la mc icp ms u pb dating of zircons from volcanic rocks and subvolcanic intrusions in ashele cu zn ore district xinjiang and its geological implications the ashele cu zn deposit is representative volcanogenic massive sulfide vms deposit in china s altay region the deposit occurs in the marine volcano sedimentary sequence of the ashele formation in the ashele basin no main ore body occurs in the stratiform form between tuff and basalt and mineralization occurs in massive densely massive banded laminated densely disseminated and veinlet forms the hydrothermal alteration and mineralization also occur in the sub dacite lots of research work has been carried out in the ashele deposit but the ages of its ore hosting volcanic rock subvolcanic rock and mineralization have not yet been clearly determined in this paper la mc icp ms zircon u pb dating was used for studying the tuff and basalt from the direct host rocks of the ore bodies sub dacite and quartz diorite in the asele cu zn district the u pb ages are 387 0 4 2 ma 388 2 3 3 ma 379 4 0 8 ma 378 5 0 9 ma for tuff basalt sub dacite and quartz diorite respectively suggesting that the ashele formation is early middle devonian in age the ashele cu zn deposit is an syngenetic deposit and no main ore body occurs between tuff and basalt the ages of the tuff and basalt suggest that the mineralization in the sedimentary exhalative period occurred in the late early devonian 388 387 ma the u pb age of the sub dacite is 379 4 0 8 ma imrplying that the magmatic hydrothermal mineralization took place in the middle devonian the u pb ages of the older zircons from tuff basalt sub dacite and quartz diorite range from 743 to 2505 ma which represent the forming age of the precambrian crystalline basement in altay region

analogies to oceanic behaviour in the continental breakup of the western woodlark basin the woodlark basin d entrecasteaux island region off northeast papua new guinea fig 1 offers a rare glimpse of the propagation of active rifting and continental breakup1 into orogenically thickened and subsequently extended lithosphere2 10 rifting continental breakup and the subsequent formation of oceanic lithosphere by sea floor spreading often involve rupture controlled by propagation of a rift tip11 20 we recognize several tectonic elements in the evolving western woodlark intracontinental system that are close geometric analogues of wholly oceanic propagators and a kinematic development that can be viewed as an extrapolation of oceanic tectonics the response to deformational stresses in the final stages of breakup in a thick relatively hot and hence weak continental lithosphere appears akin to that of oceanic lithosphere

the pinal schist of southern arizona a paleoproterozoic forearc complex with evidence of spreading ridge trench interaction at ca 1 65 ga and a proterozoic arc obduction event the pinal terrane of southern arizona is composed mostly of quartz sericite schists meta wackes metavolcanic rocks and granitoid and gneissic rocks with minor meta chert meta conglomerate and carbonate rocks various tectonic settings have been suggested for the origin of this terrane the existence of tectonic melanges within the terrane the presence of granitic rocks in close proximity to the melanges the presence of both oceanic and arc like metavolcanic rocks within the terrane the age distributions of igneous and meta igneous rocks within the terrane the deformational patterns within the terrane and the width of the terrane all support a model in which the terrane was formed as a forearc subduction complex that experienced a ridge subduction event at ca 1 65 ga the pinal terrane was likely the forearc subduction complex to the mazatzal arc terrane which lies immediately to the north regional variations in the mineralogy of metavolcanic and metasedimentary rocks exposed within the terrane together with structural data suggest the terrane may have experienced an arc obduction event in the proterozoic after the ridge subduction event if it did most of this arc terrane was eroded off the pinal terrane in the 300 m y following the obduction event and prior to deposition of the apache group at ca 1 32 ga there is a paleoproterozoic arc terrane exposed in northern mexico that may represent remnants of an obducted arc terrane high pressure high temperature metamorphic rocks e g blueschists have not been identified within the pinal terrane

