backthrusting in accretionary wedges backthrusting of the rear of the accretionary wedge arcward over the rocks of the forearc basin is a common aspect of accretionary tectonics an older assumption that backstops of accretionary wedges dip back toward the arc and that accreted material is emplaced below such backstops is contradicted by observations from many active forearcs that indicate 1 backthrusting is common 2 forearc basins are nearly ubiquitous associates of accretionary wedges and 3 forearc basement where imaged appears to diverge from the sedimentary package dipping under the wedge while the overlying sediments are often lifted up against it backthrusting may be favored where relief is high between the crest of the wedge and the surface of the forearc basin because the relief must be supported by shear stress along the backthrust we find ratios of lt lb averaging 2 64 0 8 where lt is the horizontal distance from the crest of the wedge to the front of the toe and lb is the distance from the crest of the wedge to the surface projection of the backthrust this ratio is predicted to increase with total convergence and with sedimentation because the toe undergoes higher deformation rates than the backthrust and sedimentation in the trench favors outbuilding of the toe whereas sedimentation in the forearc basin tends to cancel the gravitational component of the driving stress for the backthrust

a late cretaceous and cenozoic reconstruction of the southwest pacific region tectonics controlled by subduction and slab rollback processes abstract a cenozoic tectonic reconstruction is presented for the southwest pacific region located east of australia the reconstruction is constrained by large geological and geophysical datasets and recalculated rotation parameters for pacific australia and lord howe rise pacific relative plate motion the reconstruction is based on a conceptual tectonic model in which the large scale structures of the region are manifestations of slab rollback and backarc extension processes the current paradigm proclaims that the southwestern pacific plate boundary was a west dipping subduction boundary only since the middle eocene the new reconstruction provides kinematic evidence that this configuration was already established in the late cretaceous and early paleogene from 82 to 52 ma subduction was primarily accomplished by east and northeast directed rollback of the pacific slab accommodating opening of the new caledonia south loyalty coral sea and pocklington backarc basins and partly accommodating spreading in the tasman sea the total amount of east directed rollback of the pacific slab that took place from 82 ma to 52 ma is estimated to be at least 1200 km a large percentage of this rollback accommodated opening of the south loyalty basin a north south trending backarc basin it is estimated from kinematic and geological constraints that the east west width of the basin was at least 750 km the south loyalty and pocklington backarc basins were subducted in the eocene to earliest miocene along the newly formed new caledonia and pocklington subduction zones this culminated in southwestward and southward obduction of ophiolites in new caledonia northland and new guinea in the latest eocene to earliest miocene it is suggested that the formation of these new subduction zones was triggered by a change in pacific australia relative motion at 50 ma two additional phases of eastward rollback of the pacific slab followed one during opening of the south fiji basin and norfolk basin in the oligocene to early miocene up to 650 km of rollback and one during opening of the lau basin in the latest miocene to present up to 400 km of rollback two new subduction zones formed in the miocene the south dipping trobriand subduction zone along which the solomon sea backarc basin subducted and the north dipping new britain san cristobal new hebrides subduction zone along which the solomon sea backarc basin subducted in the west and the north loyalty south fiji backarc basin and remnants of the south loyalty santa cruz backarc basin subducted in the east clockwise rollback of the new hebrides section resulted in formation of the north fiji basin the reconstruction provides explanations for the formation of new subduction zones and for the initiation and termination of opening of the marginal basins by either initiation of subduction of buoyant lithosphere a change in plate kinematics or slab mantle interaction

seismic evidence for an earthquake nucleation phase near source observations show that earthquakes initiate with a distinctive seismic nucleation phase that is characterized by a low rate of moment release relative to the rest of the event this phase was observed for the 30 earthquakes having moment magnitudes 2 6 to 8 1 and the size and duration of this phase scale with the eventual size of the earthquake during the nucleation phase moment release was irregular and appears to have been confined to a limited region of the fault it was characteristically followed by quadratic growth in the moment rate as rupture began to propagate away from the nucleation zone these observations suggest that the nucleation process exerts a strong influence on the size of the eventual earthquake

observation of the seismic nucleation phase in the ridgecrest california earthquake sequence near source observations of five m 3 8 5 2 earthquakes near ridgecrest california are consistent with the presence of a seismic nucleation phase these earthquakes start abruptly but then slow or stop before rapidly growing again toward their maximum rate of moment release deconvolution of instrument and path effects by empirical green s functions demonstrates that the initial complexity at the start of the earthquake is a source effect the rapid growth of the p wave arrival at the start of the seismic nucleation phase supports the conclusion of mori and kanamori 1996 that these earthquakes begin without a magnitude scaled slow initial phase of the type observed by iio 1992 1995

finite faults and inverse theory with applications to the 1979 imperial valley earthquake abstract using a representation theorem from elastodynamics subsurface slip on a known fault is formulated as the solution to an inverse problem in which recorded surface ground motion is the data two methods of solution are presented the least squares method which minimizes the squared differences between theory and data and the constrained least squares method which simultaneously maintains a set of linear inequalities instabilities in the solution are effectively eliminated in both methods and the sensitivity of the solution to small changes in the data is quantitatively stated the inversion methodology is applied to 77 components of near field ground acceleration recorded during the 15 october 1979 imperial valley earthquake the faulting is constrained to propagate bilaterally away from the epicenter at an average velocity of 90 per cent of the shear wave speed on a vertical fault plane extending from the surface to 10 km depth inequality constraints are used to keep the faulting sequence physically reasonable by maintaining right lateral motion and positive slip velocity the preferred solution is stable and provides a good fit to the data it is also realistic and consistent with observed surface offsets and independent estimates of seismic moment

new insights on 3 d plates interaction near taiwan from tomography and tectonic implications abstract recent tomographic results are used to trace the south china sea and philippine sea subducting slabs south and northeast of taiwan along the manila and ryukyu trenches respectively in particular the 3 d plates interaction beneath taiwan is discussed based on a close up view of the tomographic sections together with earthquake hypocenters distribution our study indicates that 1 the east dipping south china sea slab can be followed to the north up to the latitude of hualien 2 the eurasian plate subducts beneath most part of the taiwan island down to the 670 km depth discontinuity 3 the north dipping philippine sea slab can be followed slightly west of the longitude of hualien both plates thus interact beneath northern taiwan where the arc continent collision is paroxysmal 4 slab detachment is envisaged at the northern edge of the subducted eurasian plate beneath the coastal range of taiwan which may facilitate the northwestward motion of the philippine sea plate with respect to eurasia slabs geometries obtained from tomographic sections allow us to reconstruct the late neogene plate kinematics and dynamics in this region our main conclusions are 1 the size of the original south china sea was about twice its present size 2 the subducted part of the west philippine basin i e the largest oceanic basin of the philippine sea plate extends only 400 km north of the ryukyu trench 3 slab detachment might have occurred 3 5 my ago beneath the central and northern ryukyu arc along a weak zone that is aligned with the gagua ridge an ancient plate boundary 4 the ryukyu trench has propagated westward from 126 e of longitude southeast of miyako island to the locus of the present arc continent collision along a major lithospheric tear that cut through the continent ocean boundary first and then through the continental lithosphere as a consequence the southern ryukyu margin developed progressively from east to west as a subduction zone during the last 8 my it has been elaborated onto a passive margin which corresponded to the edge of the eurasian continental shelf bordering the south china sea prior to pliocene 5 the southern okinawa trough rifted during the last 2 1 ma following the propagation of the subduction zone 6 subduction of the eurasian margin probably stopped recently beneath northern taiwan and a flip of subduction from west to east is being initiated near hualien

