the oligo miocene qom formation iran evidence for an early burdigalian restriction of the tethyan seaway and closure of its iranian gateways in the central iranian esfahan sirjan and qom basins sedimentation of the oligo miocene qom formation took place on extensive mixed carbonate siliciclastic ramps during this time both basins were positioned at the eurasian margin of the tethyan seaway which connected the western and eastern regions of the tethys ocean at least until the late burdigalian during the so called terminal tethyan event the tethyan seaway was then closed due to the collision of the african arabian and iranian eurasian plates facies analysis of the sedimentary record of both basins indicates paleoenvironments ranging from terrestrial to open marine settings including mangrove restricted inner shelf lagoon seagrass meadow reefal and deeper offshore environments recognition of eight depositional sequences and elaboration of an integrated biostratigraphic framework calcareous nannoplankton planktic and larger benthic foraminifers gastropods and pectinids allow us to construct a basin spanning stratigraphy the assignment of the recognized sea level lowstands to the ru 3 to bur 3 lowstands of the global sea level curve enables a comparison with time equivalent sections from the zagros basin which was part of the african arabian plate on the opposing southern margin of the tethyan seaway the so calibrated sections display restrictions of the tethyan seaway and interruption of the south iranian gateways between the qom basin and the proto indopacific in relation to ongoing plate collision during the early burdigalian

no flat wadati benioff zone in the central and southern central andes abstract the wadati benioff zone wbz is an approximate plane defined by earthquakes hypocentres observed in convergent plate boundaries and that usually dips at angles greater than 30 in some areas of the andes where there are gaps in volcanic activity and where heat flow is abnormally low this plane in most studies has nearly horizontal dip at a depth of about 75 100 km and it has been associated to flat subduction of the oceanic lithosphere this situation has been taken as the present day analogue of the laramide orogeny of western north america for which a flat slab episode has been proposed in the past years in this work the observed low heat flow in areas of the andes is assumed to be due to low radiogenic heat generation in geologically old and allochthonous terranes constituting large regions of western south america on the basis of geotherms obtained for areas of ecuador peru chile and argentina and of rheological results describing the partition between brittle and ductile regimes the seismic activity observed both in the lower crust and at depths of about 75 100 km is thoroughly explained at these depths earthquakes occur within the subcontinental upper mantle and then there is no flat wbz associated to subduction of the oceanic lithosphere there is evidence from recent seismological observations that the real wbz lies not horizontally and deeper in the tectonosphere

a vaporization model for iron silicate fractionation in the mercury protoplanet abstract a study has been carried out of the vaporization of a totally molten silicate magma of chondritic composition heated into the range 2500 3500 k the motivation for this was to determine the changes in the composition of the mantle that would occur in the mercury protoplanet should that body have been subjected to the high temperature phase in the evolution of the primitive solar nebula but the results are of more general interest we have used both ideal and nonideal descriptions of the magma chemistry an empirical model based on ideal mixing of complex components developed by hastie and co workers was used to describe the nonideal magma comparison of our results of the nonideal magma calculations with experimental work on vaporization of chondritic material shows generally good agreement we find that vaporization of about 70 80 of the original amount of silicate from a chondritic planet is required to produce an iron rich body with a mean uncompressed density equal to that deduced for mercury 5 3 g cm3 at this point the silicate is depleted in the alkalis feo and sio2 and enriched in cao mgo al2o3 and tio2 relative to chondritic material we also predict the production of unique trace element abundance patterns which are depleted in easily oxidized elements relative to other trace elements of similar volatilities

the late cretaceous to recent tectonic history of the pacific ocean basin abstract a vast ocean basin has spanned the region between the americas asia and australasia for well over 100 myr represented today by the pacific ocean its evolution includes a number of plate fragmentation and plate capture events such as the formation of the vancouver nazca and cocos plates from the break up of the farallon plate and the incorporation of the bellingshausen kula and aluk phoenix plates which have been studied individually but never been synthesised into one coherent model of ocean basin evolution previous regional tectonic models of the pacific typically restrict their scope to either the north or south pacific and global kinematic models fail to incorporate some of the complexities in the pacific plate evolution e g the independent motion of the bellingshausen and aluk plates thereby limiting their usefulness for understanding tectonic events and processes occurring in the pacific ocean perimeter we derive relative plate motions with 95 uncertainties for the pacific farallon vancouver kula pacific bellingshausen pacific and early pacific west antarctic spreading systems based on recent data including marine gravity anomalies well constrained fracture zone traces and a large compilation of magnetic anomaly identifications we find our well constrained relative plate motions result in a good match to the fracture zone traces and magnetic anomaly identifications in both the north and south pacific in conjunction with recently published and well constrained relative plate motions for other pacific spreading systems e g aluk west antarctic pacific cocos recent pacific west antarctic spreading we explore variations in the age of the oceanic crust seafloor spreading rates and crustal accretion and find considerable refinements have been made in the central and southern pacific asymmetries in crustal accretion within the overall pacific basin where both flanks of the spreading system are preserved have typically deviated less than 5 from symmetry and large variations in crustal accretion along the southern east pacific rise i e pacific nazca farallon spreading appear to be unique to this spreading corridor through a relative plate motion circuit we explore the implied convergence history along the north and south america s where we find that the inclusion of small tectonic plate fragments such as the aluk plate are critical for reconciling the history of convergence with onshore geological evidence

thermal evolution of mercury effects of volcanic heat piping 1 a 1d thermal evolution model of mercury is presented it accounts for stagnant lid convection mantle differentiation and inner core growth early messenger results indicate that contrary to prior conclusions drawn from mariner 10 imagery volcanism has indeed played a significant role in mercury s past to study the effects of mantle heat bypassing the stagnant lid by means of volcanic heat piping contrasting end member models are considered results show how break down of mantle convection and onset of inner core growth are influenced by the mode of heat removal structural models of present day mercury are presented their dependence on the core sulphur contents predominates that on the choice of mantle heat removal however the latter clearly controls the timing of thermal history events

a seismotectonic study of the burma and andaman arc regions using centroid moment tensor data abstract the concept of a mean slip angle is introduced this enables a classification of focal mechanisms in any region into predominantly strike slip thrust and normal categories based on this concept the harvard centroid moment tensor cmt data in the burma and andaman arc regions from 1977 to 1992 comprising 167 focal mechanism solutions are examined and categorized distinct trends on the surface and in depth sections emerge on examination of these categories for instance in the burmese arc there is a clear segregation along the slab between strike slip type mechanisms down to a depth of about 90 km and thrust events which occur exclusively below this depth in addition a study of p and t axis orientations indicates that the stress pattern in the subducted slab is different from that farther east whereas the p axes of the thrust and strike slip mechanisms in the slab show a predominantly nne trend commensurate with the direction of motion of the indian plate with respect to the eurasian plate those in the region to the east show interestingly an average e w oriented compressive regime the northern and southern parts of the andaman are region exhibit distinct tectonic patterns whereas in the southern part the disposition of focal mechanisms along the slab and the p and t axis orientations indicate active subduction the mechanisms in the northern part exhibit a peculiar segregation into clusters of thrust normal and strike slip types without conforming to the local trend of the arc a similar study in the western aleutian trench region indicates a much simpler subduction pattern

crust and upper mantle structure beneath the pacific northwest from joint inversions of ambient noise and earthquake data 1 we perform a joint inversion of phase velocities from both earthquake and ambient noise induced rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the pacific northwest we focus particularly on the areas affected by mid miocene to present volcanic activity the joint inversion combined with the high density seismic network of the high lava plains seismic experiment and data from the earthscope transportable array provides outstanding resolution for this area in oregon we find that the pattern of low velocities in the crust and uppermost mantle varies between the high lava plains physiographic province and the adjacent northwestern basin and range these patterns may be due to the presence of the brothers fault zone which separates the clockwise rotating northwest basin and range from the relatively undeformed areas further north further to the east the owyhee plateau snake river plain srp and northeastern basin and range are characterized by high crustal velocities though the depth extent of these fast wave speeds varies by province of particular interest is the mid crustal high velocity sill previously only identified within the srp we show this anomaly extends significantly further south into utah and nevada we suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid crustal sill structures within the srp

thermal convection in a volumetrically heated infinite prandtl number fluid with strongly temperature dependent viscosity implications for planetary thermal evolution parameterized models of the thermal evolution of planets are usually based on the assumption that the lithosphere convecting mantle boundary can be defined by an isotherm at a temperature below which viscosity is infinite on geologic timescales recent experimental results argue against this assumption we have investigated both the definition of the lithosphere convecting mantle boundary and the power law relation describing convecting heat transfer based on numerical experiments of thermal convection in a volumetrically heated fluid with temperature dependent viscosity other recent studies have treated only the heating from below but volumetric heating is likely to be the dominant mode of heating in planetary mantles either as a consequence of radioactive heating or as a proxy for secular cooling convection can occur either in the whole box or be located under a stagnant lid in the lid regime convection is driven by a temperature contrast depending on the rheology of the fluid and the interior temperature this result in agreement with experimental studies indicates that boundary between the stagnant lid and the convecting layer similar to the lithosphere convecting mantle boundary cannot be defined as a fixed isotherm during thermal evolution of planets the viscosity contrast in the convecting mantle remains constant not the temperature at the bottom of the lithosphere we present an example showing that the evolution of planets is strongly dependent on the criterion chosen to define the lithosphere convecting mantle boundary for reasonable values of the activation energy for thermally activated creep the temperature defining the lithosphere convecting mantle boundary the mantle temperature and the thickness of the lithosphere could be larger than expected from previous models which treat the base of the lithosphere as a fixed isotherm

geodynamic evolution of southern costa rica related to low angle subduction of the cocos ridge constraints from thermochronology abstract the late tertiary shallow subduction of the cocos ridge under the caribbean plate controlled the evolution of the cordillera de talamanca in southeast costa rica which is a mountain range that consists mainly of granitoids formed in a volcanic arc setting fission track thermochronology using zircon and apatite as well as 40ar 39ar and rb sr age data of amphibole and biotite in granitoid rocks constrain the thermal history of the cordillera de talamanca and the age of onset of subduction of the cocos ridge shallow intrusion of granitoid melts resulted in fast and isobaric cooling a weighted mean zircon fission track age 13 analyses and rb sr biotite ages of about 10 ma suggest rapid cooling and give minimum ages for granitoid emplacement in some cases 40ar 39ar and rb sr apparent ages of amphibole and biotite are younger than the zircon fission track ages which can be attributed to partial resetting by hydrothermal alteration apatite fission track ages range from 4 8 to 1 7 ma but show no correlation with the 3090 m elevation span over which they were sampled the apatite ages seem to indicate rapid exhumation caused by tectonic and isostatic processes the combination of the apatite fission track ages with subduction parameters of the cocos plate such as subduction angle plate convergence rate and distance of the cordillera de talamanca to the trench implies that the cocos ridge entered the middle america trench between 5 5 and 3 5 ma