role of the tectonic facies concept in orogenic analysis and its application to tethys in the eastern mediterranean region abstract the purpose of this paper is to advocate the use of a tectonic facies concept which aids systematic and objective analysis of orogenic belts the tectonic facies concept as discussed here focuses on the recognition of discrete tectonic settings mainly those exposed at high crustal levels in orogenic belts application of the tectonic facies concept can help field geologists to relate plate scale processes to outcrop geology tectonic facies are recognised based on knowledge of the modern oceans and margins and well constrained land settings tectonic facies analogous to sedimentary facies can be identified from integrated sets of field data and related laboratory studies a number of tectonic facies models can be recognised in this way their identification aids interpretation and comparisons of orogenic belts palinspastic reconstructions require additional information particularly age kinematic and palaeomagnetic data in this paper the tectonic facies concept is tested by application to a well documented mainly high level orogenic segment tethys in the eastern mediterranean area and is found to be an effective way both of the describing and interpreting this region other methods of tectonic analysis e g recognition of tectono stratigraphic terranes may also be found useful in certain settings and can be used in conjunction with the tectonic facies concept

petrogenesis and tectonic settings of volcanic rocks of the ashele cu zn deposit in southern altay xinjiang northwest china insights from zircon u pb geochronology geochemistry and sr nd isotopes abstract the early mid devonian ashele formation of the southern margin of the chinese altay hosts the ashele cu zn volcanogenic massive sulfide vms deposit and consists of intercalated volcanic and sedimentary rocks that have experienced regional greenschist facies metamorphism we studied the petrography zircon u pb geochronology geochemistry and sr nd isotopes of dacites and basalts in order to understand the petrogenesis of these rocks and the regional tectonic evolution two dacites yielded la mc icp ms zircon u pb ages of 402 6 ma and 403 2 ma the dacites are calc alkaline and characterized by high na 2 o k 2 o ratios 3 6 9 3 and high mg values 47 63 enrichment in large ion lithophile elements lile and light rare earth elements lree depletion in nb ta ti and p and relatively positive e nd t values 3 6 to 7 5 collectively suggesting a sanukitic magma affinity the variations in the major and trace elements of the dacites indicate that fe ti oxide plagioclase and apatite were fractionated during their petrogenesis the basalts are tholeiitic and are characterized by high mg values 66 73 and negative nb and ta anomalies the geochemical characteristics of the basalts are similar to those of n morb those characteristics together with the positive e nd t values 6 8 to 9 2 of the basalts indicate that the precursor magma was derived mainly from an n morb type depleted asthenospheric mantle in an island arc setting the geochemical similarities between the basalts and dacites indicate that they both originated from a similar depleted mantle source via partial melting under different magmatic conditions in each case possibly related to ridge subduction

magmatism as an essential driving force for formation of active metamorphic core complexes in eastern papua new guinea the d entrecasteaux islands in eastern papua new guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation the plutons do not appear to have been intruded by diapiric processes as previously suggested late relatively undeformed plutons form flat lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography it is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes the source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the woodlark seafloor spreading center into continental crust it is suggested that a localized heat source of this type which can provide heat and magmatic material to the crust is essential for the development of metamorphic core complexes

