lower silurian aeronian megafaunal and conodont biofacies of the northwestern michigan basin lower silurian llandovery aeronian carbonates of the burnt bluff group northwestern michigan basin represent a transect along a southward dipping ramp that extends from tidal flat to basin environments benthic megafaunas include an ostracod biofacies tidal flat stromatoporoid coral biofacies very shallow subtidal pentamerid crinozoan and crinozoan stromatoporoid biofacies deeper subtidal and a crinozoan sponge biofacies distal ramp and basin the crinozoan sponge biofacies which includes diverse small crinozoan ossicles 19 types of siliceous sponge spicules and at least 65 other taxa has a biota that is similar to those of silurian continental margins megafaunal biofacies indicate an early silurian gradient going from a shoreline in the north to water depths of perhaps 60 m in the south the burnt bluff group contains conodonts of the icriodella discreta icriodella deflecta zone as an onshore biofacies with panderodus kockelella ozarkodina icriodella and oulodus and an offshore biofacies with panderodus walliserodus and aspelundia rcsumc les carbonates du silurien infkrieur llandovery aeronien du groupe de burnt bluff dans la rcgion nord ouest du bassin de michigan rephentent un transect le long d une rampe inclince vers le sud s ktalant d un environnement d estran tidal a celui d un bassin les mkgafaunes benthiques incluent un biofaciks a ostracodes estran tidal un biofaciks a stromatoporoi des coraux infratidal trks peu profond des biofaciks a pentamkrides crinozoaires et crinozoaires stromatoporoi des infratidal profond et un biofaciks a crinozoaires cponges rampe distale et bassin le biofaciks a crinozoaires cponges qui inclut diverses petites plaques de crinozoaires 19 types de spicules d cponges siliceux et au moins 65 autres taxons posskde un biote qui est similaire a ceux des marges continentales siluriennes les biofaciks mkgafauniques indiquent un gradient au silurien prccoce allant d une ligne de rivage dans le nord jusqu a des profondeurs d eau atteignant peut etre 60 m dans le sud le groupe de burnt bluff contient des conodontes de la zone a icriodella discreta icriodella deflecta comme biofaciks intracdtier accompagncs de panderodus kockelella ozarkodina icriodella et oulodus et un biofaciks extrac6tier avec panderodus walliserodus et aspelundia traduit par la rcdaction

trench curvature and deformation of the subducting lithosphere summary the subduction of oceanic lithosphere is generally accompanied by downdip and lateral deformation the downdip component of strain is associated with external forces that are applied to the slab during its sinking namely the gravitational force and the mantle resistance to penetration here we present theoretical arguments showing that a tectonic plate is also subject to a predictable amount of lateral deformation as a consequence of its bending along an arcuate trench zone independently from the long term physical processes that have determined the actual curvature of the subduction zone in particular we show that the state of lateral strain and the lateral strain rate of a subducting slab depend from geometric and kinematic parameters such as trench curvature dip function and subduction velocity we also demonstrate that the relationship between the state of lateral strain in a subducting slab and the geometry of bending at the corresponding active margin implies a small component of lateral shortening at shallow depths and may include large extensional lateral deformation at intermediate depths whereas a state of lateral mechanical equilibrium can only represent a localized exception our formulation overcomes the flaws of the classic ping pong ball model for the bending of the lithosphere at subduction zones which lead to severe discrepancies with the observed geometry and style of deformation of the modern subducting slabs a study of the geometry and seismicity of eight modern subduction zones is performed to assess the validity of the theoretical relationship between trench curvature slab dip function and lateral strain rate the strain pattern within the eight present day slabs which is reconstructed through an analysis of harvard cmt solutions shows that tectonic plates cannot be considered as flexible inextensible spherical caps whereas the lateral intraslab deformation which is accommodated through seismic slip can be explained in terms of deviations from the mechanical equilibrium