the effect of oceanic crustal structure on phase changes and subduction abstract recent numerical models for the thermal regime of subduction zones are in disagreement with observations of depth to formation of eclogite which require higher temperatures than those found in the models this discrepancy may be reconciled if more accurate geometric depictions of the subducted slab are incorporated in the modelling studies the transition from 15 dip along the thrust plane to 30 along the benioff zone starts at depths of about 40 km dehydration reactions and the gabbro eclogite phase change are presumed to increase the average density of the lithosphere and to cause the steepening of the subducted lithosphere the benioff zone seismicity which starts at about 40 km depth may itself be a result of stresses caused by the phase changes since the gabbro eclogite transformation occurs in gabbroic sections of the crust thickened crustal sections beneath oceanic plateaus will affect subduction first the width of the thrust zone is reduced due to the increased dip angle because of early transformation of lower and warmer crustal sections second incomplete conversion of the entire crustal thickness results in an average density intermediate between unsubducted oceanic lithosphere and fully transformed normal lithosphere the first effect leads to decreased thrust mechanism seismicity because the coupling between the plates is reduced the second effect leads to flat lying subcontinental subduction or arcward advance of the trench in suboceanic subduction

ablative subduction a two sided alternative to the conventional subduction model subduction is conventionally modelled as a convergence and overlapping of two semirigid plates such a convergence is one sided in that one lithospheric plate descends while the other remains on the surface we suggest an alternative model one based on the dynamics of fluids in this model the viscous lower lithosphere flows downward and the brittle upper lithosphere deforms in passive response this process is potentially double sided for we find that even a buoyant plate can be dragged downward by a dense descending neighbor thus an apparent overriding plate may be worn away by a process of viscous ablation with the rate of ablation a function of plate buoyancy we call this process ablative subduction ablative subduction allows us to simply interpret observations concerning slab profiles interplate seismicity back arc tectonics and complex processes such as double subduction and subduction polarity reversal in performing experiments modelling the evolution of simple fluid slabs we find that slab profile is strongly influenced by ablation in the overriding plate when ablation is weak as when a buoyant continent borders the trench deformable slabs adopt shallow andean style profiles these profiles develop over time from an initially steep shape more vigorous ablation rates as might occur in an ocean ocean convergence yield steeper marianas style profiles thus differences between marianas style and andean style slabs may result from differences in ablation rate and subduction duration the occurrence of ablation might not be easily detected at depth because material from subducting and ablating plates adhere closely as a single slab these slabs show downdip deformations that are consistent with seismic focal mechanisms plate deformations associated with ablative subduction are also consistent with observed patterns of seismicity in particular the relative aseismicity of marianas style plate boundaries is consistent with the ablative model we may thus explain how plates can converge aseismically without requiring that these plates be decoupled ablation should be weakest in andean style subduction although pulses of rapid ablation might lead to episodes of crustal shortening in oceanic regions the manifestation of ablation may depend upon tectonic regime in an extensional area like the marianas ablation may be temporally self limiting and ablative cycles may explain conflicting observations of tectonic erosion and accretion in a compressional regime vigorous ablation might lead to observations of double subduction and subduction reversal in each case the behavior of the lower lithosphere is relatively simple what varies is the rate of ablation and the response of the upper lithosphere

k ar age and geochemistry of the sw japan paleogene cauldron cluster implications for eocene oligocene thermo tectonic reactivation abstract systematic k ar dating and geochemical analyses of paleogene cauldrons in the sanin belt of sw japan have been made to explore the relationship between the timing of their formation and the paleogene subduction history of sw japan documented in the shimanto accretionary complex we also examine the magma sources and tectonics beneath the backarc region of sw japan at the eastern plate boundary of eurasia fifty eight new k ar ages and 19 previously reported radiometric age data show that the cauldrons formed during middle eocene to early oligocene time 43 30 ma following a period of magmatic hiatus from 52 to 43 ma the hiatus coincides with absence of an accretionary prism in the shimanto belt resumption of the magmatism that formed the cauldron cluster in the backarc was concurrent with voluminous influx of terrigenous detritus to the trench as a common tectono thermal event within a subduction system the cauldrons are composed of medium k calc alkaline basalts to rhyolites and their plutonic equivalents these rocks are characterized by lower concentrations of large ion lithophile elements lile including k2o ba rb th u and li lower la yb n ratios lower initial sr isotopic ratios 0 7037 0 7052 and higher end t values 0 5 to 3 5 relative to late cretaceous to early paleogene equivalents there are clear trends from enriched to depleted signatures with decreasing age from the late cretaceous to the paleogene the same isotopic shift is also confirmed in lower crust derived xenoliths and is interpreted as mobilization of pre existing enriched lithospheric mantle by upwelling depleted asthenosphere relatively elevated geothermal gradients are presumed to have prevailed over wide areas of the backarc and forearc of the sw japan arc trench system during the eocene to oligocene newly identified late eocene low silica adakites and high mg andesites in the sanin belt and early eocene a type granites in the sw korea peninsula probably formed due to upwelling of hot asthenosphere and subduction of a young plate the backarc region was an extensional tectonic setting and some paleogene rift basins and sanin belt cauldrons occur in linear arrays the eocene oligocene sanin se korea continental arc lies on the ne extension of the east china sea basin the initial stage of which probably formed by continental arc rifting this rifting may have been triggered by upwelling of hot asthenosphere into the wedge space created by rollback of the subducted slab in response to decreased convergence rate between the pacific and eurasian plates

flat slab thermal structure and evolution beneath central mexico recent seismic and magnetotelluric experiments aimed at better characterizing the shape and state of the subducting slab and continental crust beneath central mexico exposed significant differences with conclusions of previous studies a new slab geometry is revealed in which the subducting cocos slab is perfectly flat between 120 to 290 km from the trench after which it plunges into the asthenosphere at a dip angle of 65 in sharp contrast with the previously proposed 20 dip angle seismic tomography studies show negative p wave velocity anomalies 2 to 4 in the mantle wedge beneath the mexican volcanic belt and positive anomalies 2 to 3 for the subducted cocos slab magnetotelluric experiments exposed a very low resistivity area 1 10 m located within the continental crust just below the mexican volcanic arc finally several spots of non volcanic tremors nvts have been recorded inside the continental crust above the flat slab segment while all these experiments provide a better picture of the subduction system beneath central mexico several key processes need further investigation in this study we take advantage of these new observations to better constrain the thermal structure beneath central mexico two different thermal models are computed for a mantle potential temperature t p of 1 350 and 1 450 c respectively the new thermal structures are then converted into p wave velocity anomalies and compared with the observed v p anomalies we found that a t p of 1 450 c produced larger v p anomalies that do not fit the observations however using a t p of only 1 350 c our predicted v p anomalies are positive 2 to 3 for the cold slab and negative 2 to 4 in the mantle wedge these v p estimates are consistent with the observed seismic tomography from p wave arrivals and therefore we conclude that a t p of 1 350 c is a better estimate for the mantle potential temperature beneath central mexico the new thermal model in conjunction with phase diagrams for sediments hydrated basalt and lithospheric mantle have been used to estimate the amount and location of fluids released from the subducting cocos slab several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat slab we found a good correlation between the pattern of these dehydration pulses and the location of nvts suggesting that slab dehydration is responsible for triggering the tremors we suggest that nvt bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening a process that has been going on since 15 ma ago when the cocos slab entered into a flat slab regime such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15 ma allowing the slab to easily roll back the continuous low resistivity region recorded beneath the volcanic front in central mexico might represent the evidence of slab dehydration and crust weakening over time

structure and seismogenic properties of the mentawai segment of the sumatra subduction zone revealed by local earthquake traveltime tomography 1 on 12 september 2007 an mw8 4 earthquake occurred within the southern section of the mentawai segment of the sumatra subduction zone where the subduction thrust had previously ruptured in 1833 and 1797 traveltime data obtained from a temporary local seismic network deployed between december 2007 and october 2008 to record the aftershocks of the 2007 event was used to determine two dimensional 2 d and three dimensional 3 d velocity models of the mentawai segment the seismicity distribution reveals significant activity along the subduction interface and within two clusters in the overriding plate either side of the forearc basin the downgoing slab is clearly distinguished by a dipping region of highvp 8 0 km s which can be a traced to 50 km depth with an increased vp vs ratio 1 75 to 1 90 beneath the islands and the western side of the forearc basin suggesting hydrated oceanic crust above the slab a shallow continental moho of less than 30 km depth can be inferred suggesting that the intersection of the continental mantle with the subducting slab is much shallower than the downdip limit of the seismogenic zone despite localized serpentinization being present at the toe of the mantle wedge the outer arc islands are characterized by low vp 4 5 5 8 km s and high vp vs greater than 2 0 suggesting that they consist of fluid saturated sediments the very low rigidity of the outer forearc contributed to the slow rupture of the mw 7 7 mentawai tsunami earthquake on 25 october 2010

mercury cratering record viewed from messenger s first flyby morphologies and size frequency distributions of impact craters on mercury imaged during messenger s first flyby elucidate the planet s geological history plains interior to the caloris basin displaying color and albedo contrasts have comparable crater densities and therefore similar ages smooth plains exterior to caloris exhibit a crater density 40 less than on interior plains and are thus volcanic and not caloris impact ejecta the size distribution of smooth plains craters matches that of lunar craters postdating the late heavy bombardment implying that the plains formed no earlier than 3 8 billion years ago ga at diameters less than or equal to 8 to 10 kilometers secondary impact craters on mercury are more abundant than primaries this transition diameter is much larger than that on the moon or mars a low density of craters on the peak ring basin raditladi implies that it may be younger than 1 ga

oblique plate convergence in the indo burma myanmar subduction region the indo burma myanmar subduction boundary is highly oblique to the direction of relative velocity of the indian tectonic plate with respect to the eurasian plate the area includes features of active subduction zones such as a wadati benioff zone of earthquakes a magmatic arc thrust and fold belts it also has features of oblique subduction such as an arc parallel strike slip fault sagaing fault that takes up a large fraction of the northward component of motion and a buttress the mishmi block that resists the motion of the fore arc sliver

mercury the enigmatic innermost planet abstract the planet mercury a difficult object for study by astronomical observation and spacecraft exploration alike poses multiple challenges to our general understanding of the inner planets mercury s anomalously high uncompressed density implies a metal fraction of 60 or more by mass an extreme outcome of planetary formational processes common to the inner solar system whether that outcome was the result of chemical gradients in the early solar nebula or removal by impact or vaporization of most of the silicate shell from a differentiated protoplanet can potentially be distinguished on the basis of the chemical composition of the present crust our understanding of the geological evolution of mercury and how it fits within the known histories of the other terrestrial planets is restricted by the limited coverage and resolution of imaging by the only spacecraft to have visited the planet the role of volcanism in mercury s geological history remains uncertain and the dominant tectonic structures are lobate scarps interpreted as recording an extended episode of planetary contraction issues that require global imaging to be fully examined that mercury has retained a global magnetic field when larger terrestrial planets have not stretches the limits of standard hydromagnetic dynamo theory and has led to proposals for a fossil field or for exotic dynamo scenarios hypotheses for field generation can be distinguished on the basis of the geometry of mercury s internal field and the existence and size of a fluid outer core on mercury can be ascertained from measurements of the planet s spin axis orientation and gravity field and the amplitude of mercury s forced librations the nature of mercury s polar deposits suggested to consist of volatile material cold trapped on the permanently shadowed floors of high latitude impact craters can be tested by remote sensing of the composition of mercury s surface and polar atmosphere the extremely dynamic exosphere which includes a number of species derived from mercury s surface offers a novel laboratory for exploring the nature of the complex and changing interactions among the solar wind a small magnetosphere and a solid planet recent ground based astronomical measurements and several new theoretical developments set the stage for the in depth exploration of mercury by two spacecraft missions within the coming decade