present day crustal motion along the longitudinal valley fault eastern taiwan abstract the nne striking longitudinal valley fault lvf in eastern taiwan is an extremely active high angle thrust fault it bounds the coastal range and the longitudinal valley which is considered a collision boundary between the philippine sea and the eurasian plates repeated gps data in the longitudinal valley area from 1992 to 1999 are utilized to study the spatial variation of crustal motion along the lvf with respect to penghu in the chinese continental margin velocities for stations on the western side of the lvf longitudinal valley and eastern central range are 18 35 mm yr in directions 283 311 whereas those on the eastern side of the lvf the coastal range are 28 68 mm yr in directions 303 324 a major discontinuity of about 30 mm yr on the rate of crustal motion across the longitudinal valley is attributed to the aseismic slip along the lvf as revealed by trilateration data previously to the south of fengping the block motions of the coastal range are 31 40 mm yr in 317 330 relative to the central range while the near fault motions are 13 33 mm yr in 309 336 various partitions on the left lateral strike slip and convergent components along the lvf are found in the southern longitudinal valley crustal motion is mainly accommodated on the lvf and the luyeh fault in contrast those in the central and northern longitudinal valley are partly taken up on the faults to the east of the lvf or result in the elastic deformation of the coastal range the crustal motion in the northern longitudinal valley area is likely to be distributed in the several ne striking thrusts in a horsetail pattern and obliquely cut the northern coastal range with a small portion of fault slips along the lvf data from dense deployed gps networks across the lvf can be employed to give better estimates of near fault motions and delineate the surface traces of the lvf repeated gps and leveling data from two stations on both ends of the yuli bridge that are 575 m apart clearly indicate that the surface trace of the lvf passes beneath the bridge with oblique horizontal motion of 23 mm yr in 306 and uplift rate of 24 mm yr

2003 mw6 8 chengkung earthquake and its related seismogenic structures abstract the mw 6 8 chengkung earthquake on december 10 2003 in eastern taiwan was associated with abundant aftershocks to explore the seismogenic mechanism and its causative fault system we relocated the aftershocks determined the focal mechanisms from p wave polarities and used the finite dimension source model method to identify the fault plane based on our results the earthquake can be divided into three seismic clusters one is located in the east of the southern longitudinal valley one on the chihshang fault and the other on the western side of the luyeh fault to the south both the seismic distribution and finite dimension source model method show that east dipping fault planes with thrust mechanisms coincide with previous studies based on background seismicity the third seismic cluster suggests thrust mechanism along west dipping fault plane that was not recognized by background seismicity this seismic cluster occurred along the central range fault previously proposed as active by geomorphic and geologic evidence

hypocenter locations in finite source rupture models we use a database of more than 80 finite source rupture models for more than 50 earthquakes m w 4 1 8 1 with different faulting styles occurring in both tectonic and subduction environments to analyze the location of the hypocenter within the fault and to consider the correlation between hypocenter location and regions of large slip rupture in strike slip and crustal dip slip earthquakes tends to nucleate in the deeper sections of the fault subduction earthquakes do not show this tendency ratios of the hypocentral slip to either the average or the maximum slip show that rupture can nucleate at locations with any level of relative displacement rupture nucleates in regions of very large slip d 2 3 d max in only 16 of the events in regions of large slip 1 3 d max d d max in 35 of the events and in regions of low slip d 1 3 d max in 48 of the events these percentages significantly exceed the percentages of fault area with very large 7 and large 28 slip ruptures that nucleate in regions of low slip however tend to nucleate close to regions of large slip and encounter a zone of very large slip within half the total rupture length applying several statistical tests we conclude that hypocenters are not randomly located on a fault but are located either within or close to regions of large slip

seismogenic structure in a tectonic suture zone with new constraints from 2006 mw6 1 taitung earthquake 1 a mw 6 1 earthquake occurred on april 1st 2006 near taitung eastern taiwan it produced significant coseismic ground displacements and a large number of aftershocks in the ensuing month this event provides an opportunity to diagnose the seismogenic structure in the southern longitudinal valley lv of eastern taiwan long viewed as one of the collision sutures between the philippine sea and the eurasian plates with precisely relocated main and aftershock hypocenters focal mechanisms for m 3 8 events and coseismic ground displacements from strong motion records we determine a main shock dislocation model our results indicate that the main shock occurred on a high angle fault azimuth 198 dip 77 the model comprises a fault with two segments the main shock and a large number of aftershocks are associated with the northern segment that exhibited predominantly left lateral strike slip motion in agreement with p wave first motions and waveform usgs solutions the southern segment exhibits a slightly larger thrust component in agreement with cmt solutions tectonically this event highlights a nne trending fault on the west side of the lv which is predominantly strike slip the aftershocks clustered to the east of the main shock which exhibit mainly thrust mechanisms indicate that shortening is still acting on the sedimentary materials deposited between the coastal and central ranges prior to collision as a result the southern lv is undergoing slip partitioning along different faults which has never been specified before

alternating seismic uplift and subsidence in the late holocene at madang papua new guinea evidence from raised reefs well preserved mid late holocene coral reefs are exposed in low coastal cliffs in the vicinity of the madang lagoon on the north coast of papua new guinea results from u th and 14 c dating of corals surveying and field mapping indicate several major changes in relative sea level over this period specifically there is evidence for a relative sea level fall of 4 5 m about 3000 calendar years b p followed by relative sea level rises of 1 5 m about 2400 calendar years b p and 0 5 m about 1200 calendar years b p and a subsequent relative sea level fall of 3 m some time in the past 1000 years since regional eustatic sea levels are believed to have been dropping gradually over this time frame these observed changes in relative sea level are interpreted as reflecting alternating tectonic uplift and subsidence furthermore the detailed structure and age relationships of the coral deposits indicate that both uplift and subsidence occurred rapidly most probably as coseismic events with vertical displacements of 0 5 to 4 5 m these events may be related to rupture on nw se trending reverse faults which have been mapped in the nearby adelbert range and possibly on ne trending cross faults which have been inferred from seismicity this interpretation implies a much greater degree of tectonic instability and potential seismic hazard in the region than previously recognized although the inferred coseismic vertical displacements are shown to be consistent with present day local seismicity in a broader context the study illustrates how detailed analysis of vertical changes in coral reef structure and assemblages may be used as a sensitive indicator of changing relative sea level capable of resolving century timescale events and reversals