ludlow silurian lithistid and hexactinellid sponges cape phillips formation canadian arctic abundant hexactinellid and lithistid sponges occur in ludlow silurian platform slope strata of the cape phillips formation canadian arctic islands the following are new in the anthaspidellidae miller 1889 rhodesispongia simplex n gen and sp climacospongia snowblindella n sp and climacospongia undulata n sp in the sphaerocladina schrammen 1910 caryospongia tuberosa n sp in the hexactinellida schmidt 1870 malumispongium cornwallisi n sp of the monospecific superfamily malumispongium rigby 1967 other previously described sponges recognized in the study assemblage include hindia sphaeroidalis duncan 1879 and caryospongia juglans rauff 1894 these occur in association with abundant archaeoscyphia sp in thin laterally extensive lithistid sponge biostromes which throve on platform slope sediments during the late silurian based on the dominant dendroclone forms in the skeletons of these and other genera anthaspidellidae can be divided informally into two groups i the polyclonids p

syndepositional diagenesis of modern platform carbonates evidence from isotopic and minor element data physical elemental and stable isotopic evidence documents significant syndepositional diagenesis off biogenic carbonate in modern sediment pore water halimeda aragonite and neogoniolithon high mg calcite fragments from sediment cores and experimental substrates were compared with fresh samples to determine the magnitude of minor element sr mg and stable isotopic c and o compositional shifts although no significant shift in bulk sediment mineralogy is apparent experimental substrates and natural biogenic grains exhibit significant diagenetic changes these changes include dissolution textures net loss of mass changes in minor element composition and progressive evolution toward carbon and oxygen isotopic equilibrium with p ore water our results demonstrate that carbonate sediment can undergo rapid syndepositional dissolution and reprecipitation pore waters likely are buffered chemically and isotopically by most abundant carbonate grain types in turn compositions of less abundant carbonate grains may shift toward equilibrium with respect to pore water therefore many components of shelf limestone have compositions controlled by chemically evolved pore waters rather than by seawater from which the biotic carbonate originally precipitated

lateral stretching of the philippine sea plate subducting along the nankai suruga trough focal mechanisms are investigated for shallow earthquakes h 60 km of central japan by using telemetered seismic data of microearthquakes shallow earthquakes in central and southwestern japan can be distinguished into three groups upper crustal earthquakes subcrustal earthquakes and interplate thrust earthquakes the first and the second ones being subjects of the present study most of the upper crustal earthquakes are of strike slip type or reverse faulting with e w to se nw compression whereas the subcrustal earthquakes are characterized by strike slip or normal faulting with e w to ne sw extension the subcrustal seismic zone subparallel to the nankai trough becomes progressively deeper landward to the north and progressively shallower to the east so that it emerges into a very shallow seismic activity in the izu peninsula which has been interpreted as a microcontinent of the philippine sea plate colliding against the eurasian plate on the basis of hypocentral distribution three dimensional seismic velocity structure and focal mechanisms subcrustal earthquakes are considered to occur within the subducting philippine sea plate that extends down to a depth of at least 60 km on the basis of this and other focal mechanism studies stress trajectories are drawn for the subducting philippine sea plate and for the overriding eurasian plate the overriding plate is on the whole under the e w compression which coincides with the moving direction of the pacific plate relative to the eurasian plate the subducting plate on the other hand is under lateral extension along its strike we interpret that the latter is a stress regime unique to an incipient subduction zone where the subducting plate at subcrustal depths is forced to warp downward being constrained laterally at the arc arc junctions and thereby comes under lateral stretching the lateral constraints arise either from apparent increase in flexural strength of the plate at the arc arc junction or from buoyancy of a microcontinent that resists subduction

stratigraphic controls on vertical fracture patterns in silurian dolomite northeastern wisconsin vertical opening mode fractures are mapped on quarry walls to assess the stratigraphic controls on fracture patterns in the relatively undeformed silurian dolomite of northeastern wisconsin our two stage study uses maps of vertical fractures to assess the effectiveness of various types of stratigraphic horizons e g organic partings or cycle bounding mud horizons in terminating opening mode fractures first the mechanical stratigraphy of the exposures is interpreted from the observed fracture pattern both visual inspection and a newly developed quantitative method are employed to identify effective mechanical interfaces the two methods show similar results confirming the validity of qualitative visual inspection the second stage of our study stochastically predicts mechanical stratigraphy and subsequent fracture pattern from empirical relationships between the observed sedimentary stratigraphy and the interpreted mechanical stratigraphy for example 63 of cycle bounding mud horizons within the inner middle and middle shelf facies associations serve as mechanical interfaces these empirical percentages are input to a monte carlo analysis of 50 stochastic realizations of mechanical stratigraphy comparisons of the stochastically predicted and interpreted mechanical stratigraphy yield errors ranging from 13 to 33 this method yields far better results than assuming that all stratigraphic horizons act as mechanical interfaces the methodology presented in this article demonstrates an improved prediction of fracture pattern within relatively undeformed strata from both complete characterization of sedimentary stratigraphy and understanding mechanical controls on fracturing