seismotectonics of the central segment of the indonesian arc abstract in this paper a revision of seismicity affecting the central segment of the indonesian island arc ranging between 110 and 126 e is presented using the areal and in depth distribution of seismic activity strain release maps and focal mechanisms lateral changes in the wadati benioff zone have been analyzed to determine possible boundaries between portions of lithosphere with different subduction geometries the seismicity pattern indicates that the sumbawa flores wetar sector shows different forms of behaviour with respect to the adjacent sectors these include driving mechanism inclination and continuity of the subducting slab and subduction features this area therefore seems to be isolated from the sunda and banda arcs by two principal boundaries these having a nearly n s trend in the bali region and with a nearly e w trend in the region ranging between wetar northern timor and tanibar the first boundary characterized by an absolute minimum of seismic activity at all ranges of depth has been interpreted in terms of subduction of the roo rise aseismic bathymetric ridge for the second boundary dividing a northwardly steeply dipping slab from an e w subducting slab dipping with an angle of 30 40 a tear in the upper part depth less than 300 km and a hinge fault system in the deepest part of the lithosphere have been proposed from the analysis of focal mechanisms of shallow earthquakes it was inferred that the central part of the indonesian arc is subject to a vortex shaped stress field centred on the savu basin in this model the compressive axes appear to rotate counterclockwise from sw to nne in the sumba sumbawa western flores region and clockwise from w to nnw in the timor eastern flores zone to interpret these features on the basis of seismological evidence a lateral discontinuity in the arc trench system close to sumba a collision between sumba and sumbawa and a rotation towards the north northeast of sumba have been suggested the proposed structural discontinuity trending nw se may represent a major transcurrent fault zone through which the australian continental lithosphere comes into contact with the indian oceanic lithosphere

intermediate and deep seismicity and lateral structure of subducted lithosphere in the circum pacific region in this paper we present a region by region review of the wadati benioff zone structure of most of the world s seismically active subduction zones focusing primarily on the intermediate and deep seismicity lateral changes in wadati benioff zone structure are common in every major subduction zone in this study we use these changes to define possible boundaries between portions or segments of lithosphere with differing subduction geometries although earthquake data seldom have the resolution to show conclusively whether these boundaries separate independent blocks of lithosphere the available data indicate that the active process at most of these segment boundaries is ductile deformation of the subducting plate rather than tearing we found the strongest evidence for the existence of tears where tears are geometrically necessary such as where a transform boundary terminates a trench as at the new hebrides tonga and south sandwich trenches weak evidence suggesting other tears does exist in some regions such as taiwan the japan izu bonin corner and the philippines the causes of these changes in structure are in most cases unclear only about 34 of the possible segment boundaries coincide with subducting bathymetric features some boundaries occur where there is apparent lateral strain caused by anomalous trench geometry we have designed a simple modeling procedure which incorporates published plate motion and the observed geometry of trenches and wadati benioff zones to estimate the lateral strain in subduction zones throughout the circum pacific region although no observed subduction zone has a perfectly strain free geometry there is a broad range of geometries for which the lateral strain is small indeed the observed geometry of most subduction zones involve relatively little lateral strain comparison with centroid moment tensor focal mechanisms indicates that in zones where the modeling predicted little lateral strain the mechanisms of intermediate and deep earthquakes show no effects of lateral stress and downdip stresses are clearly dominant in regions such as the mariana arc where the model predicts very large lateral extension lateral tension is very evident in the focal mechanisms in regions such as the hokkaido corner where the modeling predicts large compressional strains the plate appears to buckle and bending stresses parallel to the trench are evident in general this study finds that subducted lithosphere is remarkably cohesive and rigid and only rarely deforms by breaking or stretching

cenozoic deformation and tectonic style of the andes between 33 and 34 south latitude 1 the andes of argentina and chile between latitudes 33 and 34 s are composed from west to east of an oligocene to miocene volcanic arcs and the neogene east vergent aconcagua fold and thrust belt of the cordillera principal and the basement block faulted cordillera frontal a regional cross section suggests that shortening across the andes was achieved by thrusting along detachments at several levels in the crust while thin skinned deformation along newly formed thrusts occurred in mesozoic sequences of the eastern cordillera principal reactivation of preexisting jurassic and oligocene normal faults has resulted in additional hybrid thick and thin skinned structures in the western cordillera principal five major thrusting events are recognized in this part of the andes 1 early to middle miocene tectonic inversion of the extensional faults in the western cordillera principal 2 middle to late miocene development of the aconcagua fold and thrust belt 3 late miocene uplift of cordillera frontal 4 late miocene early pliocene out of sequence thrusting in the cordillera principal and 5 pliocene to present deformation of the foreland

andesite and dacite genesis via contrasting processes the geology and geochemistry of el valle volcano panama the easternmost stratovolcano along the central american arc is el valle volcano panama several andesitic and dacitic lava flows which range in age 5 10 ma are termed the old group after a long period of quiescence approximately 3 4 ma volcanic activity resumed approximately 1 55 ma with the emplacement of dacitic domes and the deposition of dacitic pyroclastic flows 0 9 0 2 ma these are referred to as the young group all of the samples analyzed are calc alkaline andesites and dacites the mineralogy of the two groups is distinct two pyroxenes occur in the old group rocks but are commonly absent in the young group in contrast amphibole has been found only in the young group samples several disequilibrium features have been observed in the minerals e g oscillatory zoning within clinopyroxenes these disequilibrium textures appear to be more prevalent among the old as compared with the young group samples and are most likely the result of magma mixing assimilation and or polybaric crystallization mass balance fractionation models for major and trace elements were successful in relating samples from the old group but failed to show a relationship among the young group rocks or between the old and young group volcanics we believe that the old group volcanics were derived through differentiation processes from basaltic magmas generated within the mantlewedge the young group however does not appear to be related to more primitive magmas by differentiation the young group samples cannot be related by fractionation including realistic amounts of amphibole distinctive geochemical features of the young group including la yb ratios 15 yb 1 sr y 150 and y 6 suggest that these rocks were derived from the partial melting of the subducted lithosphere these characteristics can be explained by the partial melting of a source with residual garnet and amphibole dacitic material with the geochemical characteristics of subducted lithosphere melting is generated apparently only where relatively hot crust is subducted based on recent work the young dacite genesis at el valle volcano is related to the subduction of relatively hot lithosphere

upper miocene molluscs of monti livornesi tuscany italy biotic changes across environmental gradients abstract the upper miocene mollusc assemblages of monti livornesi used as a means to study the nature of mediterranean benthic communities at the edge of the messinian salinity crisis are framed in a high resolution stratigraphic scheme and quantitatively approached by the study of historical museum collections and modern samples instead of a single assemblage from either tortonian or messinian age as previously thought this fauna comes from three consecutive shallowing upward depositional sequences bounded by regional unconformities the luppiano upper tortonian lowermost messinian rosignano and raquese units early messinian facies analysis taphonomy and quantitative paleoecology show that the luppiano assemblage is characterised by aragonitic species from a eutrophic brackish water shallow marine muddy bottom the rosignano assemblage by calcitic species from an oligotrophic coarse grained seafloor close to a coral reef and the raquese assemblage from an open marine muddy bottom published comparisons between miocene and pliocene faunal lists should be considered only crude estimates of faunal change until more is known about the distribution of species along paleoenvironmental gradients the analysis of miocene and pliocene abundance data allows us to frame the monti livornesi molluscs along carbonate siliciclastic and water depth gradients and to reach a better understanding of the effects of salinity crisis on the mediterranean biota

the role of transcurrent intra arc tectonics in the configuration of a volcanic arc summary field investigations show that plio quaternary mexican and quaternary ecuadorian volcanic arcs are coeval with intra arc transcurrent or transtensional tectonics with motions along faults parallel or subparallel to the arc this relationship can also be observed in the quaternary kamchatka arc these arcs have an anomalously wide distribution of volcanic centres which cannot be explained simply by a low dip in the subducting lithosphere comparison with the intra arc tectonics and geometry of other quaternary arcs reveals that strike slip fault zones parallel to the arc favour magma rising in a broad surficial zone and may channel magma feeding mainly huge stratovolcanoes

volcanism on mercury evidence from the first messenger flyby the origin of plains on mercury whether by volcanic flooding or impact ejecta ponding has been controversial since the mariner 10 flybys 1974 75 high resolution images down to 150 meters per pixel obtained during the first messenger flyby show evidence for volcanic vents around the caloris basin inner margin and demonstrate that plains were emplaced sequentially inside and adjacent to numerous large impact craters to thicknesses in excess of several kilometers radial graben and a floor fractured crater may indicate intrusive activity these observations coupled with additional evidence from color images and impact crater size frequency distributions support a volcanic origin for several regions of plains and substantiate the important role of volcanism in the geological history of mercury

crustal thickness of the moon new constraints from gravity inversions using polyhedral shape models a new method is presented for estimating crustal thickness from gravity and topography data on the moon by calculating analytically the exterior gravitational field for a set of arbitrarily shaped polyhedra relief along the crust mantle interface can be inverted for that satisfies the observational constraints as this method does not rely upon filtering the bouguer anomaly which was required with previous inversions performed in the spherical harmonic domain and as the dramatic variations in spatial quality of the lunar gravity field are taken into account our crustal thickness model more faithfully represents the available data using our model results we investigate various aspects of the prominent nearside impact basins the crustal thickness in the central portion of the orientale and crisium basins is found to be close to zero suggesting that these basins could have conceivably excavated into the lunar mantle furthermore given our uncertain knowledge of the density of the crust and mantle it is possible that the humorum humboldtianum nectaris and smythii basins could have excavated all the way through the crust as well the crustal structure for most of the young impact basins implies a depth diameter ratio of about 0 08 for their excavation cavities as noted in previous studies however the crustal structure of imbrium and serenitatis is anomalous which is conceivably a result of enhanced rates of post impact viscous relaxation caused by the proximity of these basins to the procellarum kreep terrane impact basins older than smythii show little or no evidence for crustal thinning suggesting that these ancient basins were also affected by high rates of viscous relaxation resulting from higher crustal temperatures early in the moon s evolution the lithosphere beneath many young basins is found to be supporting a downward directed force even after the load associated with the mare basalts is removed and this is plausibly attributed to superisostatic uplift of the crust mantle interface those basins that are close to achieving a pre mare isostatic state are generally found to reside within or close to the procellarum kreep terrane