deep structure of an arc continent collision earthquake relocation and inversion for upper mantle p and s wave velocities beneath papua new guinea we examine mechanisms of lithospheric growth and modes accommodating subcrustal convergence following the cessation of subduction by analyzing the seismicity and upper mantle velocities in the new guinea arc continent collision earthquakes in the papua new guinea png region from 1967 to 1984 have been relocated in a joint inversion with seismic velocities using p and s wave arrival times recorded by seismographs in png a total of 957 well recorded earthquakes were chosen for use in three dimensional velocity inversions from an initial catalog of 12 960 events based on the stability of initial hypocenter relocations constant velocity blocks were defined as irregular polyhedra to give fine detail in heavily sampled areas without introducing a large number of poorly controlled parameters and to exploit a priori knowledge of the geometry of velocity anomalies spherical geometry was used to accurately calculate long ray paths up to 1500 km in length most of the 700 best located hypocenters from the png data set are in the upper mantle beneath the finisterre huon fh ranges the newly accreted island arc terrane beneath the papuan peninsula and in the new britain seismic zone an oceanic subduction system these hypocenters show a well defined seismic zone dipping vertically or steeply to the north beneath the northern fh ranges from 125 to 250 km depth continuous along strike with the new britain seismic zone to the east which shows earthquakes to 600 km depth the steeply dipping intermediate depth seismicity flattens near 100 km depth beneath the fh ranges and forms a subhorizontal seismic zone beneath the fh ranges by contrast seismicity continues upward to the surface beneath new britain where the arc has not yet collided with new guinea the 100 km deep flat zone does not continue south of the fh ranges but appears to be truncated below the eastern and southern boundaries of the island arc terrane and no clear evidence for a south dipping seismic zone could be found the relationship of the flat part of the slab 100 km beneath the fh ranges and surface faulting is difficult to discern the apparent surface suture bounding the south side of the fh ranges is 100 km directly above the southern termination of intermediate depth seismicity nowhere in png could clear evidence for arc polarity reversal be found from seismicity all of the well located subcrustal earthquakes beneath the papuan peninsula lie in a narrow horizontal line between 125 and 175 km depth that follows the center of the peninsula and are distinctly separated from the fh seismic zone to the north these events do not require a southward subducting plate beneath the papuan peninsula an alternate and reasonable explanation for these events is that they are a result of unstable thickening of the lithosphere information on lateral heterogeneity in the mantle is limited because large variations in crustal velocities dominate the observed pattern of travel time residuals allowing for only crustal heterogeneity reduces travel time residual variances by 40 from a simple one dimensional structure while allowing for additional mantle heterogeneity results in 10 further reduction for the various parameterization s attempted here inversions with several different block geometries show low velocities at 35 to 171 km km depth beneath the bismarck sea back arc basin and high velocities just south of the intermediate depth fh seismic zone the pattern of lateral heterogeneity supports the inference from seismicity of north dipping slabs beneath both northeast new guinea and new britain

breakup of young oceanic lithosphere in the upper part of a subduction zone implications for the emplacement of ophiolites it is investigated by means of numerical modelling whether the interaction between young oceanic lithosphere and a subduction zone can lead to the breakup of a young and thin oceanic plate first a thermal model of the upper part of a subduction zone the region between the trench and the volcanic arc is presented temperatures are modelled for situations in which the age of the subducting oceanic lithosphere gradually decreases culminating in the arrival of a spreading ridge at the trench temperatures are used to infer the strength of material within the subducting plate from a pressure and temperature dependent rheology next it is investigated by means of finite element modelling whether the forces acting upon young subducting oceanic lithosphere recently created at a spreading center in the vicinity of a subduction zone can lead to its breakup our modelling results show that very young oceanic lithosphere may indeed break up during the early phase of its subduction whether breakup occurs depends upon the spreading velocity and the length of the ridge segment that interacts with the trench if breakup occurs some young oceanic lithosphere situated in the upper part of the subduction zone and in the region between the ridge and the trench will be detached from the other parts of the subducting plate the detached sheet of thin oceanic lithosphere will not be subducted to mantle depths instead subduction of the trailing plate at the opposite side of the ridge can lead to the incorporation of this sheet into the forearc region our modelling provides a mechanism to incorporate thin oceanic lithosphere into an arc trench region and eventually upon closure of the ocean basin into an orogenic belt it is shown that a large subset of ophiolites those with a harzburgite dominated mantle section has properties in good agreement with an emplacement history in which the breakup of young oceanic lithosphere in the upper part of a subduction zone is the first phase

ground motions from subduction zone earthquakes a total of 258 horizontal components of accelerogram data recorded at soil sites during thrust normal and strike slip earthquakes occurring in seven subduction zones around the pacific ocean were analyzed the results of statistical analyses of 5 per cent damped pseudo velocities psv computed from these accelerograms at 10 periods t between 0 1 and 4 sec indicated no significant differences in the average psv levels of the thrust normal and strike slip data from the northern honshu zone each of these groups of northern honshu data which together comprised one half of the total data base were fit equally well by the same attenuation model the analyses also revealed that at intermediate periods 0 8 t 3 sec the psv observed at stiff soil sites within the northern honshu nankai kuril mexico and alaska zones were on the average significantly greater than the psv observed at similar sites in the peru northern chile and new britain bougainville zones this observation indicates that differences in the source and or travel path characteristics between the two groups accounted for the differences in psv independent evidence supporting source differences is the correlation noted between psv in this period band and the characteristic source complexities for these zones which hartzell and heaton inferred from the teleseismic data of the larger magnitude earthquakes certain anomalous tectonic characteristics of the new britain bougainville zone were noted that may have contributed in some systematic manner to the relatively unusual spectral characteristics of the ground motions recorded on stiff soil sites within this zone some of the differences in the psv among the zones at intermediate and long periods were probably the result of differences in local geologic characteristics geology greatly influenced the mexican psv data from soft soil sites to a lesser extent geology probably affected the alaskan data which were recorded mostly on deep stiff soils and the new britain bougainville data recorded at yonki which is underlain by softer soil deposits