tectonic evolution and copper gold metallogenesis of the papua new guinea and solomon islands region papua new guinea and the solomon islands are in one of the most prospective regions for intrusion related mineral deposits however because of the tectonic complexity of the region and the lack of comprehensive regional geological datasets the link between mineralization and the regional scale geodynamic framework has not been understood here we present a new model for the metallogenesis of the region based on a synthesis of recent studies on the petrogenesis of magmatic arcs and the history of subduction zones throughout the region combined with the spatio temporal distribution of intrusion related mineral deposits and six new deposit ages convergence at the pacific australia plate boundary was accommodated from at least 45 ma by subduction at the melanesian trench with related melanesian arc magmatism the arrival of the ontong java plateau at the trench at ca 26 ma resulted in cessation of subduction immediately followed by formation of cu au porphyry epithermal deposits at 24 20 ma throughout the melanesian arc late oligocene to early miocene tectonic reorganization led to initiation of subduction at the pocklington trough and onset of magmatism in the maramuni arc the arrival of the australian continent at the pocklington trough by 12 ma resulted in continental collision and ore deposit formation from 12 to 6 ma this is represented by cu au porphyry deposits in the new guinea orogen and epithermal au systems in the papuan peninsula from 6 ma crustal delamination in papua new guinea related to the prior pocklington trough subduction resulted in adiabatic mantle melting with emplacement of diverse cu and au porphyry and epithermal deposits within the papuan fold and thrust belt and papuan peninsula from 6 ma to the present day subduction at the new britain and san cristobal trenches from ca 10 ma resulted in an escalation in tectonic complexity and the onset of microplate tectonics in eastern papua new guinea and the solomon islands this is reflected in the formation of diverse and discrete geodynamic settings for mineralization within the recent to modern arc setting primarily related to upper plate shortening and extension and the spatial relationship to structures within the subducting slab

continuous monitoring of an active fault in a plate suture zone a creepmeter study of the chihshang fault eastern taiwan abstract data from continuously monitored creepmeters across the active chihshang fault in eastern taiwan are presented the chihshang fault is an active segment of the longitudinal valley fault the main suture between the converging philippine and eurasian plates in taiwan since the 1951 earthquake mw 7 0 no earthquake larger than magnitude 6 0 occurred in the chihshang area at least during the last 20 years the chihshang fault underwent a steady creep movement resulting in numerous fractures at the surface five creepmeters were installed in 1998 at two sites tapo and chinyuan within the chihshang active fault zone one year results from august 1998 to july 1999 show a horizontal shortening of 19 4 0 3 mm and 17 3 0 7 mm at tapo and chinyuan respectively these annual shortening rates are in a good agreement with other estimates of strain rate independently obtained from geodetic measurements and geological site investigation the creepmeter measurements were made on a daily basis providing accurate information on the previously unknown evolution of creep during the year the records of fault creep at the tapo site thus revealed close seasonal correlation with average rainfall the period of high creep rate coincides with the wet season whereas that of low creep rate coincides with the dry season also in comparison with the tapo site the creep behaviour as a function of time is complex at the chinyuan site possible factors of irregularity are under investigation thermal effect acting on the concrete basement of the creepmeters earth tide effect water table variations in a nearby rice field and rainfall the comparison between gps measurements across the longitudinal valley 31 mm year of horizontal displacement and the creepmeter measurement across the chihshang fault zone 17 19 mm year of horizontal displacement suggests that there exists other shortening deformation across the active fault zone in addition to those we have measured from the creepmeters

geomorphology of the southernmost longitudinal valley fault implications for evolution of the active suture of eastern taiwan in order to understand fully the deformational patterns of the longitudinal valley fault system a major structure along the eastern suture of taiwan we mapped geomorphic features near the southern end of the longitudinal valley where many well developed fluvial landforms record deformation along multiple strands of the fault our analysis shows that the longitudinal valley fault there comprises two major strands the luyeh strand on the west has predominantly reverse motion the peinan strand on the east has a significant left lateral component between the two strands late quaternary fluvial sediments and surfaces exhibit progressive deformation the luyeh strand dies out to the north where it steps to the east and joins the peinan strand to become the main strand of the reverse sinistral longitudinal valley fault to the south the luyeh strand becomes an e w striking monocline this suggests that the reverse motion on the longitudinal valley system decreases drastically at that point the longitudinal valley fault system is therefore likely to terminate abruptly there and does not seem to connect to any existing structure further to the south this abrupt structural change suggests that the development of the longitudinal valley suture occurs through discrete structural jumps rather than by a continuous northward maturation

structural evolution of a modern arc continent collision in papua new guinea the finisterre mountains and western solomon sea of northern papua new guinea are the site of an active oblique arc continent collision comparison of structures along the length of the collision zone provides a history of its structural evolution here we compare the results of structural mapping in the finisterre mountains where collision is in an advanced state with side scan sonar images and potential field data from the western solomon sea which illuminate the structure of the juvenile collision zone an accretionary wedge complex composed of a southwestward younging stack of imbricate thrust sheets developed along a westward extension of the modern new britain trench prior to collision this accretionary wedge crops out in the finisterre mountains as the erap structural complex consequences of collision were doubling of the crustal thickness to 50 52 km rapid up lift of the finisterre terrane and initiation of a major out of sequence thrust that displaced the finisterre volcanics over younger sediments of the accretionary wedge this out of sequence thrust is mapped in the finisterre mountains as the wongat thrust and it is recognized in the western solomon sea on side scan sonar images a second generation out of sequence thrust formed after substantial uplift had subaerially exposed the collision zone this thrust crosscuts the wongat thrust forming a window of accretionary wedge sediments bounded to both the ne and sw by finisterre volcanics this younger out of sequence thrust is the gain thrust of the finisterre mountains at least 15 km of crustal shortening has occurred across the wongat thrust at 146 30 e longitude and the gain thrust has accommodated up to 7 4 km of shortening in the same area it is probable that the well lithified finisterre volcanics serve as the backstop for the accretionary wedge collision complex that is forming in the collision zone of the western solomon sea potential field models indicate that this backstop is arcward dipping thrusting of the back stop material over the accretionary wedge as we observe along the wongat thrust is expected for wedges with arcward dipping backstops a series of ne sw trending cross faults perpendicular to the dominant structural trend exist in the collision zone these faults have components of left lateral strike slip and se down dip slip motion it is likely that the finisterre terrane is being segmented along these faults with the segmentation caused by differential resistance to convergence between the downgoing continental crust of the collision zone and the oceanic crust of the subduction zone to the east the origin of the faults is uncertain previous authors believed that they formed during paleogene volcanic activity in the finisterre terrane the colinearity of several of these cross faults with major mapped faults on the downgoing plate raises the possibility that the finisterre range cross faults are upper plate manifestations of lower plate structures and as such reveal strong interplate coupling in the collision zone

new empirical relationships among magnitude rupture length rupture width rupture area and surface displacement source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude m surface rupture length subsurface rupture length downdip rupture width rupture area and maximum and average displacement per event the resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment moment magnitude subsurface rupture length downdip rupture width and average surface displacement each source parameter is classified as reliable or unreliable based on our evaluation of the accuracy of individual values only the reliable source parameters are used in the final analyses in comparing source parameters we note the following trends 1 generally the length of rupture at the surface is equal to 75 of the subsurface rupture length however the ratio of surface rupture length to subsurface rupture length increases with magnitude 2 the average surface displacement per event is about one half the maximum surface displacement per event and 3 the average subsurface displacement on the fault plane is less than the maximum surface displacement but more than the average surface displacement thus for most earthquakes in this data base slip on the fault plane at seismogenic depths is manifested by similar displacements at the surface log linear regressions between earthquake magnitude and surface rupture length subsurface rupture length and rupture area are especially well correlated showing standard deviations of 0 25 to 0 35 magnitude units most relationships are not statistically different at a 95 significance level as a function of the style of faulting thus we consider the regressions for all slip types to be appropriate for most applications regressions between magnitude and displacement magnitude and rupture width and between displacement and rupture length are less well correlated and have larger standard deviation than regressions between magnitude and length or area the large number of data points in most of these regressions and their statistical stability suggest that they are unlikely to change significantly in response to additional data separating the data according to extensional and compressional tectonic environments neither provides statistically different results nor improves the statistical significance of the regressions regressions for cases in which earthquake magnitude is either the independent or the dependent parameter can be used to estimate maximum earthquake magnitudes both for surface faults and for subsurface seismic sources such as blind faults and to estimate the expected surface displacement along a fault for a given size earthquake