mesozoic cenozoic denudation history of the patagonian andes southern chile and its correlation to different subduction processes fission track ft analysis is applied to assess the mesozoic and cenozoic thermal and denudational history of the patagonian andes between 44 and 51 s and the geologic and geomorphic response of late cenozoic subduction of the active chile rise mid oceanic spreading center on the overriding plate seventy two ft ages from 43 samples are presented zircon ft ages indicate fast post intrusion cooling of cretaceous parts of the patagonian batholith and previously unreported miocene magmatic rocks south of 48 s metamorphic basement rocks to the east of the batholith are constrained as having been deposited and metamorphosed in the early carboniferous and late permian apatite ft data reveal initiation of accelerated cooling and denudation at ca 30 ma at the western margin of southern continental south america followed by an up to 200 km eastward migration of the locus of maximum denudation that ceased at ca 12 8 ma at the position of the present day main topographic divide this migration is proposed to be related to either coeval eastward migration of the retroarc deformation the effects of subduction erosion in the overriding plate at the peru chile trench or less likely shallowing of the angle of subduction east of the divide 3 km of denudation has occurred since the late cretaceous enhanced denudation is interpreted to be the result of increased tectonic uplift driven by a large increase in convergence rates at ca 28 26 ma that triggered orographically enhanced precipitation on the west side of the patagonian andes allowing increased erosion by fluvial incision and mass transport processes the actual process of spreading center subduction had remarkably little influence on denudation in the upper plate and indeed coincides with a slowdown in denudation

cretaceous to early paleogene tectonic evolution of the northern central andes 0 18 s and its relations to geodynamics the tectonic phases subsidence patterns and location of the magmatic arc along the peruvian margin during late cretaceous and paleogene times are compared with the main geodynamic parameters absolute plate motion convergence velocity and direction age of the subducted slab and accretional events for the same periods during the late cretaceous long termed compression seems to be controlled by the absolute trenchward motion of the overriding plate and to a minor extent by the young age of the subducted lithosphere short lived contractional or extensional phases are mainly linked to the acceleration or deceleration of the convergence between the oceanic and continental plates and probably to changes in the convergence direction periods of high and low convergence rates coincide with increased and decreased subsidence rates along the margin respectively and seem to be independent of compressional tensional regimes during the paleogene in addition to the mentioned processes the eastward shift of the magmatic belt and subsidence in the forearc zone are interpreted as the result of the subduction erosion of the margin this process seems to be related to the decrease of the dip of the benioff zone expressed by the drastic widening of the magmatic arc

cenozoic plate kinematics subduction and magmatism south american andes revised plate reconstructions and enlarged isotopic age data sets strongly support three mechanisms for controlling the configuration and associated volcanism and tectonism of the andean subduction zone these include age of subducted plate convergence rate and interaction with hotspot traces from 50 to 25 ma in the andes the age of oceanic plate subducted progressively decreased apparently resulting in a9gradual decrease in subduction angle and consequent eastward shift in the loci of magmatic activity a major reorganization of plate motions at 25 ma resulted in an increased convergence rate normal to the chilean andes and was accompanied by subsequent broadening of the locus of magmatic activity into adjacent portions of bolivia and argentina convergence rates changed only slightly across the peruvian margin and no clear effect on subsequent magmatism is indicated interaction of the andean subduction zone with the juan fernandez and easter nazca hotspot traces apparently resulted in lower angle subduction and consequent gaps in magmatic activity as predicted continuations of the traces correspond with regions lacking dated volcanic rocks the current gaps in igneous activity correspond with contemporary subduction of the two traces and this association apparently occurred in the past as well

the tectonic expression slab pull at continental convergent boundaries examination of five thrust belt systems developed at continental subduction boundaries suggests that they comprise two distinct groups that display pronounced and systematic differences in structural style topographic elevation denudation metamorphism postcollisional convergence and foredeep basin geometry and facies the distinctive geological features developed within each thrust belt group appear to be causally linked to the relative rates of subduction and convergence via the magnitude of horizontal compressional stress transmitted across the subduction boundary at subduction boundaries where the rate of overall plate convergence is less than the rate of subduction termed here retreating subduction boundaries the transmission of horizontal compressive stress across the plate boundary is small and regional deformation of the overriding plate is by horizontal extension the tectonic expression of these retreating subduction boundaries includes topographically low mountains little erosion or denudation low grade to no metamorphism little to no involvement of crystalline basement in shortening little to no postcollisional convergence anomalously deep foredeep basins and a protracted history of flysch deposition within the adjacent foredeep basin analysis of deflection and gravity data across three retreating subduction boundaries apennine carpathian and hellenic systems shows that subduction is driven by gravitational forces acting on dense subducted slabs at depths between about 40 and 80 km carpathians 50 and 150 km apennines and 50 and 250 km hellenides the total mass anomalies represented by the slabs are approximately 3 1012 6 1012 and 12 1012 n m respectively the slabs are partially supported by flexural stresses transmitted through the subducted lithosphere to the foreland and partially supported by dynamic viscous stresses in the asthenosphere at subduction boundaries where the rate of overall plate convergence is greater than the rate of subduction termed here advancing subduction boundaries the transmission of horizontal compressive stress across the plate boundary is large and regional deformation of the overriding plate is by horizontal shortening the tectonic expression of these advancing subduction boundaries includes topographically high mountains antithetic thrust belts large amounts of erosion and denudation exposure of high grade metamorphic rocks at the surface extensive deformation of crystalline basement to midcrustal depths protracted postcollisional convergence tens of millions of years and a protracted history of molasse deposition within the adjacent foredeep basins analysis of gravity and deflection data across two advancing subduction boundaries developed within the continental lithosphere western to eastern and southern alps and himalayas shows that the thrust sheets have been translated for great distances over the foreland lithosphere relative to the point at which the subduction forces are applied thus obscuring any flexural and gravity signals from the subducted slab however it appears that far field stresses presumably related to global plate motions drive most of the convergent motion across these subduction boundaries the concept that orogenic belts formed above retreating subduction boundaries have recognizable tectonic signatures that differ from those of orogenic belts formed above advancing subduction boundaries suggests that it may be possible to interpret the plate boundary settings in which ancient orogenic belts evolved appendix b is available with entire article on microfiche order from the american geophysical union 2000 floridaavenue n w washington d c 20009 document t92 004 2 50 payment must accompany order

controls of subduction geometry location of magmatic arcs and tectonics of arc and back arc regions most variation in geometry and angle of inclination of subducted oceanic lithosphere is caused by four interdependent factors combinations of 1 rapid absolute upper plate motion toward the trench and active overriding of the subducted plate 2 rapid relative plate convergence and 3 subduction of intraplate island seamount chains aseismic ridges and oceanic plateaus anomalously low density oceanic lithosphere cause low angle subduction under conditions of low angle subduction the upper surface of the subducted plate is in contact with the base of the overlying plate the wedge of low density asthenosphere is replaced by subducted lithosphere and the width of the arc trench gap either is significantly increased or a magmatic arc is not developed within the overlying plate the fourth factor is age of the subducting lithosphere subduction of young lithosphere produces two opposing tendencies 1 low angle subduction and increased arc trench distance owing to its low density and 2 decreased arc trench distance owing to its higher temperature two factors of secondary importance contribute to variation in subduction zone geometry and arc trench distance accretion of sediment in trenches depresses the upper portion of the subducting oceanic plate and causes the trench axis to migrate seaward prolonged subduction thickens the upper plate depresses the isotherms in the subducted plate and may create a broader arc both factors increase the arc trench gap the four primary factors also control development of other tectonic elements such as regional subsidence for example the amazon basin and a portion of the cretaceous interior seaway of western united states intra arc extension for example the basin and range province foreland fold and thrust belts and laramide style tectonics

facies development along the tide influenced shelf of the burdigalian seaway an example from the ottnangian stratotype early miocene middle burdigalian abstract herein we report quantitative micropaleontological benthic foraminifers dinoflagellate cysts calcareous nannoplankton sedimentological grain size analysis and geophysical background gamma radiation analyses from ottnang schanze the stratotype for the regional ottnangian stage central paratethys lower miocene middle burdigalian the revealed trends in bathymetry primary productivity bottom water oxygenation and water energy allow exemplary insights into the paleoenvironment of the terminal burdigalian seaway several facies of a eutrophic environment are distinguished that reflect a transition from a suboxic outer neritic to upper bathyal towards a better oxygenated middle neritic setting under the influence of storm events and currents a comparison with available data from upper austria and bavaria consistently shows the regressive trend during the late early ottnangian in upper austria the deep water facies from the lower part of the stratotype represents the most distal sediments the upper part together with localities closer to the northern coast records inner to middle neritic environments that are heavily affected by tidal currents the facies distribution results from the progradation of a tide influenced environment along the northern shelf of the north alpine foreland basin heralding the closure of the burdigalian seaway the available age estimate for the stratotype constrains the onset of the regressive phase to 18 ma

the development of slabs in the upper mantle insights from numerical and laboratory experiments we have performed numerical and laboratory experiments to model subduction of oceanic lithosphere in the upper mantle from its beginnings as a gravitational instability to the fully developed slab a two dimensional finite element code is applied to model newtonian creep in the numerical experiments scaled analog media are used in the laboratory a sand mixture models the brittle crust silicone putty simulates creep in the lower crust and mantle lithosphere and glucose syrup is the asthenosphere analog both model approaches show similar results and reproduce first order observations of the subduction process in nature based on density and viscosity heterogeneities in a stokes flow model subduction nucleates slowly and a pronounced slab forms only when the viscosity contrast between oceanic plate and mantle is below a threshold we find that the subduction velocity and angle are time dependent and increase roughly exponentially over tens of millions of years before the slab reaches the 670 km discontinuity the style of subduction is controlled by the prescribed velocity of convergence the density contrast between the plates and the viscosity contrast between the oceanic plate and the mantle these factors can be combined in the buoyancy number f which expresses the ratio between driving slab pull and resisting viscous dissipation in the oceanic plate variations in f control the stress in the plates the speed and the dip of subduction and the rate of trench retreat reproducing the contrasting styles of subduction observed in nature the subduction rate is strongly influenced by the work of bending the lithosphere as it subducts