the tertiary arc chain in the western pacific abstract the arcs bordering the pacific plate on the western and southwestern pacific rim are reconstructed since their initiation in the eocene and oligocene they occur in a zone forming an arc chain from the western pacific tropics to the eastern margin of australia they are the bonin mariana yap palau halmahera north new guinea west melanesia solomon vanuatu and tonga kermadec arcs designated here the tertiary arc chain they are associated with the formation and consumption of backarc basins reversals of arc polarity and episodic subduction has occurred in some of them the tertiary arc chain is characterized by four major stages in its evolution which can be seen characteristically in some of the arcs the first stage is the occurrence of the arc chain from the middle eocene to earliest oligocene the second stage is the formation of the backarc basins from the early to late oligocene the third stage is the occurrence of double arcs on the inner side of the arc chain in the early to middle miocene and the fourth stage is the reversal of arc polarities due to collisions since the late miocene the backarc basins associated with the arcs of the tertiary arc chain have fixed limits of duration in their evolution the backarc basins initially form 15 million years after the initiation of the volcanic arc several to 10 million years after the initial opening backarc spreading terminates approximately 20 million years after the cessation of the backarc spreading a second phase of opening occurs in the backarc region in the case of arc collision reversal of the arc polarity occurs if there is oceanic crust on the backarc side and opening of a backarc basin occurs within several million years these occurrences and durations have a variation of ca 3 5 million years

continental breakup and uhp rock exhumation in action gps results from the woodlark rift papua new guinea we show results from a network of campaign global positioning system gps sites in the woodlark rift southeastern papua new guinea in a transition from seafloor spreading to continental rifting gps velocities indicate anticlockwise rotation at 2 2 7 myr relative to australia of crustal blocks north of the rift producing 10 15 mm yr of extension in the continental rift increasing to 20 40 mm yr of seafloor spreading at the woodlark spreading center extension in the continental rift is distributed among multiple structures these data demonstrate that low angle normal faults in the continents such as the mai iu fault can slip at high rates nearing 10 mm yr extensional deformation observed in the d entrecasteaux islands the site of the world s only actively exhuming ultra high pressure uhp rock terrane supports the idea that extensional processes play a critical role in uhp rock exhumation gps data do not require significant interseismic coupling on faults in the region suggesting that much of the deformation may be aseismic westward transfer of deformation from the woodlark spreading center to the main plate boundary fault in the continental rift the mai iu fault is accommodated by clockwise rotation of a tectonic block beneath goodenough bay and by dextral strike slip on transfer faults within and surrounding normanby island contemporary extension rates in the woodlark spreading center are 30 50 slower than those from seafloor spreading derived magnetic anomalies the 0 5 ma to present seafloor spreading estimates for the woodlark basin may be overestimated and a reevaluation of these data in the context of the gps rates is warranted

tectonics of the new guinea region the new guinea region evolved within the obliquely and rapidly converging australian and pacific plate boundary zone it is arguably one of the most tectonically complex regions of the world and its geodynamic evolution involved microplate formation and rotation lithospheric rupture to form ocean basins arc continent collision subduction polarity reversal collisional orogenesis ophiolite obduction and exhumation of ultra high pressure metamorphic rocks we describe the major onshore and offshore tectonic and geologic components including plate boundaries seismicity faults and magmatism and we integrate these with emerging ideas about mantle dynamics to evaluate the cenozoic tectonic evolution of new guinea future research opportunities to resolve the mantle structure beneath new guinea will enable mantle dynamics to be linked to lithospheric and surface processes virtually all plate tectonic and mantle processes have been active in the new guinea region throughout the cenozoic and as such