large scale duplexes within the new britain accretionary wedge a possible example of accreted ophiolitic slivers a multichannel seismic line acquired in the western solomon sea images reflectors within the new britain accretionary wedge which we interpret as accreted duplexes of the downgoing oceanic plate the seaward edge of the largest duplex is located just 6 km from the toe of the accretionary wedge suggesting recent incorporation into the wedge these data therefore provide an excellent opportunity to determine the dynamics of oceanic basement accretion the oceanic basement is represented by a high amplitude low frequency reflector that can be traced up to 40 km arcward of the toe of the accretionary wedge on several seismic lines in this survey another prominent reflector similar in character to the basement reflector lies approximately one second higher in at least one seismic section this reflector is likely to be the top of a basement duplex structures imaged above the basement duplex define a preduplex accretionary wedge whereas those at the toe indicate a new episode of postduplex accretion we have tested this interpretation using both magnetic modeling and interval velocity analysis magnetic models of the wedge that include basement duplexes provide a good match to the observed magnetic data and provide limits on the size and interval velocities of the slivers interval velocities between the two strong reflectors calculated using stacking velocities and constrained through magnetic modeling exceed 5000 m s and are consistent with our interpretation of ophiolitic slivers within the accretionary wedge

interactions between a transform fault and arc volcanism in the bismarck sea papua new guinea 1 we present a new interpretation of the geological evolution of the western branch of the bismarck sea seismic lineation bssl region offshore the northwestern coast of papua new guinea from mapping and interpretation of bathymetry and backscatter data acquired aboard r v kilo moana in 2004 at present the schouten islands parallel the papua new guinea coast but this distribution results from the left lateral displacement of some of these volcanic islands by the bismarck sea seismic lineation the trend of the islands and seamounts was initially aligned n s and then displaced by the bssl wei island lies on a large submarine edifice that may have formed as part of a leaky transform subsequent to forming wei island was bisected and its pieces were displaced 45 km using this distance together with the strike slip rate for the bismarck sea seismic lineation predicted from the north bismarck plate south bismarck plate pole the division occurred approximately 385 000 years ago

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

a classification of thrust fronts variations in structural style at thrust fronts are examined to determine whether they represent different stages of typical thrust sheet development or unique events as orogenic stresses declined thrust fronts are classified as buried or emergent in which either the sole thrust died out in the subsurface or emerged to the synorogenic surface buried thrust fronts are subdivided into two types type 1 fronts end in broad zones of low strain type 2 fronts end in zones of high strain emergent thrust fronts may display significant horizontal displacement strongly emergent or limited displacements of about 1 km weakly emergent tip stick thrust fronts exhibit high strains and out of sequence thrusting different types of thrust fronts are linked by transitional tip li e anticlines and strike slip zones most thrust front types represent different stages of typical thrust sheet development but those in type 1 display atypical development unique to thrust fronts

shallow seismicity of arc continent collision near lae papua new guinea abstract in northeastern new guinea the narrow ramu markham valley rmv separates island arc rocks to the north from those of continental origin to the south and appears to be the western onland extension of the new britain trench to explore the tectonic processes at the leading edge of the island arc during collision we operated a portable seismic network for six weeks near the city of lae at the eastern end of the rmv we observed a narrow near vertical belt of seismicity between 10 and 30 km depth that we call the lae seismic zone lsz starting at the rmv in the southwest and trending northeasterly cutting across surface geologic structure the truncation of the lsz along a steep plane by the ramu markham fault zone rmfz and earthquake first motions suggest that the earthquakes occur in the hanging wall of a steep n dipping fault that crops out at the rmfz we also consider that the lsz is within the lower plate of a gently dipping thrust below 20 km depth the microearthquake zone is truncated by a gently ne dipping plane coinciding in depth and dip with nodal planes of recent large mb 5 6 and 6 0 thrust earthquakes we suggest that the huon peninsula is being emplaced onto the australian plate along a gently 25 dipping thrust fault that is 20 km deep beneath lae the rmfz may be a steeply dipping thrust fault that connects with this gently n dipping thrust but accommodates little convergence at present the lsz trends nearly perpendicular to an anticlinal range which appears to be sheared in a left lateral sense p wave first motions for earthquakes in the lsz with steep 70 to 90 dip nodal planes that strike parallel to the lsz suggest a component of south side up displacement also hence the crustal block south of the lsz may be rising relative to the huon peninsula and the rapid quaternary uplift rates estimated for the lae coastal region may be higher than the uplift rate of the huon peninsula as a whole we suggest that the lsz reveals a tear of small offset in the huon terrane but may be similar to a structure that produced a magnitude 7 earthquake near madang in 1970

cenozoic tectonics of new guinea major hydrocarbon discoveries have been made in eastern and westernmost new guinea and there is great potential for additional discoveries although the island is a type locality for arc continent collision during the cenozoic the age number and plate kinematics of the events that formed the island are vigorously argued the northern part of the island is underlain by rocks with oceanic island arc affinities and the southern part is underlain by the australian continental crust based on regional sedimentation patterns it is argued herein that the cenozoic tectonic history of the island involves two distinct collisional orogenic events the first cenozoic event the peninsular orogeny of oligocene age 35 30 ma was restricted to easternmost new guinea emergent uplifts that shed abundant detritus resulted from the subduction of the northeastern corner of the australian continent beneath part of the inner melanesian arc this collision uplifted the papuan ophiolite and formed the associated mountainous uplift that was the primary source of siliciclastic sediments that largely filled the aure trough between the oligocene and miocene the paleogeography of the region was similar to present day new caledonia the continental crust under central and western new guinea remained a passive margin the second event the central range orogeny began in the latest middle miocene when the bulldozing of australian passive margin strata first created emergent uplifts above a north dipping subduction zone beneath the western part of the outer melanesian arc the cessation of carbonate shelf sedimentation and widespread initiation of siliciclastic sedimentation on top of the australian continental basement is dated at about 12 ma this collision emplaced the irian ophiolite and created the present mountainous topography forming the spine of the island