benchmark data of a changing sea palaeogeography palaeobiogeography and events in the central paratethys during the miocene abstract the reconstruction and interpretation of terrestrial ecosystems and vegetational patterns in the central european miocene have to take into account the influence of a vast landlocked water body namely the paratethys sea as a northern appendage of the early mediterranean sea it spanned a north south gradient of at least 4 latitude and has been suggested to represent some kind of palaeothermometer which reflected slight expansions or restrictions of climatic belts due to its vulnerable marine connections it was also highly susceptible to major global sea level fluctuations which are reflected in phases of endemism hence a fairly continuous record of marine nearshore assemblages throughout the miocene reflects an extraordinary interplay of sea level fluctuations changes in climate immigrations and blooms in autochthonous elements whilst biostratigraphic implications of these patterns were recognised early in palaeontology a biogeographic model is still lacking the intermingling of palaeogeographic terms with those restricted to biogeography is still commonly used a situation which can be overcome by integrating data from different biota into a new palaeobiogeographic scheme herein a data set of 1809 species level taxa gastropoda and foraminifera serves as the base for interpretations gastropods in particular turned out to be of greatest value for the differentiation of palaeobiogeographic units due to sensitive reactions to all environmental parameters and occupying a wide range of ecological niches based on the excellent gastropod record the proto danubian province early danubian p danubian p balatonian p and a proto caspian subprovince are defined the biogeographic reorganisations are associated with five major biotic events within the gastropod faunas these big five comprise four extinctions the late ottnangian extinction event loee the mid badenian extinction event mbee the badenian sarmatian extinction event bsee the sarmatian pannonian extinction event spee and one exceptional immigration origination event termed the early badenian build up event ebbe foraminifera are less valuable for palaeobiogeographic purposes in the area and time slice studied endemisms are generally lower and turn over rates less dramatic they are however very useful for stratigraphic correlations with the mediterranean and atlantic areas this fairly well calibrated marine biostratigraphy of the central paratethys serves as stratigraphic backbone and control for continental stratigraphy

geochronology petrogenesis and tectonic significance of peraluminous granites from the chinese altai nw china abstract the paleozoic granites in the chinese altai are important for the study of tectonic evolution and crustal growth in the central asian orogenic belt caob four representative peraluminous granitic intrusions were selected for systematic studies of zircon u pb and hf isotopic compositions and whole rock geochemical and nd sr isotopic analyses these rocks have high asi alumina saturation index al 2 o 3 cao na 2 o k 2 o 1 01 1 46 molecular ratios with 0 6 5 6 wt of normative corundum and are characterized by moderately negative eu anomalies eu eu 0 38 0 98 and strong depletion in ba nb and sr elements our data suggest that these intrusions were emplaced from 419 to 393 ma consistent with a period of intensive magmatic activities and high temperature metamorphism in the chinese altai while in situ zircon hf isotopic analyses for these granites give predominantly positive e hf t values 0 8 to 12 8 a few inherited zircons yield negative e hf t values from 12 5 to 1 53 the u pb age and hf isotopic data of these inherited zircons are similar to that of the widespread metasediments in addition the peraluminous granitic rocks have near zero or negative e nd t values 3 3 to 0 5 and relatively high initial 87 sr 86 sr ratios 0 7079 0 7266 distinct from those of the i type granites in the study region but similar to the early paleozoic habahe sediments these isotopic compositions suggest that the newly accreted metasediments of habahe group may be the major source rock of the peraluminous granites the geochemical compositions indicate that their precursor magmas were derived from a relatively shallow crustal level p 5 kbar and zircon saturation temperatures suggest that these granitic intrusions were emplaced at 672 861 c the peraluminous granitic magmas were generated by dehydration melting of newly accreted materials which were possibly brought to at least middle crustal depth by subduction related processes in an active margin and were subsequently molten by strikingly high ambient temperature probably caused by upwelling of the hot asthenosphere associated with ridge subduction in the paleozoic

tectonic segmentation of the burmese indonesian arc abstract several lines of evidences which include change in trend and offset in arcs bathymetry and sedimentation faulting in the region change in composition and trend of the line of volcanoes spatial distribution of earthquakes and change in dip of the benioff zone suggest a tectonic segmentation of the burmese indonesian arc two transverse boundary zones viz the north andaman boundary zone and the sunda boundary zone separating segments of the arc are identified larger than usual thickness of the oceanic crust beneath the ninetyeast ridge reduces the average density of the lithosphere carrying the ridge the buoyancy effect caused by the reduced average density inhibits the subduction of an approximately triangular shaped lithosphere between the burmese indonesian arc and the ninetyeast ridge which in turn causes tectonic segmentation along the arc the model predicts and some available focal mechanism solutions confirm left lateral northeast faulting within the sunda boundary zone

volcanism on mercury a new model for the history of magma ascent and eruption 1 we use the identification of volcanism on mercury together with lobate flow front like topography crustal composition information and data on the stress state and history of the lithosphere to derive a new model for the ascent and eruption of magma on mercury we find that extrusion is likely to be dominated by high effusion rate events initial emplacement of dikes in the crust and extrusions to the surface will result in a denser crust favoring even more extrusive volcanism this will be followed by cooling associated with planetary thermal evolution that will rapidly decrease the ability of magma to reach the surface resulting in a decline and termination of volcanism following the emplacement and deformation of regional smooth plains these predictions and the resulting history can be tested with observations from missions to mercury

cenozoic evolution of the savu basin indonesia forearc basin response to arc continent collision abstract a seismo stratigraphic analysis of the savu basin relates the evolution of a complexly evolved forearc basin to the late miocene recent collision with australia initial early miocene subsidence of the outer forearc basin and the subsequent development of a middle late miocene volcanic proto arc located in the south of the basin both resulted from the late oligocene early miocene eastward propagation of the java timor trench block faulting of the relatively thick continental forearc basement probably determined the rather irregular and scattered pattern of middle late miocene volcanic centres just north of the accretionary prism the forearc basement is flexed down and reshaped into a trenchward dipping backstop that facilitates backthrusting of the accretionary prism the southern forearc basement probably acted as a barrier against compression the thickness of this continental basement is suggested to be a critical factor in the response of the forearc to continent collision owing to its strength the savu basin did not react to collision by the development of northward verging inter arc thrusts in contrast it responded to the underthrusting of continental crust by reactivation of the basement ridges this resulted in the differentiation of the forearc basin into a presently extinct and uplifted miocene south savu basin and a pliocene recent active north savu basin the simultaneous late miocene recent uplift of large segments of the outer forearc and subsidence of the north savu basin is interpreted to result from tectonic underplating of continental material and uplift caused by the buoyancy of the subducted continental crust the relatively recent decoupling of this crust from the oceanic slab and the subsequent isostatic rebound may superimpose the initial effects of post collision extension on arc continent collision

spherical shell tectonics effects of sphericity and inextensibility on the geometry of the descending lithosphere the shape of a deep seismic zone is thought to represent that of the descending slab of lithosphere the lithosphere before subduction is a spherical shell and the shape of the descending slab is the result of the deformation of the spherical lithosphere at the subduction zone upon bending a spherical shell often deforms in a very different way from a simple plate we examine whether the actual shape of the descending slab can be explained by a simple bending of an inextensible spherical shell which shows little surface deformation under moderate stress this examination is made region by region for most of the subduction zones in the world by means of an analogue method the lithosphere is simulated by an inextensible spherical shell made of polyvinyl chloride resin the wadati benioff zone is shaped by plaster by referring to the reliable hypocentral data selected from the international seismological centre isc bulletins the spherical shell is forced to fit the miniature of the wadati benioff zone fitting is first attempted only by bending if a good fit is not attainable and if a discontinuity or gap in seismic activity is observed in the relevant region the spherical shell is torn along this discontinuity or gap and the goodness of fit is reexamined the results of the analysis are summarized as follows 1 the shape of the wadati benioff zone can be simulated largely by a simple bending of a spherical shell without surface deformation 2 in almost all of the regions of poor fit with bending a good fit can be achieved by tearing the spherical shell along the trace of low seismicity the sphericity of the lithosphere and the inextensibility upon deformation are the two essential factors in controlling the slab shape this means that the lateral constraint is most important for understanding the geometry of the downgoing slab of lithosphere and the stress state within it further several problems related to the deformational characteristics of the spherical lithosphere are also reviewed and discussed in connection with subduction tectonics

mesozoic episodic magmatism in south korea and its tectonic implication 1 ne trending granitoid batholiths in south korea are part of the huge phanerozoic granitoid belt in east asia the crystallization ages of granitoid plutons show three episodes of mesozoic magmatism in south korea triassic 248 210 ma jurassic 197 158 ma and late cretaceous early tertiary 110 50 ma with a late jurassic early cretaceous 50 m y hiatus the granitoid ages and kinematic analyses of the mesozoic structures in south korea suggest that the middle jurassic 180 170 ma dextral strike slip ductile shear zones formed by the northwest orthogonal subduction of the izanagi plate whereas the cretaceous sinistral faults formed by the northward oblique subduction of the same plate which generally agree with proposed reconstruction of plate motions albeit uncertain around east asia among the models proposed to explain magmatic lull in a continental arc shallow angle subduction associated with high convergence rate and subduction of oceanic plateaus might be the probable cause for the magmatic hiatus in south korea and japan the huge width 500 km of the east asian granitoid belt may be explained by oceanward retreat of subduction zone in the cretaceous in association with change in the movement direction of the subducting plate

earthquake occurrence processes in the indo burmese wedge and sagaing fault region abstract earthquakes in the indo burmese wedge and sagaing fault regions occur in response to the partitioning of the india sunda motion along these two distinct boundaries under the accretionary wedge of the indo burmese arc majority of the earthquakes occur in the depth range of 30 60 km and define an eastward gently dipping seismicity trend surface that coincides with the indian slab the dip of the slab steepens in the east direction and earthquakes occur down to a depth of 150 km though the slab can be traced up to the 660 km discontinuity although these features are similar to a subduction zone the nature of the earthquakes and our analysis of their focal mechanisms suggest that these earthquakes are of intra slab type which occur on steep plane within the indian plate and the sense of motion implies a northward relative motion with respect to the sunda plate thus these earthquakes and the stress state do not support active subduction across the indo burmese arc which is also consistent with the relative motion of india sunda plates the absence of inter plate earthquakes lack of evidence of the occurrence of great earthquakes in the historical records and non seismogenic nature of the plate interface under the accretionary wedge suggest that seismic hazard due to earthquakes along the plate boundary may be relatively low however major intra slab earthquakes at shallow and intermediate depths may still cause damage in the sediment filled valley regions of manipur and cachar in india and chittagong and sylhet regions of bangladesh in the sagaing fault region earthquakes occur through dextral strike slip motion along the north south oriented plane and the stress state is consistent with the plate motion across the sagaing fault

tibet uplift and erosion abstract the 5 km high tibetan plateau is an outstanding topographic feature on the earth today its horziontal extent elevation and location cause significant effects on modern atmospheric circulation and climate so the history of uplift of the surface of tibet is linked to cenozoic climate changes at local regional and perhaps global scales geological and geophysical studies of the plateau are contributing data on the present and past deformation of the tibetan lithosphere that has formed the plateau primarily during the cenozoic the principal of isostasy then can be used to estimate the elevation history of the surface for a given deformation history different parts of tibet probably had different uplift histories but presently available data are not sufficient for distinguishing many contrasts in one scenario cretaceous and early cenozoic north south distributed shortening of tibetan crust and mantle lithosphere probably caused significant uplift of the surface relative to sea level to perhaps half of the present elevation by the early miocene thinning of the high density mantle portion of the lithosphere during the miocene may then have allowed the thick tibetan crust to rise close to its present elevation perhaps higher before 8 ma since then slow east west extension of tibet probably reduced the crustal thickness slightly and may have caused the elevation of the plateau to decrease during the late cenozoic erosion of tibet unlike narrow mountain belts has been unable to match the uplift of a broad plateau orographic precipitation and efficient river networks concentrate erosion on the edges while the interior is protected from significant erosion despite its lofty elevation the southern edge of the plateau the himalaya has suffered a minimum of 25 km of denudation since the miocene while central tibet shows little or no sign of major erosion since that time the gangdese arc in southern tibet was rapidly eroded during the mid miocene when 4 km of rock apparently were removed from the surface as shown by mineral cooling ages this pulse of erosion was probably caused by a combination of local thrust system movement and changes in base level and precipitation due to relative elevation changes between the gangdese and the himalaya to the south the modern long wavelength flatness of tibet is unlikely to have been caused by erosion and indicates viscous flow at some level of the lithosphere has been acting to level the plateau surface