shallow dips of normal faults during rapid extension earthquakes in the woodlark d entrecasteaux rift system papua new guinea earthquakes in the woodlark d entrecasteaux rift system reveal changes in fault geometry along strike as extension progresses from continental to oceanic modes in one segment evidence is found for earthquakes on shallow dipping normal faults to examine these trends locations of 147 earthquakes are redetermined and focal mechanisms for 21 events are derived from waveform inversion where seafloor spreading occurs east of 152 e most earthquakes illuminate a ne sw strike slip fault north of the oceanic rift system along the woodlark rise elsewhere earthquakes occur along a well defined rift zone and focal mechanisms reveal normal faulting with north south t axes and depths less than 10 km most normal faulting earthquakes show high angle fault planes dips 40 50 but several are consistent with slip on moderately low angle planes dipping 15 35 these low angle nodal planes are only found in a region between 150 5 e and 152 5 e where the transition occurs from seafloor spreading to continental rifting a seismic reflection profile near 151 e reveals a major normal fault that dips to the north at 25 35 just landward of the oceanic rift tip that fault and parallel structures are excellent candidates for the fault that slipped in at least two of the earthquakes studied hence normal faults dipping 30 are likely to be seismically active in at least one part of this rift system in this region just landward from the rift tip the highest extension rates are expected 40 mm yr and strain appears localized on a single system of normal faults because shallow dips for normal faults are seen near the rift tip but not elsewhere along strike high strain rates may be an important factor in allowing normal faults to slip at unusually shallow dips perhaps high shear traction at the base of the upper crust localized by nearby spreading sufficiently alters the local stress regime

southeast papuan crustal tectonics imaging extension and buoyancy of an active rift southeast papua hosts the world s youngest ultra high pressure uhp metamorphic rocks these rocks are found in an extensional setting in metamorphic core complexes competing theories of extensional shear zones or diapiric upwelling have been suggested as driving their exhumation to test these theories we analyze the cdpapua temporary array of 31 land and 8 seafloor broadband seismographs seismicity shows that deformation is being actively accommodated on the core complex bounding faults offset by transfer structures in a manner consistent with overall north south extension rather than radial deformation rayleigh wave dispersion curves are jointly inverted with receiver functions for crustal velocity structure they show crustal thinning beneath the core complexes of 30 50 and very low shear velocities at all depths beneath the core complexes on the rift flanks velocities resemble those of normal continents and increase steadily with depth there is no evidence for velocity inversions that would indicate that a major density inversion exists to drive crustal diapirs also low density melt seems minor within the crust together with the extension patterns apparent in seismicity these data favor an extensional origin for the core complexes and limit the role of diapirism as a secondary exhumation mechanism although deeper mantle diapirs may be undetected a small number of intermediate depth earthquakes up to 120 km deep are identified for the first time just northeast of the d entrecasteaux islands they occur at depths similar to those recorded by uhp rocks and similar temperatures indicating that the modern seismicity occurs at the setting that generates uhp metamorphism

the opening of the woodlark basin subduction of the woodlark spreading system and the evolution of northern melanesia since mid pliocene time abstract magnetic anomaly and seismological data define segments of active seafloor spreading and associated magnetic lineations trending ene in the woodlark basin the total opening rate has been approximately 6 cm yr for the last 1 m y spreading rates diminish by over 10 from east to west along the woodlark spreading system implying a pole of current opening 15 20 to the west commencement of seafloor spreading in the basin has apparently been time transgressive beginning prior to 3 5 m y in the east and at successively later times to the west earthquake focal mechanisms and geological evidence suggest that the land areas bounding the western end of the woodlark basin are undergoing tensional deformation we believe that eventually the woodlark basin plate boundary will propagate westward through the d entrecasteaux islands into the papuan peninsula hitherto unreported shallow seismicity associated with the northern margin of the ne trending section of the woodlark rise probably reflects partial decoupling of the woodlark and solomon basins possibly due to mechanical difficulties in subducting the young woodlark lithosphere analysis of the relative motions between the solomon indo australian and pacific plates shows that the woodlark spreading system has been subducted at high rates 10 cm yr beneath the solomon islands during the opening of the woodlark basin several tectonic and geological features limited to the region of interaction of the woodlark basin with the solomon trench and arc may be symptomatic of ridge subduction these features include high heat flow in the solomon trench which shoals to 4 km low levels of seismicity and only shallow hypocenters and voluminous eruptions of high k 2 o tio 2 olivine basalts and basaltic andesites extremely close to the trench axis this close association in space and time of an unusual volcanic suite with ridge subduction implies a strong dependence of the petrogenesis on the tectonic regime a combination of this study of the woodlark basin and the previous study of the bismarck basin taylor 1979 provides a reconstruction of the positions of the continents ocean basins and island chains in northern melanesia for mid pliocene time in accepting the existence of a solomon plate we can explain the trench like structure off the trobriand margin of new guinea the occurrence of late cenozoic calc alkaline volcanism along the papuan peninsula and the presence of intermediate depth seismicity beneath the north papuan peninsula the rapid changes in relative motions along or across the new ireland solomons chain over the past 3 5 m y may explain the spatial and temporal changes in igneous activity observed on these islands