a stepped karst unconformity as an early silurian rocky shoreline in guizhou province south china abstract where succeeded by marine strata karst unconformities signify a former rocky coastline such relationships may help sort out relative sea level changes and aspects of local geography controlling facies distribution an exceptional example of an early silurian karst shore is well exposed near the village of wudang in central guizhou province near the capital city of guiyang in south china here the lower silurian kaochaitien formation oversteps 63 m of paleotopographic relief in limestones belonging to the llanvirn guniutan formation and caradoc to early ashgill huanghuachong formation ordovician the corresponding rise in sea level took place coeval with tectonic uplift as confirmed by a regionally diachronous relationship in the ordovician silurian boundary across a 250 km track from central to northern guizhou province the change in sea level also fits with a global rise of sea level in late aeronian later llandovery early silurian time borings of the ichnofossil trypanites are reported from the karst surface of the huanghuachong formation and silurian strata fill sink holes in this unit over 5 m deep the silurian karst shoreline near wudang is integrated with other regional data to construct a paleogeographic map covering the northern half of guizhou province

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

geological evidence for the geographical pattern of mantle return flow and the driving mechanism of plate tectonics tectonic features at the earth s surface can be used to test models for mantle return flow and to determine the geographic pattern of this flow a model with shallow return flow and deep continental roots places the strongest constraints on the geographical pattern of return flow and predicts recognizable surface manifestations because of the progressive shrinkage of the pacific averaging 0 5 km2 yr over the last 180 m y this model predicts upper mantle outflow through the three gaps in the chain of continents rimming the pacific caribbean drake passage australian antarctic gap in this model upper mantle return flow streams originating at the western pacific trenches and at the java trench meet south of australia filling in behind this rapidly northward moving continent and providing an explanation for the negative bathymetric and gravity anomalies of the australian antarctic discordance the long continued tectonic movements toward the east that characterize the caribbean and the easternmost scotia sea may be produced by viscous coupling to the predicted pacific outflow through the gaps and the caribbean floor slopes in the predicted direction if mantle outflow does pass through the gaps in the pacific perimeter it must pass beneath three seismic zones central america lesser antilles scotia sea none of these seismic zones shows foci below 200 km mantle material flowing through the caribbean and drake passage gaps would supply the mid atlantic ridge while the java trench supplies the indian ocean ridges so that deep mantle upwellings need not be centered under spreading ridges and therefore are not required to move laterally to follow ridge migrations the analysis up to this point suggests that upper mantle return flow is a response to the motion of the continents the second part of the paper suggests a possible driving mechanism for the plate tectonic process which may explain why the continents move this hypothetical driving mechanism has four aspects 1 the lower mantle convects 2 this convection drives the continents by drag on their deep roots 3 return flow in the upper mantle provides a volumetric balance for the motion of the continental masses without passing under them and 4 oceanic plates are effectively decoupled from the asthenosphere and are driven largely by slab pull this mechanism accounts for the opening of the atlantic the ability of spreading ridges to stay on the midlines of oceans and the penetration of india into asia following their collision continental motions strongly imply lower mantle upwelling and divergence beneath the atlantic and southeast indian oceans and convergence and subsidence along the tethyan belt this picture disagrees with the concept derived from hot spot studies of an undeforming absolute reference framework at depth but weaknesses in current hot spot theory would make a rejection of the present model on this ground premature if the present model is generally correct a fairly simple pattern of lower mantle convection with as few as four cells may explain much of the tectonic complexity of the earth