seismicity and tectonics of the subducted cocos plate we have examined teleseismic earthquake locations reported by the international seismological centre isc for the middle america region and selected 220 as the most reliable these hypocenters and other data are used to delineate the deep structure of the subducted cocos plate the results indicate that the subducted plate consists of three major segments segment i extends from the panama fracture zone to the nicoya peninsula the structure of this segment is poorly defined segment ii is the largest and best defined segment this segment consists of two parts iia and iib part iia extends from the nicoya peninsula to western guatemala and is very well defined and continuous in structure its strike follows the curvature of the trench and dips at about 60 part iib extends from western guatemala to orizaba mexico the dip of this part of the segment decreases slightly toward the northwest and its strike is more northward than that of the trench segment iii extends from orizaba to the rivera fracture zone and is not well defined due to a lack of earthquake activity beneath about 100 km its orientation differs markedly from segment ii and strikes somewhat more westward than the trench between parts iia and iib of segment ii the subducted plate seems to be continuous bending smoothly to accommodate the change in geometry local network data from costa rica suggest there may be a tear between segments i and ii between segments ii and iii there is a gap in the hypocenters which makes it difficult to define the boundary the change in geometry between these two segments indicates that there may be a tear and two strike slip focal mechanisms in the region support this conclusion we find no convincing evidence supporting the existence of segments smaller than the three described above if there is smaller scale segmentation in the shallow part of the subducting plate the plate must still maintain enough continuity to appear continuous at greater depths there is no evidence for any major tear in the subducted plate associated directly with either the tehuantepec ridge or the orozco fracture zone the shallow subduction at the northwestern end of segment ii may be related to the bouyancy of the tehuantepec ridge the cocos ridge is probably directly responsible for the change in geometry between segments i and ii and may even be slowing or stopping subduction in segment i the structure of the subducted plate in segment ii and the changes in the character of volcanism along the arc can be related to the relative motion of the north american and caribbean plates the present geometry of part iib of segment ii is more consistent with the probable configuration of the trench about 7 ma ago than with the present configuration indicating that the north american plate is overriding the subduction zone appendices 2 3 and 4 are available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 009 2 50

how the state of stress varies in the wadati benioff zone indications from focal mechanisms in the wadati benioff zone beneath sumatra and java summary this study concentrates on the detailed determination of the state of stress in the wadati benioff zone in the lithosphere subducting under sumatra and java the orientations of p and t axes of earthquake focal mechanisms hcmt are used to define a system of domains in which a uniform state of stress is assumed the method of gephart forsyth is then used to determine the state of stress in each domain we succeeded in delimiting eight domains three si siii in the sumatra region and five ji jv in the java region domains with similar states of stress occur in both regions in similar positions the maximum compression 1 is perpendicular to the trench in domains si sii and jii that is within the depth range 0 165 km the orientation of 1 is almost parallel to the trench in domains siii and jiii depth range 25 225 km the boundary between domains sii and siii and jii and jiii is not distinct covering the depth range 40 165 km it seems that the focal mechanisms belonging to domains sii and siii and similarly to domains jii and jiii occur in different stress layers and that we observe an overlap of earthquakes with different focal mechanisms from two different stress state layers parallel to the wadati benioff zone in domains jiv and jv the states of stress correspond to the stress pattern observed in other subduction zones slab dip parallel extension is observed in domain jiv depth range 225 315 km and slab dip parallel compression is observed in domain jv deeper than 400 km

neocene and quaternary extent and geometry of the subducted pacific plate beneath north island new zealand implications for kaikoura tectonics abstract the extent and geometry of the obliquely subduced oceanic pacific plate beneath north island new zealand for five million year intervals through the mid miocene to quaternary are presented in a series of maps and cross sections these show that the subducted plate progressively increased its extent from ne to sw beneath the north island and in the more northern regions where it was first emplaced concomitantly increased its dip from 10 to 50 the changing extent and geometry of the subducted slab has been established from the age pattern of orogenic andesites and from the geochemical k 2 o h parameter of depth of magma generation the radiometric dates show a migration of the volcanic front back towards the trench at an average rate of 20 km my the trenchward migration is explained by a model of increasing slab dip which is corroborated by the k 2 o data calibrated against the presently active arc taupo volcanic zone with the exception of northern coromandel peninsula the andesitic magmas were generated at 85 100 km depth the interpretation of the dates adopted here indicates that the subducted slab originated at the ne sw trending kermadec hikurangi trench and implies a different and much simpler evolution of the australia pacific plate boundary in the vicinity of north island than other recent models subduction geometry has been found elsewhere to be a principal influence upon the state of stress and deformational style in an over riding plate the possibility is explored that the timing nature and pattern of the neogene to quaternary kaikoura orogeny in north island is due to this influence apart from the effect of oblique subduction in eastern north island there is an accord between the onset of deformation and the emplacement sequence of the shallow slab beneath north island and between the change in subduction geometry and a progressive north to south change in northern north island from compression to extension

pectinid coquinas and their palaeoenvironmental implications examples from the early miocene of northeastern egypt abstract the early miocene burdigalian section at gebel gharra sw suez egypt represents a mixed carbonate siliciclastic regime and is composed of a series of fining and deepening upward sequences mass occurrences of scutellid echinoids and pectinid bivalves are remarkable features within this section five pectinid horizons occurring in two superpositioned fining upward sequences are qualitatively and quantitatively studied in detail the taxonomic composition of respective pectinid accumulations clearly support a deepening upward interpretation with decreasing pectinid diversity decreasing convexity shell thickness and sculpturing the taphonomic parameters of the pectinid beds in combination with the study of co occurring biota and frequency and density of shell encrustations and bioerosion clearly point to different mechanisms for shell accumulations namely as composite concentrations and reworked event concentrations

relationships between plate motions and late cretaceous to paleogene magmatism in southwestern alaska four major late cretaceous to paleogene magmatic belts have been recognized in southwestern alaska and tentative boundaries have been drawn for each the belts indicate minimum ages by which the tectonostratigraphic terranes they overprint must have been linked together though there has been significant postmagmatic tectonism the kuskokwim mountains belt 73 60 m y b p the alaska range belt 74 55 m y b p and the gulf of alaska belt 62 57 m y b p all may have been related to a period of rapid north northeastward motion of the kuia plate from about 74 to 56 m y b p the angle of convergence was low with respect to the present orientation of the belts suggesting the possibility of postmagmatic rotation of the belts a higher angle of convergence may have existed with respect to the alaska range belt a probable arc if postmagmatic rotation resulted from 300 400 km of right lateral displacement on the denali fault the gulf of alaska belt is in an anomalous forearc position with respect to the alaska range belt but probably formed by a convergence related mechanism the kuskokwim mountains belt is in an apparent back arc position but the mechanism of its origin is uncertain it may have been part of an unusually broad alaska range arc or less likely it may have been a separate arc magmatic quiescence occurred throughout south western alaska during a period of plate reorganization from about 56 to 43 m y b p a period which began with extremely rapid northward motion of the kuia plate magmatism resumed when the present period of rather slow northwestward pacific plate motion began at about 43 m y b p the aleutian arc probably formed at this time and has been episodically active in about the same location ever since

circum mediterranean oligo miocene biogeographic evolution the gastropods point of view based on studies in iran qom basin esfahan sirjan basin zagros mountains in turkey mut and sivas basins in the mesohellenic basin and in northeastern egypt a new palaeobiogeographic concept for the oligocene and miocene in the circum mediterranean area with special emphasis on the distribution patterns of gastropod faunas is presented a very strict biogeographic terminology is proposed to avoid the common fusion of geographic faunistic and geodynamic terms in the palaeontologists jargon our main interest focuses on the affinities and the faunistic interactions between the early mediterranean and the early indo pacific regions during the oligo miocene especially the role of the early indo pacific faunas as the mythic centre of origination of early neogene european mollusc faunas as often vaguely indicated in the literature is critically evaluated this leads to the establishment of a mediterranean iranian province and a western indian eastern african province as palaeobiogeographic subunits of the western tethys region during the oligocene due to major geodynamic and related biogeographic changes in the early miocene the western tethys region no longer existed and was replaced by the proto mediterranean atlantic region for the area of the paratethys the palaeobiogeographic unit danubian province which includes the protocaspian subprovince is proposed furthermore the underrated transatlantic communication of the oligo miocene nearshore mollusc faunas is demonstrated altogether this paper is a contribution to the discussion on the biogeographic concepts classification and nomenclature in palaeontology as initiated by westermann palaeogeogr palaeoclimatol palaeobiogeogr 158 2000 1 13 163 2000 49 68 2002 elsevier science b v all rights reserved

constraints on 2004 sumatra andaman earthquake rupture from gps measurements in andaman nicobar islands the 26 december 2004 sumatra andaman earthquake mw 9 0 9 3 is the greatest earthquake of the modern seismological era the rupture characteristics of the earthquake particularly in the andaman nicobar region are not well resolved from seismological or far field geodetic data here in this article we present campaign mode global positioning system gps measurements of coseismic displacements at 13 sites in the andaman nicobar islands before and after the 2004 sumatra andaman earthquake these measurements provide improved estimates of rupture characteristics in the region coseismic horizontal ground displacement of 1 5 6 5 m towards the southwest and coseismic vertical displacement mostly subsidence of 0 5 2 8 m occurred along the andaman nicobar islands with maximum displacements in the nicobar islands we estimate coseismic slip under the andaman and nicobar islands as 3 8 7 9 m and 11 15 m respectively the length of the rupture is estimated to be about 1500 km with a width varying from 120 km under middle andaman island to 160 km under great nicobar island gps measurements during january 11 22 2005 from port blair suggest rapid afterslip in the postseismic period limited gps data available from 1995 measurements at two sites in andaman provide evidence of strain accumulation that varied significantly in the 10 yr preceding the earthquake d 2005 elsevier b v all rights reserved