subduction versus accretion of intra oceanic volcanic arcs insight from thermo mechanical analogue experiments we perform thermo mechanical laboratory experiments designed to explore the behaviour of the volcanic arc during intra oceanic arc continent collision following oceanic subduction and subsequent back arc opening the overriding oceanic lithosphere is made of two layers representing the oceanic crust and the lithospheric mantle this lithosphere carries a volcanic arc and is thinned and weakened beneath both the arc and the back arc basin the subducting plate contains three parts one layer oceanic and two layer crust and mantle continental lithosphere with a continental margin between them when the continental margin reaches the trench and starts subducting the overriding plate undergoes growing horizontal compression and finally fails in the vicinity of the back arc spreading centre which is the weakest part of this plate the failure can result in subduction of the whole arc plate comprised between the trench and the back arc spreading centre during subduction of the arc plate the mantle part of this plate subducts completely while the behaviour of the arc crust depends on its thickness and strength which is a function of composition and temperature we tested four cases with different arc crust thicknesses and composition rheology with total lithosphere thickness in the arc being constant three types of tectonic evolution have been obtained complete arc subduction complete arc accretion and partial arc subduction accretion the result is largely controlled by the crustal thickness of the arc a thin arc thickness equivalent to v16 km in nature made of the same strong material as the oceanic crust subducts completely without leaving any trace at the surface on the contrary a thick arc equivalent to v26 km in nature made of the same material is scraped off and accreted to the overriding plate the lower crust of such an arc is hotter therefore its strength at moho depth and coupling between crust and mantle are small in addition the thick arc has a high isostatic relief and hence a greater mechanical resistance to subduction therefore the arc is scraped off if the arc is made of a weaker continental like material or contains a weak layer low friction interface it is completely or partially scraped off even if it is small when there is no back arc opening before collision no thin and weak lithosphere in the rear of the arc the overriding plate fails in the arc area which may result in a complete fore arc block subduction with the volcanic arc remaining at the surface the obtained models are compared with mountain belts with nearly no trace of arc activity oman with accreted arc kohistan and with small remnants of subducted arc southern tibet 2003 elsevier science b v all rights reserved

determining fault geometry from the distribution of coseismic fault slip related to the 2006 taitung earthquake eastern taiwan on 1 april 2006 the taitung earthquake m w 6 1 occurred in taiwan at the boundary between the philippine sea and eurasian plates where high convergence rates contributed to the development of plio pleistocene orogeny in the region from the joint inversion of seismic and geodetic data we identified the event s fault geometry and reconstructed the distribution of coseismic fault slip we modeled fault geometries with increasing complexity and selected the model that best reproduced all datasets simultaneously even though the earthquake magnitude was moderate rupturing occurred in two steps the initial rupture was generated on a listric north south trending fault for which dip decreases with increasing depth and was immediately followed by movement along a perpendicular structure that cross cuts the main fault at 5 km south of the earthquake hypocenter the average slip along the rupture was 30 cm with a maximum of 87 cm oblique reverse fault movement was characterized by a predominant left lateral component the amount of slip is well constrained for offsets of more than 5 cm with an associated uncertainty of 32 for slip amounts greater than 5 cm uncertainties on rake and rupture time are 11 and 0 54 s respectively the rupture propagated from the hypocenter bilaterally moving slightly faster toward the south 2 5 0 4 km s than to the north 1 7 0 1 km s to the south the rupture was rapidly transmitted upward at the junction with the cross cutting east west segment whereas in the north the rupture remained confined to the lower segment of the main fault from global positioning systems gps and seismic data time window 1 min we infer that the cross cutting segment was activated following coseismic rupture on the main north south fault yet close enough in time to be associated with coseismic movement acquired by gps daily solutions

double sided onshore offshore seismic imaging of a plate boundary super gathers across south island new zealand abstract onshore offshore seismic refraction profiling allows for the determination of crustal and mantle structures in the transition between continental and oceanic environments islands and narrow landmasses have the unique geometry of allowing for double sided onshore offshore experiments that favor the construction of composite super gathers using the acquisition of onshore offshore and ocean bottom seismometer receiver gathers land explosion shot gathers and near vertical incidence multichannel seismic mcs profiling a number of sites at plate boundaries are amenable to the application of double sided onshore offshore imaging including the indo australian pacific transform boundary on south island new zealand by comparing the ratio of island width to mantle refraction pn maximum crossover distance using nondimensional distances we provide an indicator of raypath coverage for crustal illumination islands or narrow land masses whose widths are less than twice their maximum crossover distance are candidates for double sided onshore offshore experiments the sight south island geophysical investigation experiment in new zealand is located where the width of south island is sufficiently narrow with respect to its crustal thickness that a double sided onshore offshore experiment allows for complete crustal imaging of the associated plate boundary

basin dimensions and morphology as controls on amplification of tidal motions the early miocene north hungarian bay following the late aquitanian sea level fall tide influenced deposition started in the north hungarian bay an embayment in the paratethys open to the north east the relatively narrow funnel shape of the bay supported amplification of tidal movements resulting in the generation of strong tidal currents the length and the depth of the north hungarian bay and the connecting seaway through east slovakia fell into the tidal amplification window and thus fulfilled the conditions needed for resonant amplification of semidiurnal m2 tides tide influenced deposits were formed at both sides of the north hungarian bay they reflect dominant currents in opposite directions and of different strengths at either side of the basin this difference was the result of bottom tide interactions cyclonic anticlockwise residual currents were induced above the asymmetrical central depression in the bay and were superimposed upon the tidal currents producing an anticlockwise time and velocity asymmetrical current system the north hungarian bay and other examples show that amplification of tidal motions and formation of tide influenced deposits may occur if basin dimensions pass through the tidal amplification window this window represents ideal conditions for resonant or amphidromic amplification of tidal currents it determines an ideal length depth or width depth ratio relative to the wavelength of the astronomical tides thus signs of strong tidal influence in fossil basin fills can be used to reconstruct the dimensions length depth and width of such basins

fmnear determination of focal mechanism and first estimate of rupture directivity using near source records and a linear distribution of point sources abstract the fmnear method is based on both the waveform inversion of near source seismic records and on a linear finite source model the primary source parameters that are determined are the moment magnitude m w the double couple focal mechanism strike dip and rake and the distribution of seismic moment along strike which provides a first order estimate of rupture length and directivity source depth is also explored an advantage of the approach is the use of the finite source model directly within the search of the focal mechanism this strategy allows the inclusion of strong motion records at short distances for moderate to very large earthquakes m w 5 5 however for moderate to small earthquakes m w m w 4 5 to 9 0 test cases incorporate continental events recorded by a large number of well distributed stations and less favorable cases in its present form the fmnear approach is very well adapted to continental earthquakes surrounded by seismic stations but it can also give informative results in more difficult configurations the fmnear method is now ready for fully automated determinations and is already implemented in near real time online material details on inversion parameters flow charts of the fmnear method figures of waveform modeling and sensitivity for selected events and table of velocity models

convergent margin extension associated with arc continent collision the finsch deep papua new guinea the finsch deep is an asymmetric rhomboidal basin with a maximum depth of 5400 m situated to the north of the solomon sea triple junction papua new guinea anomalous slip vector azimuths regional plate velocities earthquake hypocenters and bathymetric depth relative to surrounding terranes all indicate that the finsch deep is not a passive feature developed behind the subduction front our interpretation of all available data including seismological studies detailed bathymetry side scan character and seismic sections suggests that the finsch deep has developed due to n s extension in the transition zone from continental collision west of the solomon sea triple junction to oceanic subduction to the east the key mechanisms thought to drive convergent margin extension within the new guinea collision are slab pull and oblique subduction researchers studying other convergent margins have recognized underplating and tectonic erosion as driving mechanisms for extension though we have no evidence to either support or discount their presence within the new guinea collision