generalized inverse of a reduced gravity primitive equation ocean model and tropical atmosphere ocean data a nonlinear 2 layer reduced gravity primitive equations pe ocean model is used to assimilate sea surface temperature sst data from the tropical atmosphere ocean tao moored buoys in the tropical pacific the aim of this project is to hindcast cool and warm events of this part of the ocean on seasonal to interannual timescales the work extends that of bennett et al who used a modified zebiak cane coupled model they were able to fit a year of 30 day averaged tao data to within measurement errors albeit with significant initial and dynamical residuals they assumed a 100 day decorrelation timescale for the dynamical residuals this long timescale for the residuals reflects the neglect of resolvable processes in the intermediate coupled model such as horizontal advection of momentum however the residuals in the nonlinear pe model should be relatively short timescale errors in parameterizations the scales for these residuals are crudely estimated from the upper ocean turbulence studies of peters et al and moum the assimilation is performed by minimizing a weighted least squares functional expressing the misfits to the data and to the model throughout the tropical pacific and for 18 months it is known that the minimum lies in the data subspace of the state or solution space the minimum is therefore sought in the data subspace by using the representer method to solve the euler lagrange el system although the vector space decomposition and solution method assume a linear el system the concept and technique are applied to the nonlinear el system resulting from the nonlinear pe model by iterating with linear approximations to the nonlinear el system as a first step the authors verify that sequences of solutions of linear iterates of the forward pe model do converge the assimilation is also used as a significance test of the hypothesized means and covariances of the errors in the initial conditions dynamics and data a strong constraint inverse solution is computed however it is outperformed by the weak constraint inverse a cross validation by withheld data is presented as well as an inversion with the model forced by the florida state university winds in place of a climatological wind forcing used in the former inversions

correlation of lower silurian strata from the michigan upper peninsula to manitoulin island the same approach in using sea level curves to refine time rock correlations between widely separated regions also has an important application in resolving the tangle of dual stratigraphic nomenclature that often develops in neighboring regions divided by political boundaries a good example is the lower silurian of the michigan upper peninsula and manitoulin island ontario four peaks in sea level fluctuation are recorded in both areas by coeval pentamerid communities stratigraphically intermixed with coral algal and ostracode vermiform communities indicative of shallower water conditions despite many similarities between the rock units containing these fossil communities most are identified by different formation names on opposite sides of the u s canadian border this study details the eastward thinning of strata across the boundary area the cordell dolomite of michigan is extended to manitoulin island but not the underlying schoolcraft dolomite due to the pinching out of beds that define its u

fine structure of the dipping seismic zone and subduction mechanics in the shumagin islands alaska microearthquake data from a permanent telemetered array have been used to elucidate the structure and tectonics of the subduction zone in the shumagin islands alaska the shallow microseismicity is characterized by active interplate thrusting in the 25 45 km depth range arcward of this interplate activity microearthquakes in the overlying plate have a strike slip mechanism the near horizontal p axis of this mechanism is oriented in the direction of plate convergence suggesting that the plate interface at shallow depths is currently efficiently coupled below 45 km depth the dipping seismic zone appears to be double planed the upper plane dips at 32 from 45 to 100 km depth where it exhibits a kneelike bend below the volcanic front the lower plane begins at about 65 km depth where it is separated from the upper plane by some 25 km and appears to converge with the upper plane at about 120 km depth the inferred geometry of the plate interface suggests that the double planed portion of the dipping seismic zone is a product of elastic unbending of the subducted plate a composite fault plane solution for events in the upper plane indicates however that downdip tension is currently present rather than the downdip compression that would be expected from unbending it is proposed that because of locking of the plate interface at shallower depths slab pull is currently overprinting the unbending stresses which predominate at times when the plates are unlocked this model is consistent with previous interpretations that there is a high probability of a great earthquake in the shumagin islands seismic gap within the next decade or two