empirical relationships among the convergence rate of plates rollback rate of trench axis and island arc tectonics laws of convergence rate of plates abstract the data on 27 recent arcs which were subducted by normal oceanic lithosphere were compiled among the compression extension rates for island arcs v a compression is defined as positive and the kinematic components of absolute plate motions parallel to the direction of backarc compression extension v on oceanward velocity of the overriding plate v sn continentward velocity of the subducting plate and v tn rollback rate of the trench axis the equations v a v on v sn 7 2 cm y 1 v tn 7 2 cm y v sn 2 are found to hold as for the convergent zones associated with a wadati benioff zone deeper than 200 km equation 1 suggests that backarc spreading and crustal shortening play the role of compensating the deviation of the convergence rate from the critical value of 7 2 cm y the equation 2 means that any subducting oceanic lithosphere has the potentiality of making the trench axis rollback at the rate of 7 2 cm y and that the virtual rollback rate v tn is determined by the balance between the potential rollback rate 7 2 cm y and v sn the present author calls the kinematic relationships represented by equations 1 and 2 laws of convergence rate of plates the convergent zones of plates associated with the wadati benioff zone shallower than 200 km also show a tendency that the greater and smaller values of v on v sn correspond to the backarc compression and extension however the critical value is 3 4 cm y instead of 7 2 cm y

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

the distribution of deep convection over ocean and land during the asian summer monsoon abstract tle highly reflective cloud hrc dataset is a daily index of organized deep convection at one degree resolution from 17 years of polar orbiting satellite imagery these data are used to analyze and discuss the climatological geographical distribution of deep convection observed over the asian summer monsoon season and its component months june july august and september intraseasonal variations of convection for selected regions am examined using normalized pentad time series of regional median hrc values we also compare hrc data over two regions western coastal india and western coastal burma thailand with the results from a two dimensional numerical model consisting of a simple differentially heated land ocean system which predicts that a preponderance of deep convection occurs over the coastal zone the buma thailand regional comparison supports the model result comparison of the model with the western coastal india region is less conclusive which may be due to the limitations of

collision related break up of a carbonate platform eratosthenes seamount and mud volcanism on the mediterranean ridge preliminary synthesis and implications of tectonic results of odp leg 160 in the eastern mediterranean sea abstract drilling of the eratosthenes seamount south of cyprus documented incipient collision of the african and eurasian plates the oldest sediments recovered mid cretaceous shallow water limestones are overlain by upper cretaceous to lower oligocene pelagic carbonates with several hiatuses following uplift a carbonate platform was established in the miocene eratosthenes was then below eustatic sea level during the messinian desiccation crisis the platform subsided to bathyal depths during the lower pliocene associated with localized breccia deposition further subsidence occurred in late pliocene early quaternary coeval with strong surface uplift of southern cyprus subsidence and break up of eratosthenes was achieved by a combination of flexural loading and normal faulting in addition the milano and napoli mud volcanoes were drilled on the northern flank of the mediterranean ridge accretionary complex south of crete a mainly extrusive sedimentary origin is indicated multiple debris flows include clasts of sandstone and limestone of at least partly miocene age both mud volcanoes are dated as 1 ma old and have been active episodically hydrocarbon gas is associated with both mud volcanoes while methane hydrates clathrates exist locally at milano the driving force of mud volcanism is overpressuring caused by incipient plate collision messinian evaporites may have acted as a localized seal material escaped through a zone of backthrusting against rigid cretan crust to the north