trench motion slab geometry and viscous stresses in subduction systems summary a semi analytic 3 d model for subduction within a newtonian viscous upper mantle provides a dynamically consistent means of computing viscous stress trench motion and slab geometry in subduction systems although negative slab buoyancy provides the basic driving force for subduction slabs that extend from the surface to the base of the upper mantle are oversupported by viscous stresses in the shallow 100 150 km mantle and undersupported by viscous stresses at greater depth in the upper mantle these deeper parts of the subduction system act as an engine for subduction while shallower parts act as a brake on trench motion trench migration rates and slab geometry reflect a competition between these two effects during steady state subduction trench migration rates vary approximately linearly with slab buoyancy and model rates of trench motion are in good agreement with the range of observed rates for a two layer upper mantle viscosity of 2 10 20 pa s above 300 km and 5 10 20 pa s below steady state slab dip increases as slab density decreases especially for very low density slabs which dip significantly more steeply than high density slabs the horizontal velocity at the top of the lower mantle measured relative to the foreland has a very large effect on trench migration rates rivalling or even exceeding that of slab buoyancy slab width parallel to the trench also has a significant effect on trench migration rates due to the viscous pressure of toroidal flow around the slab the stiffness of the subducting lithosphere does not exert a significant effect on trench migration rates or slab geometries for rigidities compatible with oceanic lithosphere very stiff slabs with elastic plate thicknesses more that 40 km or viscosities in the range of 10 25 10 26 pa s subduct significantly more slowly than weak slabs with trench migration rates in the range of half to a third that of weak slabs large unexpected effects on trench migration rates and slab geometry are exerted by the structure and density of the frontal prism and overriding plate indicating that local geology can exert important constraints on slab dynamics during non steady state subduction rates of trench migration respond rapidly as variably buoyant lithosphere penetrates into the asthenosphere in the absence of other driving forces for convergence trench migration rates can change by a factor of two or more in as little as 2 3 myr for example from 35 to 70 mm yr 1 when an oceanic piece of slab follows a continental one into the subduction system subduction of variable buoyancy lithosphere is accompanied by changes in slab dip with depth and through time

assessing the role of slab rheology in coupled plate mantle convection models reconstructing the 3d structure of the earth s mantle has been a challenge for geodynamicists for about 40 yr although numerical models and computational capabilities have substantially progressed parameterizations used for modeling convection forced by plate motions are far from being earth like among the set of parameters rheology is fundamental because it defines in a non linear way the dynamics of slabs and plumes and the organization of lithosphere deformation in this study we evaluate the role of the temperature dependence of viscosity variations up to 6 orders of magnitude and the importance of pseudo plasticity on reconstructing slab evolution in 3d spherical models of convection driven by plate history models pseudo plasticity which produces plate like behavior in convection models allows a consistent coupling between imposed plate motions and global convection which is not possible with temperature dependent viscosity alone using test case models we show that increasing temperature dependence of viscosity enhances vertical and lateral coherence of slabs but leads to unrealistic slab morphologies for large viscosity contrasts introducing pseudo plasticity partially solves this issue producing thin laterally and vertically more continuous slabs and flat subduction where trench retreat is fast we evaluate the differences between convection reconstructions employing different viscosity laws to be very large and similar to the differences between two models with the same rheology but using two different plate histories or initial conditions

early burdigalian infill of the puchkirchen trough north alpine foreland basin central paratethys facies development and sequence stratigraphy abstract to improve the prediction of gas and oil in strata along the tectonically imbricated southern margin of the puchkirchen trough north alpine foreland basin central paratethys a better understanding of facies distribution and stratigraphic control of the basin is essential the present study provides bio and chemofacies analyses and a biostratigraphic evaluation for the pelitic hall formation from the borehole hochburg 1 in the central part of the puchkirchen trough a statistical evaluation of benthic foraminiferal assemblages together with geochemical proxy records toc sulphur hydrogen index 13 c toc 15 n tn reveals a succession of early burdigalian depositional environments following a major subaqueous erosional hiatus conglomeratic sands at the base of the section which contain reworked chattian and aquitanian foraminiferal assemblages record the reactivation of a basin axial channel system during the early burdigalian eustatic sea level rise the channel was cut off from its sediment sources on the shelf and a deepening bathyal environment was established agglutinated foraminiferal assemblages with abundant bathysiphon filiformis developed that were adapted to an unstable environment with frequent deposition of turbidites the middle part of the hall formation is characterized by prograding high relief clinoforms which are fed by the wachtberg delta of the paleo inn river high sedimentation rates are interpreted and increased input of terrestrial derived organic matter is documented with a strong correlation of toc and toc s and low hi values as well as frequent occurrences of ammodiscus spp and other opportunistic agglutinating foraminifers a renewed transgression reestablished a eutrophic and suboxic bathyal environment followed by the development of an oxygenated outer middle neritic shelf environment heralding the ultimate infill of the deep water basin based on a comparison of the revealed development to existing sequence stratigraphic models for the puchkirchen trough three sequences and their corresponding systems tracts can be identified for the lower middle and upper hall formation biostratigraphic evidence from benthic foraminifers and calcareous nannoplankton indicates that they correspond to the regional substages of the middle and upper eggenburgian and lower ottnangian and to global 3rd order sequences bur 1 3 the observations suggest a primary control of eustatic sea level on the puchkirchen trough rather than alpine tectonics during the burdigalian

does mantle convection currently exist on mercury abstract the full equations of convective motion over the age of the solar system can be solved in a few hours 2d to several weeks 3d with numerical simulations in cartesian or spherical geometry using the 2d cartesian geometry code conman we model the thermal evolution of mercury our preferred calculation has an initial rayleigh number of 8 10 4 no mantle internal heating and a constant core heat flux of 9 75 mw m 2 in a non newtonian rheology these values all fall within the range of previously estimated parameters from 1d calculations and allow for weak mantle convection to persist until present supporting the outer core dynamo explanation of mercury s intrinsic magnetic field further the peak dynamic topography and geoid anomalies from our model are about 179 m and 37 m respectively values that are not unrealistic and should be well within the range of observation by the messenger spacecraft future spacecraft observations yielding new and more accurate data will help us better understand the thermal evolution of mercury and the origin of its magnetic field

nature of opaque components on mercury insights into a mercurian magma ocean 1 analysis of mariner 10 and messenger data sets reveal the importance of opaque components on mercury s surface a global darkening agent suggested to be ilmenite or other fe ti bearing opaque mineral has been invoked to explain the lower albedo of mercury relative to the lunar highlands separately a low reflectance material lrm has been recognized as one of three dominant color terrains we present laboratory reflectance spectra of ilmenite size separates and other candidate fe ti bearing oxide minerals these oxides cannot sufficiently darken mercury without violating neutron spectrometer constraints on surface iron content the spectra of all samples exhibit negative spectral slopes shortward of 500 nm consistent with the lrm we review models of crystallization of an feo poor mercurian magma ocean and show that lack of a plagioclase flotation crust could lead to a thin quench crust with near surface layers of incompatible and ti rich late stage cumulates consistent with mercury s albedo and lrm citation riner m a p g lucey s j desch and f m mccubbin 2009 nature of opaque components on mercury insights into a mercurian magma ocean geophys res lett 36 l02201 doi 10 1029 2008gl036128

plio quaternary geodynamic evolution of a segment of the peruvian andean cordillera located above the change in the subduction geometry the cuzco region abstract the cuzco region which is located above a change in subduction geometry appears to be characterized by a variable plio quaternary tectono sedimentary evolution essentially located along the major fault system that separates the high plateaux from the eastern cordillera after the higher surface formation of the high plateaux a set of neogene basins were filled by miocene fluvio torrential series and by plio pleistocene fluvio lacustrine deposits the neogene series have been affected by compressional tectonic forces attributed to the late miocene this compression is followed by roughly e w trending syn sedimentary extensional tectonics attributed to the pliocene it is related to reactivation of the pre existing major faults basin evolution and volcanic activity concentrated along the faults in the early pleistocene fluvio lacustrine deposits are affected by syn and post sedimentary compressional tectonism it is characterized by shortening that trends both n s and e w and produces folding and faulting of the sedimentary cover extensional tectonism trending roughly n s has been taking place from the middle pleistocene to the present it is coeval with shoshonitic volcanic activity and with sedimentation of fluvio lacustrine terraces torrential fans and moraines quaternary and active normal faults due to this tectonism are located in a narrow zone more than 100 km long between the high plateaux and the eastern cordillera and two 15 km long fault sectors in the eastern cordillera characteristic pleistocene scarps 400 m or more high are due to the cumulative normal offset and there are also little scarps with heights ranging between 2 and 20 m which are related to holocene fault reactivations recent fault reactivation on the cuzco fault system during the april 5 1986 earthquake m b 5 3 is due to the n s trending extension this state of stress located at a mean elevation of roughly 3730 m is generally homogeneous to different scales the active cuzco normal faults may be a consequence of adjustment between the compensated western cordillera and the undercompensated eastern cordillera this latter being uplifted higher than its isostatic equilibrium due to compression acting on its eastern edge the variation of the state of stress during the plio quaternary is in agreement with the variations of the compressional boundary forces it may be explained by variation of the convergence rate or by the variation of pull slab forces

computer based mollusc stratigraphy a case study from the eggenburgian lower miocene type region ne austria abstract the eggenburg group comprises the type deposits of the lower miocene central paratethys regional stage eggenburgian in ne austria these marine to estuarine deposits transgress in northwestern direction either on palaeozoic rocks of the bohemian massif or on oligocene to lower miocene fluviomarine sediments they are known from three palaeo geographic units situated on the southeastern margin of the bohemian massif these are 1 the southern marginal zone 2 the northwestern n s striking horn basin and 3 the northeastern crystalline plateau termed the eggenburg bay the present study investigates distribution patterns of the exceptionally diverse mollusc fauna composed of 321 taxa species and subspecies within and between those units hierarchical cluster analysis is employed using ward s method and squared euclidean distance calculation a comprehensive database with 1900 sets based on field museum and literature investigations allows compilation of 50 taxonomic lists samples for 30 different localities and six lithostratigraphic units four marine and two brackish formations the resulting grouping pattern implies the existence of four groups three corresponding with palaeo geography and one including low diversity taxon poor samples the analysis of the diversity distribution supports the interpretation that the lower diversity of samples from marine formations stems from secondary distortions such as diagenesis or insufficient sampling hence the samples characterised by a species richness value lower than half of the maximum species richness value of the corresponding formation were excluded from analysis the resulting grouping pattern verified the secondary distortion premise the pattern remained the same except that the low diversity cluster became restricted to samples of brackish origin among the marine samples palaeoecological differences between two fully marine formations from the eggenburg bay were of minor importance as they could not be clearly separated in contrast the individual palaeogeographic regions were shown to be taxonomically coherent but inconsistent with one other as palaeobiogeographic processes were implausible in such a small geographic area the explanation of these differences is based on the time factor including speciation extinction and faunal migration processes the relative stratigraphic position of the horn basin between the marginal zone and the eggenburg bay is implied by hierarchically ordered similarities in order to also indicate their chronological ordinance the index fossil distribution was analysed the chronological pattern is unequivocally provided by the abundant presence of the lower eggenburgian pectinid chlamys gigas schlotheim which is restricted to the marginal zone and the horn basin and by the abundant upper eggenburgian to lower ottnangian pectinids flexopecten palmatus lamarck and chlamys holgeri geinitz which are restricted to the eggenburg bay this refinement of the regional stratigraphic pattern shed light on the eggenburgian marine flooding progression early in the transgression the southern marginal zone was first marine flooded at the same time the horn basin developed an estuarine environment the next phase installed the fully marine environment in the horn basin at the same time the eggenburg bay still a mainland represented a southward pointing cape it first became flooded at the beginning of the upper eggenburgian