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

shear concentration in a collision zone kinematics of the chihshang fault as revealed by outcrop scale quantification of active faulting longitudinal valley eastern taiwan abstract repeated measurements of active deformation were carried out at three sites along the active chihshang fault a segment of the longitudinal valley fault zone of eastern taiwan the present day plate boundary between the philippine sea plate and eurasia reliable annual records of displacement along an active fault were obtained based on detailed surveys of faulted concrete structures along the active chihshang fault striking n18 e we determined average motion vectors trending n37 w with an average shortening of 2 2 cm yr thus the transverse component of motion related to westward thrusting is 1 8 cm yr whereas the left lateral strike slip component of motion is 1 3 cm yr the fault dips 39 45 to the east so that the vertical displacement is 1 5 3 cm yr and the actual oblique offset of the fault increases at a rate of 2 7 3 7 cm yr this is in good agreement with the results of regional geodetic and tectonic analyses in taiwan and consistent with the n54 w trend of convergence between the northernmost luzon arc and south china revealed by gps studies our study provides an example of extreme shear concentration in an oblique collision zone at chihshang the whole horizontal shortening of the longitudinal valley fault 2 2 cm yr on average occurs across a single narrow fault zone so that the whole reverse slip about 2 7 3 7 cm yr depending on fault dip was entirely recorded by walls 20 200 m long where faults are tightly localized this active faulting accounts for more than one fourth 27 of the total shortening between the luzon arc and south china recorded through gps analyses further surveys should indicate whether the decreasing shortening velocity across the fault is significant revealing increasing earthquake risk due to stress accumulation or not revealing continuing fault creep and weak behaviour of the chihshang fault

stratigraphic constraints on the development and timing of arc continent collision in northern papua new guinea abstract the finisterre mountains of northern papua new guinea are the site of an active southeast propagating collision between the finisterre terrane and the australian continental margin the clastic sedimentary sequence on the southern flank of the range records the history of the collision the age of initial collision can be deduced from the second of two provenance shifts that occur in the sequence as revealed by a study of 69 dated sandstones the first shift at about 16 18 ma was from volcanolithic sediments to mixed provenance sediments rich in quartz and metasedimentary lithic fragments these latter sediments were probably derived from the orogenic belt active along the australian continental margin at that time at 3 0 3 7 ma the volcanic source was rejuvenated initial collis on at this time uplifted the finisterre terrane causing the provenance shift here we break the stratigraphic sequence into three units the sarawaget beds the erap structural complex and the leron formation the sarawaget beds are middle eocene to early miocene volcanolithic sandstones that were deposited as clastic aprons un the flanks of the finisterre volcanic are volcanic activity ceased in the early miocene the erap complex is divided into two clastic subunits the sukurum and nariawang units and one basaltic subunit the gorambampan unit the sukurum unit consists of turbidites deposited south of the extinct finisterre arc from the latest early miocene through the early pliocene much of the sediment in the unit was derived from low to medium grade metasedimentary units on the active northern margin of the australian continent renewed subduction i the late miocene or pliocene occurred along the new britain trench just south of the remnant finisterre arc closing an eastward widening small ocean basin the sukurum unit probably represents an accretionary prism that formed at this subduction zone scattered seamounts that were scraped off the downgoing plate are preserved as the gorambampan unit initial collision of the finisterre terrane with the continental margin at 3 0 3 7 ma uplifted the paleogene volcanic rocks which were rapidly eroded and deposited as the volcanolithic turbidites of the nariawang unit southeastward propagation of the collision tip caused incorporation of the nariawang unit into the accretionary wedge rapid uplift raised the terrane foreland above sea level sometime after 1 3 mo the fluvial lacustrine and marginal marine deposits of the leron formation accumulated in a terrane foreland basin that developed south of the collision zone further propagation of the thrust front toward the foreland incorporated the leron formation into the fold and thrust belt rapid uplift of the finisterre range and voluminous volcanolithic sedimentation continue to the present in the finisterre range arc continent collision is marked by a provenance shift from metasedimentary and quartzose detritus to volcanic detritus in another modern arc continent collision on taiwan collision has caused a shift from volcanic to metasedimentary detritus we attribute this discrepancy to the presence of the finisterre terrane an extinct arc in the forearc region of the finisterre arc continent collision and the absence of a similar terrane in the forearc area on taiwan the very different evolution of sandstone provenance in the active taiwan and finisterre collision zones shows that local factors can profoundly influence the provenance histories of collision zones no one model of sediment provenance evolution can characterize all collision zones whether modern or ncient

rapid magmatic processes accompany arc continent collision the western bismarck arc papua new guinea new u th ra major and trace element and sr nd pb isotope data are presented for young lavas from the new britain and western bismarck arcs in papua new guinea new britain is an oceanic arc whereas the latter is the site of an arc continent collision building on a recent study of the manus basin contrasts between the two arcs are used to evaluate the processes and timescales of magma generation accompanying arc continent collision and possible slab detachment all three suites share many attributes characteristic of arc lavas that can be ascribed to the addition of a regionally uniform subduction component derived from the subducting altered oceanic crust and sediment followed by dynamic melting of the modified mantle however the western bismarck arc lavas diverge from the pb isotope mixing array formed by the new britain and the manus basin lavas toward elevated 208pb 204pb we interpret this to reflect a second and subsequent addition of sediment melt at crustal depth during collision 238u and 226ra excesses are preserved in all of the lavas and are greatest in the western bismarck arc high mg andesites with high sr y ratios in the westernmost arc are attributed to recent shallow mantle flux melting at the slab edge data for two historical rhyolites are also presented although these rhyolites formed in quite different tectonic settings and display different geochemical and isotopic compositions both formed from mafic parents within millennia

quantitative analysis of surface coseismic faulting and postseismic creep accompanying the 2003 mw 6 5 chengkung earthquake in eastern taiwan 1 on the basis of field observations and geodetic measurements we analyzed coseismic and postseismic deformation of the 2003 mw 6 5 chengkung earthquake across the rupture trace of the chihshang fault which lies along the suture zone between the philippine sea plate and the eurasian plate in eastern taiwan at three of our investigation sites along the chihshang fault the earthquake deformation was exhibited by fresh fractures within the 150 to 200 m wide surface fault zone reactivating preexisting fractures in most cases in addition to daily recorded creep meter data geodetic measurements including leveling distance and angle electronic distance meters and gps measurements were carried out during surveys 20 25 days before 20 25 days after and 120 125 days after the earthquake the near fault surface deformation is mainly characterized by anticlinal folding in the hanging wall and minor gentle synclinal folding in the footwall the geodetic data show that within our 150 to 250 m wide networks the coseismic deformation of the main shock produced only about 1 2 cm of horizontal shortening and vertical offset across the fault zone a larger additional displacement of about 7 9 cm for both the horizontal shortening and the vertical offset occurred as postseismic creep during the 120 125 days following the main shock we interpret the predominant folding and the rather large postseismic creep as a result of strong velocity strengthening along the fault plane near the surface which caused locking or coupling effect during the coseismic rupturing the depth of locked segment along which the coseismic and postseismic slip decreased dramatically upward is estimated to be 25 100 m depending on the site we interpreted the velocity strengthening and coupling effect at the shallow level as viscoelastic behavior of unconsolidated deposits in the footwall and melange mudstone in the hanging wall by incorporating far fault continuous gps data it was found that minor but significant deformation occurred outside of the surface ruptures zone of the chihshang fault in addition a possible back thrust or back fold might have occurred in the hanging wall we note that the chengkung earthquake occurred during the dry season this is consistent with the fact that the mechanical coupling of the fault at shallow depth is higher during drier periods inferred from the previous seasonal creep data at outcrop scale regarding damage of construction such as a concrete retaining wall most of the coseismic and postseismic horizontal shortening was essentially absorbed by multiple distributed brittle ruptures which deserves attention in terms of earthquake hazard mitigation