mineral potential mapping with a restricted boltzmann machine abstract a restricted boltzmann machine can be trained to encode and reconstruct training samples from a training sample population with an unknown complex probability distribution small probability samples can be differentiated from the training sample population due to their comparatively larger encoded and reconstructed errors in mineral potential mapping mineral potential areas usually take up only a small proportion of the whole mineral exploration region and have specific evidence map patterns assume that the whole mineral exploration region is divided into uniform cells and a restricted boltzmann machine is then trained on all the cells as a small proportion of the total training cells mineral potential cells have much larger encoded and reconstructed errors compared to non mineral potential cells based on the encoded and reconstructed errors of the training cells asc and ase are thus defined as two mineral potential indicators for mineral potential mapping with the trained restricted boltzmann machine the altay district in northern xinjiang in china which is linked with a complex geological setting is chosen as a case study area restricted boltzmann machines with 12 visible units and differing hidden units are constructed and trained on 9582 training cells in the study area the asc ase posterior probability and deposit bearing probability are used to predict the mineral potential of each grid cell in the study area the auc area under the curve a measure of aggregated classification performance in machine learning is applied to measure the mineral potential mapping performance of these four mineral potential indicators the results show that the four mineral potential indicators perform similarly well therefore a restricted boltzmann machine can be trained to map mineral potentials and the asc and ase are two feasible indicators for the mineral potential mapping with the trained restricted boltzmann machine

anisotropy beneath a highly extended continental rift we have employed shear wave splitting techniques to image anisotropy beneath the d entrecasteaux islands in southeastern papua new guinea our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid ocean ridge tip we image the transition from continental to oceanic extension a dense shear wave splitting data set from a 2010 to 2011 passive source seismic deployment is analyzed using single and multichannel methods splitting delay times of 1 1 5 s are observed and fast axes of anisotropy trending n s parallel to rifting direction predominate the results this trend is linked to lattice preferred orientation of olivine primarily in the shallow convecting mantle driven by up to 200 km of n s continental extension ahead of the westward propagating woodlark rift this pattern differs from several other continental rifts that evince rift strike parallel fast axes and is evident despite the complex recent tectonic history we contend that across most of this rift the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid ocean ridge like mantle fabric stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region

tectonically controlled origin of three unusual rock suites in the woodlark basin we propose alternative mechanisms for the origin of three unusual rock suites high mg andesites nati basalts and arclike rocks that have been dredged from the woodlark basin southwest pacific ocean we show that the high mg andesites and nati basalts are associated with an unusually cool ridge environment the cooling is due to increased hydrothermal circulation stimulated by an unusually high crustal permeability the high permeability is mainly due to cracking of the woodlark basin lithosphere as it passes over the flexural bulge in front of the subduction zone although local processes such as faulting in fracture zones may also make some contribution this increased convective cooling affects magma dynamics and chemistry at the ridge crest the high mg andesites occur where the hydrothermal circulation lowers the temperatures in the upper oceanic crust causing the magma chamber to sit in the mantle rather than in the crust and promoting the interaction of basalt magma with harzburgite the nati basalts are also the indirect result of increased crustal cooling which causes anomalously low degrees of partial melting of their depleted mantle source the arclike rocks are caused by interaction with volatiles from lithosphere that was emplaced beneath the edge of the basin less than 6 m y ago by a now inactive subduction zone to the north of the currently active one simple thermal models indicate that this formerly subducting plate is still cold enough to retain volatiles and to remain seismically active as the old plate loses volatiles they rise into the mantle convection cell which feeds the woodlark basin ridge crest because ridge subduction increases the likelihood of ophiolite obduction these observations and explanations of woodlark basin tectonics are potentially important for ophiolites

volcanic evolution in eastern papua abstract late mesozoic and cenozoic volcanic rocks in eastern papua record a complex series of volcanotectonic events which reflect interaction between the indo australian and solomon sea plates basement formations of upper cretaceous and eocene submarine basalt are comparable to those characteristic of sea floor spreading centers and are thought to have originated during volcanic activity associated with spreading in the coral sea basin arc trench type andesitic volcanism was prominent during the late cenozoic but shows no clear relationship to a subduction event an alternative explanation links the development of thickened crust and consequent crust mantle interaction with the generation of andesitic magmas the tectonic environment of eastern papua during the late cenozoic was one of block faulting and uplift associated with crustal tension the presence of quaternary peralkaline rhyolites suggests that this environment is now being replaced by active rifting

spreading mode of backarc basins in the western pacific abstract a number of phases are suggested in backarc spreading extension of the arc lithosphere is followed by spreading and formation of oceanic crust in the backarc basin in some basins a third phase of rifting with a reorientation of the spreading direction occurs during the second and third phases spreading is governed by rotation about a nearby pole and spreading axes are oblique to the transform faults this contrasts with the geometry of mid oceanic ridges where the ridges are on great circles perpendicular to the transform faults three spreading models are defined according to the various angular relations between transform faults and spreading axes backarc basins facing the pacific plate tend to open with axes spaced among adjacent ridge segments on projections parallel to the spreading axes type i spreading mode in some cases new spreading occurs in the countiguous area forming overlapped spreading axes type ii spreading mode initial spreading to form a backarc basin tends to spread with both spaced and overlapped axes type iii spreading mode inner backarc basins intercalated with other basins between the pacific plate also tend to spread with type iii spreading mode spreading modes and possible mechanisms for these formations in the backarc basins in the western pacific are discussed

large earthquake doublets and fault plane heterogeneity in the northern solomon islands subduction zone in the solomon islands and new britain subduction zones the largest earthquakes commonly occur as pairs with small separation in time space and magnitude this doublet behavior has been attributed to a pattern of fault plane heterogeneity consisting of closely spaced asperities such that the failure of one asperity triggers slip in adjacent asperities we analyzed body waves of the january 31 1974 m w 7 3 february 1 1974 m w 7 4 july 20 1975 14 37 m w 7 6 and july 20 1975 19 45 m w 7 3 doublet events using an iterative multiple station inversion technique to determine the spatio temporal distribution of seismic moment release associated with these events although the 1974 doublet has smaller body wave moments than the 1975 events their source histories are more complicated lasting over 40 seconds and consisting of several subevents located near the epicentral regions the second 1975 event is well modeled by a simple point source initiating at a depth of 15 km and rupturing an approximate 20 km region about the epicenter the source history of the first 1975 event reveals a westerly propagating rupture extending about 50 km from its hypocenter at a depth of 25 km the asperities of the 1975 events are of comparable size and do not overlap one another consistent with the asperity triggering hypothesis the relatively large source areas and small seismic moments of the 1974 doublet events indicate failure of weaker portions of the fault plane in their epicentral regions variations in the roughness of the bathymetry of the subducting plate accompanying subduction of the woodlark rise may be responsible for changes in the mechanical properties of the plate interface

how continents break up insights from papua new guinea the woodlark basin in the western pacific forms a continuous system of active continental rifting evolving to well developed seafloor spreading thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin magnetic isochrons indicate a single euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins in agreement with rigid plate tectonic models continental rifting initiated geologically synchronously at 6 ma along the length of the protomargins within a deforming plate boundary zone strain localization and seafloor spreading however developed in a time transgressive fashion from east to west within this zone of deformation spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the solomon and coral seas the transition to spreading occurred after a rather uniform degree of continental extension 200 40 km both early and late stage rifting involved high and low angle normal faults we identify distinct styles in the transition from rifting to spreading which we refer to as nucleation propagation and stalling these breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating the initial spreading segments achieved much of their length at nucleation within rift basins separated along strike by accommodation zones with subsequent lengthening by spreading propagation into rifting continental crust this early propagation and the subsequent development of transform faults between initially nontransform spreading segment offsets produced rift and spreading segmentation boundaries that are not simply correlated the spreading centers nucleated approximately orthogonal in strike to the opening direction but as the protomargins were oblique to this direction nucleation jumps occurred in order to maintain the new spreading centers within the protomargins thus stepwise spreading nucleation in order to remain within a rheologically weak zone rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis is the dominant form of the rifting to spreading transition in the woodlark basin