magnetic lineations in marginal basins of the western pacific this paper separates the small oceanic basins around the western perimeter of the pacific ocean into marginal basins that have formed through back arc extension and those that apparently have not and reviews our knowledge of magnetic lineation patterns observed in possible and probable back arc basins magnetic lineations in these basins resemble lineations commonly associated with the world s mid oceanic spreading systems indicating that similar processes of crustal accretion occur in both tectonic environments in some back arc basins of the southwestern pacific magnetic lineation and other evidence suggest that back arc basins can evolve through the interaction and growth of multi plate systems because of the small time and space characteristics of back arc basins compared with the world s major oceanic spreading systems tectonic conditions favourable for the generation of back arc basins are either relaxed rapidly or easily interrupted models proposed to account for back arc basin development include a local models where back arc extension is mechanically driven by the downgoing slab and b global plate kinematics models where conditions favourable for back arc extension are governed by the motion of the overriding plate relative to the trench axis

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

subduction of the nazca plate beneath peru as determined from seismic observations a detailed reinvestigation of seismicity in the southern half of peru using data from a local seismic network and a focal mechanism study using p wave first motions reveal that for the first 100 km of descent the slab as defined by this seismicity enters at a normal dip angle near 30 and that below this depth it is bent to a nearly horizontal angle this horizontal slab extends eastward for about 300 km and then dips steeply below the thick continental lithosphere scsp observations are found to be consistent with the profile of the subducting plate as determined from the local seismicity beneath the region from southern peru to northern chile the nazca plate descends with an almost constant dip angle 30 down to at least 300 km depth between the 30 dipping plate in southern peru and the horizontal profile beneath central peru there is a contortion over a 100 km wide continuous lateral section of the subducting plate we infer that at least in the upper 150 km depth range the descending nazca plate is contorted rather than torn

new and extraordinary early cambrian sponge spicule assemblage from china the fossil record of siliceous sponges compared with that of other skeleton secreting metazoa is poorly known based as it is on disarticulated spicules and sporadically preserved body fossils abundant spicules recovered from lower cambrian strata in shaanxi china essentially double the known morphological diversity of siliceous sponges for that interval of geologic time these fossils along with a comparable coeval fauna from south australia have a remarkably modern aspect thereby demonstrating that the principal siliceous sponge groups and styles of body architecture were established quickly in the earliest phanerozoic as part of the cambrian explosion and that they inhabited a variety of low energy relatively deep water settings the similarity of spicule shape and variation to that of younger assemblages reflects a conservative architecture for the siliceous sponges

mechanical stratigraphic controls on fracture patterns within carbonates and implications for groundwater flow abstract groundwater flow in low matrix permeability carbonate rocks is largely controlled by fracture networks the stratigraphic features that control fracture initiation and termination within a sequence of sedimentary rock strata define the mechanical stratigraphy of the sequence we investigate the effectiveness of various types of stratigraphic horizons in terminating opening mode fractures in two different carbonate rock sequences a relatively homogeneous dolomite sequence in door county wi and an interbedded chalk and marl sequence within the austin chalk tx additionally we present analog and numerical modeling results that delineate the specific mechanisms that facilitate fracture termination the combination of model results and empirical relationships between observed sedimentary features and mechanical stratigraphy shows 1 fractures terminate at weak contacts e g thin organic layers shallowly buried contacts or thick fine grained units adjacent to thin fractured beds 2 fractures propagate across strong contacts e g intracycle contacts between different lithology and thin fine grained units adjacent to thick fractured beds and 3 fractures step over at moderate strength contacts we use these guidelines to predict fracture network from sedimentary stratigraphy by qualitatively assessing the mechanical stratigraphy of a portion of the relatively complex cretaceous shelf margin sequence at sant corneli spain this predictive demonstration illustrates the utility of assessing the mechanical stratigraphy of subsurface strata within which fractures are not directly observable we conclude that for a variety of carbonate mechanical stratigraphic sequences dominant fluid flow characteristics such as horizontal high flow zones and flow compartmentalization can be evaluated using fracture spacing and connectivity within fracture networks that is predicted from sedimentary stratigraphy although the resulting heterogeneous flow networks do not rely on every fracture present they are highly dependent on the mechanical stratigraphy