the interior structure of mars implications from snc meteorites two end member models of mars present interior structure are presented the first model is optimized to satisfy the geochemical data derived from the snc meteorites in terms of the bulk chondritic ratio fe si 1 71 while the second model is optimized to satisfy the most probable maximum value c 0 366 x m p r p 2 of the polar moment of inertia factor hydrostatic equilibrium and stationary heat transfer are assumed and the basic differential equations for the mechanical and thermal structure are solved numerically together with an isothermal murnaghan birch type equation of state truncated in eulerian strain at forth order we obtain the radial distribution of mass hydrostatic pressure gravity temperature and heat flow density along with the corresponding density stratification viscosity profiles and the global seismic velocity structure of model mars the first model being consistent with the geochemical requirement produces c 0 357 x m p r p 2 whereas the second model commensurate with the geophysical constraint gives fe si 1 35 the calculated central pressure is about 40 gpa in both models and the central temperature is in the 2000 to 2200 k range the model calculations suggest a fe ni fes core a little less than one half of the planetary radius in size surrounded by a silicate mantle subdivided into lower spinel and upper olivine layers and overlain by a 100 to 250 km thick basaltic crust and a surface heatflow density of 25 to 30 mw m 2 in both models the pressure in the mantle is not sufficient for the spinel to perovskite transition to occur the present thermal lithosphere is estimated to be about 500 km thick and to be subdivided into a 300 km thick outermost rheological lithosphere and an underlying thermal boundary layer of mantle convection given the core sulfur content of 14 wt as derived from snc meteorites the martian core is found to be entirely molten implying the nonoperation of a self sustained dynamo due to the absence of sufficiently vigorous convection

seismo tectonics of the andaman nicobar islands abstract the epicentres of 345 earthquakes that occurred between january 1967 and december 1982 in a region bounded by 4 n to 16 n latitudes and 90 e to 98 e longitudes were plotted in order to study the seismicity of this region most of the earthquakes in this region are shallow focus the greatest focal depth being about 250 km in the northern part of this region north of 10 5 n tne earthquake foci are aligned along two branches which strike northeastwards no intermediate depth earthquakes are associated with the eastern branch south of 10 5 n the foci are located along a zone striking north northwest focal mechanism solutions are reported for 24 earthquakes that occurred in this region during this period the first motions of p and pkp phases were used to determine the mechanism solutions five of these events occurred in northern sumatra and the rest occurred in the andaman nicobar islands the mechanism solutions of the shallow focus earthquakes in northern sumatra are characterized by a predominant component of thrust and strike slip faulting the inferred orientation of slip vectors deduced from mechanisms of shallow focus events is nearly northeast mechanism solutions of 13 shallow focus events that occurred in the andaman nicobar islands are characterized by thrust normal and strike slip faulting the orientation of slip vectors deduced from thrusting mechanisms is consistent with the underthrusting of the indian ocean under the andaman nicobar islands in a northeasterly direction normal faulting mechanism solutions of these islands suggest that they are tectonically similar to island arcs the results reported in this study show for the first time using mechanism solutions the existence of underthrusting along the andaman nicobar islands for which there is ample geological evidence

cocos nazca slab window beneath central america abstract integration of petrologic and tectonic data favours a model of slab window formation beneath central america in the pliocene pleistocene central america has been the site of voluminous cenozoic arc volcanism the cocos and nazca plates which are subducting beneath central america are diverging along the east trending cocos nazca spreading ridge since 25 ma the americas have advanced about 1800 km west over the ridge transform system since at least 8 ma plate integrity and the ridge transform configuration have been preserved during convergence resulting in subduction of the spreading ridge and development of a slab window the panama fracture zone an active transform fault is the part of the ridge transform system currently being subducted the ridge transform system formerly adjoining the northern end of the panama fracture zone is likely to have been left stepping we use present day plate motions to design a slab window to fit known variations in igneous composition hypocentre distribution and mantle anisotropy the modeling demonstrates that subduction of ridge segments and resultant slab window development began between 6 and 10 ma cessation of ridge subduction occurred between 1 and 3 ma when subduction of the panama fracture zone is considered to have begun the slab window is continuing to expand and migrate northeastward below the central american volcanic arc the absence of a wadati benioff zone from southeastern costa rica through panama corresponds to the position of the slab window within this region dacitic and rhyolitic volcanic rocks have adakitic compositions and are thought to result from anatexis of the young buoyant crust which forms the trailing edges of the slabs bounding the window basalts in this area were derived from an enriched ocean island type mantle source whereas basalts from the rest of the arc in nicaragua el salvador and guatemala are mainly derived from slab modified depleted mantle characteristic of volcanic arcs the presence of ocean island type mantle beneath southern costa rica and panama is explained by eastward flow of enriched asthenosphere from the galapagos plume head through the slab window and into the volcano source region eastward transfer of asthenosphere is consistent with global plate motion studies and seismic anisotropy in the asthenosphere beneath the nazca and caribbean plates the flow of peridotite is a consequence of progressive shrinkage of the pacific mantle reservoir and concurrent expansion of the atlantic mantle reservoir

late paleogene extensional collapse of the cordilleran foreland fold and thrust belt the cordilleran foreland fold and thrust belt collapsed and spread to the west during a middle eocene to early miocene ca 49 20 ma episode of crustal extension the sedimentary and structural record of this event is preserved in a network of half grabens that extends from southern canada to central utah extensional structures superposed on this allochthonous terrain are rooted to the physical stratigraphy structural relief and sole faults of preexisting thrust fold structures the sole faults dip 3 6 west above an undeformed precambrian crystalline basement and accommodated tectonic transport of a thick up to 20 km eastward tapering hanging wall during regimes of crustal shortening and extension the chronology of tectonism for the foreland fold and thrust belt is established here by dating latest thrusting and initial normal faulting and is best defined where thrusts and normal faults are linked by common detachment surfaces dated movement on two extensionally reactivated thrusts the lewis thrust of northwest montana and southeast british columbia and the medicine butte thrust of southwest wyoming and northeast utah suggests that the hiatus between the end of crustal shortening in the early or early middle eocene and the start of extension in the early middle eocene was brief lateral spreading and extensional basin formation in the cordilleran foreland fold and thrust belt were partly concurrent with formation of metamorphic core complexes and regional magmatism conceptually linking extensional processes that were simultaneously deforming both the hinterland and foreland of the late paleogene cordilleran orogenic wedge is accomplished by applying the extensional wedge coulomb critical taper model the rapid drop in north america pacific plate convergence rate and or steepening of the subducted oceanic slab at ca 50 ma resulted in a large reduction in east west horizontal compressive stress in the cordillera as a result the cordilleran orogenic wedge was left unsupported and it gravitationally collapsed and horizontally spread west until a new equilibrium was established at ca 20 ma subsequently crustal extension and magmatism during the basin and range event ca 17 0 ma overprinted much of this earlier phase of extension

the geometry of the wadati benioff zone under southern central america and its tectonic significance results from a high resolution local seismographic network abstract we present here a detailed geometry of the wadati benioff zone under costa rica obtained from seismicity recorded by a dense local seismographic network jointly operated by the costa rica volcanological and seismological observatory national university and the charles f richter seismological laboratory university of california santa cruz underneath the nicaragua costa rica border the wadati benioff zone smoothly contorts from steep to shallow dip angles nw to se but does not show evidence of a brittle tear as postulated by others however further to the se ne of puerto caldera costa rica the wadati benioff zone does show a segmentation the quesada sharp contortion at intermediate depths h 70km nw of this sharp contortion the deepest portion of the seismically active slab dips at about 80 and reaches maximum depths ranging from 200 km near the nicaragua costa rica border to 135 km under ciudad quesada to the se the deeper portion of the wadati benioff zone dips at about 60 and the seismicity does not extend below depths ranging from 125 km behind the volcanic arc to 50 km east of quepos in southern costa rica east of 83 55 w we find no evidence of the wadati benioff zone deeper than 50 km the obtained geometry and other known tectonic features related to the subduction of the cocos plate under the caribbean plate along the southern terminus of the middle america trench nicaragua and costa rica correlate well with along trench variations in age of the subducted cocos plate some of these tectonic features are 1 the shallowing of middle america trench bathymetry from nw to se 2 variations in the energy release within the subducted slab 3 differences in coupling between cocos and caribbean plates 4 the termination of the central america volcanic chain in central costa rica 5 distinct stress field variations on the overriding caribbean plate the subduction of the cocos ridge under southern costa rica is partially responsible for some of these features shallowing of the middle america trench in southern costa rica and for the high uplift rates of the outer arc however as the presence of the panama fracture zone limits the subducted extension of the cocos ridge to less than 100 km from the trench we propose that the overall geometry of the wadati benioff zone is controlled by abrupt along trench changes in the age of the subducted cocos plate

pop up tectonics of the shillong plateau in northeastern india insight from numerical simulations article i nfo the shillong plateau in northeastern india represents one of the most seismically active pop up structures within the peninsular shield area in order to constrain the role of the inferred oldham fault in the northern boundary of the plateau we performed 2 d finite element method fem simulations for convergent displacement caused by northeastward movement of the indian plate with respect to the eurasian plate various rock properties density poisson s ratio young s modulus cohesion and angle of internal friction and the mohr coulomb failure criterion are used to evaluate failure and faulting patterns two plane strain models with appropriate boundary conditions were also calculated the predicted maximum compressive stress 1 shows a preferred orientation that helps explain the tectonic environment and the fault pattern the best fit model suggests that a compressive stress regime is dominant in the study area everywhere except for the uppermost part of the crust where extensional stress dominates with increased progressive convergent displacement the modeled 1 are predicted to rotate counterclockwise around the fault zones the simulation results suggest that the oldham fault does not have a significant role in the development of stress and deformation distribution in the area we also infer that the tectonically induced deformation in both the plateau and the adjoining areas is restricted to mainly within the crust b30 km

seismicity and tectonic stress field of a part of the burma andaman nicobar arc abstract the nature of instrumentally recorded seismicity for a part of the burma andaman nicobar arc bounded by latitudes 2 and 24 n and longitudes 91 and 99 e was investigated based on the spatial distribution of the earthquakes and their focal mechanism solutions four tectonic units were identified from the south going northward these are the sumatra trench region characterized by strike slip and thrust faulting the andaman spreading ridge characterized by normal and a few strike slip faults the andaman nicobar ranges where faulting is mostly thrust type and the indo burman ranges region dominated by thrust faulting the stress orientations in these segments and their changes with depth were studied in relation to the tectonics of the region the direction of maximum compression in the burma andaman nicobar region is ne sw to n s compatible with the postulated motion of the indian plate changes in stress orientations are attributable to local tectonics