active arc continent collision earthquakes gravity anomalies and fault kinematics in the huon finisterre collision zone papua new guinea the huon finisterre island arc terrane is actively colliding with the north edge of the australian continent the collision provides a rare opportunity to study continental accretion while it occurs we examine the geometry and kinematics of the collision by comparing earthquake source parameters to surface fault geometries and plate motions and we constrain the forces active in the collision by comparing topographic loads to gravity anomalies waveform inversion is used to constrain focal mechanisms for 21 shallow earthquakes that occurred between 1966 and 1992 seismic moment 1017 to 3 1020 n m twelve earthquakes show thrust faulting at 22 37 km depth the largest thrust events are on the north side of the huon peninsula and are consistent with slip on the ramu markham thrust fault zone the northeast dipping thrust fault system that bounds the huon finisterre terrane thus much of the terrane s crust but little of its mantle is presently being added to the australian continent the large thrust earthquakes also reveal a plausible mechanism for the uplift of pleistocene coral terraces on the north side of the huon peninsula bouguer gravity anomalies are too negative to allow simple regional compensation of topography and require large additional downward forces to depress the lower plate beneath the huon peninsula with such forces plate configurations are found that are consistent with observed gravity and basin geometry other earthquakes give evidence of deformation above and below the ramu markham thrust system four thrust events 22 27 km depth directly below the ramu markham fault outcrop are too deep to be part of a planar ramu markham thrust system and may connect to the north dipping highlands thrust system farther south two large strike slip faulting earthquakes and their aftershocks in 1970 and 1987 show faulting within the upper plate of the thrust system the inferred fault planes show slip vectors parallel to those on nearby thrust faults and may represent small offsets in the overriding plate these faults along with small normal faulting earthquakes beneath the huon finisterre ranges and a 25 along strike rotation of slip vectors demonstrate the presence of along strike extension of the accreting terrane and along strike compression of the lower plate

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

volcano collapse and tsunami generation in the bismarck volcanic arc papua new guinea during a cruise on the r v kilo moana in 2004 we mapped 12 debris avalanches from volcanoes in the bismarck volcanic arc estimated their sizes and computed the size of potential tsunami run up in major local population centers from these features we used the towed side scan instrument hawaii mr1 the hull mounted em120 system for swath bathymetry and backscatter intensity a shallow penetration chirp system several bottom camera tows and selected cores we calibrate our computations with the known tsunami run up of the ritter collapse even the small collapses may have had significant run up on near by coastlines had any of the collapses we have identified occurred in modern times each would affect a presently populated region of the coastline to a moderate or significant degree

a synthesis of the geologic evolution of taiwan abstract the island arc of taiwan is composed of cenozoic geosynclinal sediments more than 10 000 m thick lying on a pre tertiary metamorphic basement pleistocene to miocene andesitic islands surround the main island and are related mostly to arc magmatism the penghu island group in the taiwan strait is covered with pleistocene flood basalt neogene shallow marine clastic sediments are exposed mainly in the western foothills with pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands a thick sequence of paleogene to miocene argillitic to slaty metaclastic rocks underlies the western central range and forms the immediate sedimentary cover on the pre tertiary metamorphic complex to the east which represents an older mesozoic arc trench system the coastal range in eastern taiwan is a neogene andesitic magmatic arc including also a large variety of volcaniclastic and turbiditic sediments cenozoic taiwan is the site of arc continent collision where the luzon arc on the philippine sea plate overrides the chinese continental margin on the eurasian plate east and northeast of taiwan the polarity of subduction changes whereby the oceanic philippine sea plate is subducting beneath the ryukyu arc system on the eurasian plate continent arc collision in taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well defined plate boundary or subduction zone

late miocene to recent formation of the aure moresby fold thrust belt and foreland basin as a consequence of woodlark microplate rotation papua new guinea the aure moresby fold thrust belt and aure moresby foreland basin are located in the eastern gulf of papua gop papua new guinea png and formed during late miocene recent as the result of large scale counterclockwise rotation of the 355 000 km2 woodlark microplate to document the structure stratigraphy and age of convergent deformation along the poorly studied western edge of the rotating woodlark microplate we integrate results of 2 538 km of previously unpublished 2 d seismic reflection data with onshore geologic and gps studies from previous workers the late miocene aure moresby fold thrust belt is a 400 km long northwest trending fold belt exposed onshore in papua new guinea that plunges to the southeast where continuous folds and northeast dipping thrusts can be imaged in the subsurface for more than 250 km the arcuate trend of the aure moresby fold thrust belt along the southwestern coast and offshore areas of the papuan peninsula parallels the shape of the adjacent offshore aure moresby foreland basin and the strike of the transpressional segment of the left lateral owen stanley fault zone osfz passing along the center of the papuan peninsula as the osfz becomes more transtensional east of 148 e folds of the aure moresby fold thrust belt along southern coast of the peninsula become less prominent and the adjacent aure moresby foreland basin transitions into an undeformed cenozoic passive margin setting these observations of convergent an left lateral deformation along the aure moresby fold thrust belt are consistent with 1 counterclockwise rotation of the woodlark microplate known from regional gps studies 2 coeval opening of the woodlark basin along its southern edge in the late miocene and 3 rapid subduction at the new britain trench along its northern edge the kinematics of the rotating woodlark microplate are driven by slab pull forces acting on the actively subducting northern edge of the microplate

structure and evolution of the magnitogorsk forearc basin identifying upper crustal processes during arc continent collision in the southern urals the southern urals of russia contain a well preserved example of a paleozoic arc continent collision in which the intraoceanic magnitogorsk volcanic arc and its forearc basin sediments accreted to the east european craton during the devonian the magnitogorsk arc records the evolution from incipient intraoceanic subduction to a mature arc and by comparing its surface geological features with those in active arc continent collision settings it is possible to identify upper crustal processes that were active in the southern urals the arc edifice can be divided into western and eastern volcanic fronts that were active during different stages of arc evolution and for which two distinct phases of forearc basin development can be recognized the late lower to middle devonian aktau formation represents a remnant of the intraoceanic to collisional forearc basin to the irendyk volcanic front whereas the middle devonian to lower carboniferous ulutau koltubanian and zilair formations were deposited in a suture forearc basin to the east magnitogorsk volcanic front it was not until the late devonian that these two basins were joined structural mapping combined with reflection seismic profiling shows these basins to be affected by open nonlinear volcanic basement cored synsedimentary folds the karamalytash anticline appears to have the geometry of a growth fold that formed during deposition of sediments in the suture forearc basin the forearc region is affected by minor thrusting that involves the volcanic basement although it is not clear if these thrusts reactivate preexisting trench parallel faults synsedimentary deformation slumping and olistostrome development were common throughout the suture forearc basin history but were especially widespread during the late devonian when the full thickness of the continental crust is interpreted as having arrived at the subduction zone

gps and seismological constraints on active tectonics and arc continent collision in papua new guinea implications for mechanics of microplate rotations in a plate boundary zone 1 the island of new guinea is located within the deforming zone between the pacific and australian plates that converge obliquely at 110 mm yr new guinea has been fragmented into a complex array of microplates some of which rotate rapidly about nearby vertical axes we present velocities from a network of 38 global positioning system gps sites spanning much of the nation of papua new guinea png the gps derived velocities are used to explain the kinematics of major tectonic blocks in the region and the nature of strain accumulation on major faults in png we simultaneously invert gps velocities earthquake slip vectors on faults and transform orientations in the woodlark basin for the poles of rotation of the tectonic blocks and the degree of elastic strain accumulation on faults in the region the data are best explained by six distinct tectonic blocks the australian pacific south bismarck north bismarck and woodlark plates and a previously unrecognized new guinea highlands block significant portions of the ramu markham fault appear to be locked which has implications for seismic hazard determination in the markham valley region we also propose that rapid clockwise rotation of the south bismarck plate is controlled by edge forces initiated by the collision between the finisterre arc and the new guinea highlands