high resolution determination of the benioff zone geometry beneath southern peru the benioff zone geometry beneath southern peru is determined using 32 months of arrival time data from a local seismic network our data set includes 2476 located events of which 205 are chosen to be master events earthquakes with the most reliable locations all the events are then relocated using sourceregion dependent station corrections derived from the station residuals of the master events we find that the benioff zone in the more southerly region of our study area dips at a nearly constant 30 to a depth of at least 200 km while in the more northerly region this trend is apparent down to a depth of only 100 km at which depth the benioff zone becomes nearly horizontal there are enough earthquakes in our data set in the transition zone between the regions to show conclusively that the deformation of the benioff zone is continuous i e there is no discontinuity in the seismicity that might suggest a tear in the subducting plate these results corroborate those of hasegawa and sacks 1981 which used only about a third of the data now available

thermal stresses in subducting lithosphere can explain double seismic zones the discovery of double planed zones of intermediate depth seismicity associated with subducting oceanic lithosphere refs 1 7 and l s h and k h j in preparation has generated much interest in the origin and distribution of stresses in descending plates several mechanisms have been proposed to explain why double zones occur8 here we investigate the role of thermal stresses for the state of stress in a descending plate and test whether they can account for the observed characteristics of double seismic zones by modelling the descending slab as a thin elastic plate and using the temperatures in the slab estimated by toksoz et al 9 we find that the polarities of thermally generated stresses agree with those obtained from the focal mechanisms of earthquakes in the two zones thermal stresses also can explain the separation between the two zones of seismicity at shallow depths and their gradual merging at greater depth

three dimensional miniature of earthquake distribution abstract we present a simple method to create a high resolution three dimensional miniature of hypocentral distribution a pile of transparent thin acryl plates to each of which are attached spherules with hypocentral marks are put into an acryl box filled with a certain liquid the coincidence of the refractive indices between the acryl plates and the liquid provides high transparency and high resolution of the miniature and enables us to observe it from every direction we modeled the distribution of crustal and subcrustal earthquakes beneath central japan into a miniature with dimensions of 36 28 12 cm observation of this miniature makes it possible to comprehend the complex three dimensional distribution of the hypocenters both clearly and intuitively

lithospheric deformation within the molucca sea arc arc collision evidence from shallow and intermediate earthquake activity this paper presents the results of a local earthquake survey in the molucca sea arc arc collision zone of eastern indonesia although intermediate depth earthquakes define zones dipping to the west beneath the sangihe arc and to the east beneath the halmahera volcanic arc only shallow activity less than 60 km depth occurred beneath the central part of the collision zone where the majority of activity is observed the concentration of earthquake foci in the 10 to 50 km depth range in a limited region beneath the talaud mayu ridge suggests that convergence between the arcs proceeds by shortening within basement of the intervening molucca sea plate rather than by slip along shallow dipping planes between the arcs and the subducted slabs published focal mechanism solutions suggest that high angle reverse faulting beneath the talaud mayu ridge generates almost all of the seismic energy release within the collision zone the predominance of shallow reverse and strike slip faulting at the axis of the bilaterally subducting molucca sea lithosphere suggests that stresses are not due to bending alone and have a large horizontal regional component perpendicular to the island arcs the geometry and state of stress within the molucca sea lithosphere requires an external driving force for the convergence between the two arcs and a strong degree of coupling between the island arcs and the subducting molucca sea plate at least at the present stage of collision

sedimentology of an early silurian carbonate ramp the manitoulin formation southern ontario the manitoulin formation is a pervasively dolomitized lower silurian carbonate unit that was deposited in the michigan basin and locally in the appalachian basin the formation reaches a maximum thickness of 11 1 m in southern ontario and can be subdivided into eight facies and four regionally correlatable facies assemblages owing to the relatively continuous transition of shallow to deeper water facies from the northern to southern portions of the study area the manitoulin formation is interpreted as having formed on a carbonate ramp with a southerly component of dip our study suggests that the algonquin arch which is transected by the outcrop belt did not significantly influence deposition or separate the michigan and appalachian basins depending on its location on the ramp the manitoulin formation shows evidence of varying degrees of episodic storm events alternating with fair weather processes such as wave shoaling sediment reworking and bioturbation apart from bioturbation these fair weathe

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins