock both the maximum and minimum compressive stress axes plunge at 45 dipping faults can be oriented for either reverse or normal faulting in this post mainshock stress field depending on their dip explaining the observed normal faulting aftershocks without requiring a complete reversal of the stress field the significant stress rotations imply near complete stress drop in the mainshocks with 80 of the pre mainshock stress relieved in the tohoku and maule earthquakes and in the northern part of the sumatra andaman rupture the southern part of the sumatra andaman rupture relieved 60 of the pre mainshock stress the stress axes rotated back rapidly in the months following the tohoku and maule mainshocks and similarly in the southern part of the sumatra andaman rupture a rapid postseismic rotation is possible because the near complete stress drop leaves very little background stress at the beginning of the postseismic reloading in contrast there has been little or no postseismic rotation in the northern part of the sumatra andaman rupture over the 7 years since the mainshock all m 8 0 subduction earthquakes since 1990 with an adequate number of pre and post mainshock events were evaluated and not all show similar coseismic stress rotations deeper earthquakes exhibit smaller coseismic stress rotations likely due to increasing deviatoric stress with depth

catastrophic initiation of subduction following forced convergence across fracture zones although the formation of subduction zones plays a central role in plate evolution the processes and geological settings that lead to the initiation of subduction are poorly understood using a visco elastoplastic model we show that a fracture zone could be converted into a self sustaining subduction zone after approximately 100 km of convergence modeled initiation is accompanied by rapid extension of the over riding plate and explains the inferred catastrophic boninitic volcanism associated with eocene initiation of the izu bonin mariana ibm subduction zone using global plate reconstructions we suggest that ibm nucleation was associated with a change in plate motion between 55 and 45 ma we estimate that the forces resisting ibm subduction initiation were substantially smaller than available driving forces

shell beds as tools in basin analysis the jurassic of kachchh western india skeletal concentrations are ubiquitous in bathonian oxfordian shallow water sediments of the pericratonic basins of kachchh and rajasthan western india by analysing the biofabric of the concentrations and the taphonomic signatures of individual components it is possible to distinguish between the final concentration process recorded by the biofabric and events taking place prior to final deposition recorded by taphonomic signatures the skeletal concentrations are grouped in nine genetic types ranging from fair weather wave concentrations to storm concentrations and condensed concentrations they mainly reflect biological and hydraulic processes and in some instances permit estimates of their duration and show a rough zonation along a bathymetric gradient plotted against the stratigraphic sections the skeletal concentrations show in accordance with other sedimentological data three orders of bathymetric trends the first two orders are shallowing deepening cycles which possibly correspond to eustatic changes in sea level the third order represents parasequences with a strongly asymmetric sedimentary record skeletal concentrations are a useful additional tool in basin analysis but are best used in combination with other data