geodynamic processes of taiwan arc continent collision and comparison with analogs in timor papua new guinea urals and corsica abstract the taiwan arc continent collision involves four geodynamic processes intra oceanic subduction initial arc continent collision advanced arc continent collision and arc collapse subduction these processes now occur simultaneously in 19 24 30 n but have operated sequentially southward since the late miocene although the geological and geophysical features appear to change progressively from north to south across the island they are distinct within individual tectonic regimes using the present scenario as a basis for comparison it is suggested that timor is at the initial arc continent collision stage while papua new guinea and urals proceeded to the advanced arc continent collision stage however corsica represents an even more advanced stage than in taiwan where a previously accreted arc has collapsed and been totally removed

post 8 ma reconstruction of papua new guinea and solomon islands microplate tectonics in a convergent plate boundary setting papua new guinea and the solomon islands are located in a complex tectonic setting between the converging ontong java plateau on the pacific plate and the australian continent here we present a late neogene to quaternary plate tectonic reconstruction for this region the reconstruction was performed using gplates software and is based on data derived from multiple geological datasets including seafloor magnetic isochrons 3d models of subducted slabs paleomagnetic data deformation patterns and the spatio temporal distribution of arc magmatism the reconstruction shows that since ca 6 ma the crustal elements that comprise papua new guinea and the solomon islands began interacting in advance of the impending collision between the ontong java plateau and australian continent leading to the inception of regional microplate tectonics and escalation in tectonic complexity the bismarck sea initially formed as a back arc basin behind the new britain arc but was later modified during arc continent collision following collision the west bismarck sea developed into a transpressional zone associated with subduction of the caroline plate and north bismarck microplate at the new guinea trench farther east seafloor spreading occurred in response to the clockwise rotation of the south bismarck microplate relative to the australian plate the solomon sea plate was subjected to anticlockwise rotation relative to papua new guinea primarily due to the west dipping subduction at the new britain trench which resulted in initiation of subduction at the trobriand trough and extension in the woodlark basin from ca 5 ma crustal shortening in the upper plate which now forms the solomon islands resulted from the resistance of the relatively hot and buoyant woodlark basin lithosphere to subduction at the san cristobal our reconstruction shows that tectonism at convergent plate boundaries could involve an intricate relationship between microplate rotations seafloor spreading and subduction segmentation over timescales considerably 10 ma by unraveling the tectonic evolution of this complex region we provide insights into the development of microplate tectonics at convergent margins in general and the possible diagnostic geological records that might be preserved in ancient orogenic systems

seismotectonics of new guinea a model for arc reversal following arc continent collision the structure and evolution of the northern new guinea collision zone is deduced from international seismological center isc seismicity 1964 1985 new and previously published focal mechanisms and a reexamination of pertinent geological data a tectonic model for the new guinea margin is derived which illustrates the sequential stages in the collision and suturing of the bewani toricelli adelbert finisterre huon new britain arc to central new guinea followed by subduction polarity reversal in the west east of 149 e the solomon plate is being subducted both to the north and south bringing the new britain and trobriand forearcs toward collision west of 149 e the forearcs have collided and together they override a fold in the doubly subducted solomon plate lithosphere which has an axis that is parallel to the strike of the ramu markham suture and that plunges westward at an angle of 5 beneath the coast ranges of northern new guinea active volcanism off the north coast of new guinea is related to subduction of the solomon plate beneath the bismarck plate active volcanism of the papuan peninsula and quaternary volcanism of the new guinea highlands are related to slow subduction of the solomon plate beneath the indo australian plate along the trobriand trough and the trough s former extension to the west respectively from 144 148 e seismicity and focal mechanisms reveal that convergence between the sutured bismarck and indo australian plates is accommodated by thrusting within the finisterre and adelbert ranges and compression of the new guinea orogenic belt together with basement involved foreland folding and thrusting to the south the finisterre block overthrusts the new guinea orogenic belt whereas the adelbert block is sutured to new guinea and overthrusts the oceanic lithosphere of the bismarck sea along the new guinea trench west of 144 e seismicity defines a southward dipping wadati benioif zone and focal mechanisms indicate oblique subduction only this oldest westernmost portion of the collision has progressed past suturing to a full reversal in subduction polarity

seismic and geological controls on earthquake induced landslide size abstract landslide size controls the destructive power of landslides and is related to the frequency of occurrence with larger landslides being less frequent than smaller ones for this reason the analysis of landslide size is essential for landslide hazard assessment we analyse six earthquake induced landslide inventories with earthquake magnitude ranging between 6 6 and 7 9 mw papua new guinea 1993 chichi 1999 northridge 1994 niigata chuetsu 2004 iwate miyagi nairiku 2008 wenchuan 2008 for each inventory we developed magnitude frequency curves to analyse the size distribution of landslides as a function of ground motion distance from the seismic source both fault trace and epicentre local relief and lithology for three earthquakes we observed a clear relationship between the landslide size and ground motion with larger landslides associated with higher ground motion we investigate different possible causes for such observation and propose that the main mechanical reason is that stronger shaking induces higher stresses that may overcome the strength which increases with depth triggering larger landslides we also show that landslide size decreases with distance from the fault trace whereas this trend is not clear for distance from the epicentre local relief does not seem a first order control on landslide size for the earthquake induced landslides considered here some lithologies do influence landslide size but we were unable to identify a general behaviour for different lithologies

mechanical role of backstops in the growth of forearcs we use numerical and laboratory modeling to analyze the mechanical role of backstops in the overriding plate at subduction zones a backstop is defined as a region within a forearc that has significantly greater shear strength than the sediment lying farther trenchward it can be thought of as the bulldozer behind an accretionary wedge we calculate the stress and displacement fields within forearcs for various backstop models using the finite element method and we simulate deformation over a backstop using a small scale laboratory model in this way we model the effects of the mechanical properties and geometry of a backstop on forearc structures we find that the growth of an outer arc high the development of an inner deformation belt with landward vergence and the seemingly paradoxical presence of an undeformed forearc basin within an otherwise highly deformed forearc can be explained by assuming the existence of a geometrically simple backstop with geologically reasonable contrasts in mechanical properties compared to the sediments just trenchward of it the structures produced in our numerical models are quite insensitive to the rheology boundary conditions and exact mesh geometry employed the general types of structures observed in the laboratory models depend only weakly upon the strength and the geometry of the backstop these results suggest that a detailed picture of an underlying backstop cannot be determined from surface information alone backstops in which the contact with the accretionary wedge dips arcward rather than trenchward however should produce only slightly different forearc structures with less development of both the outer arc high and the landward vergent inner deformation belt although natural forearcs are far more complex than our simple models they exhibit many of the same features indicating that relatively simple backstop mechanics may be a very important factor in the overall growth of forearcs

differential preservation in the geologic record of intraoceanic arc sedimentary and tectonic processes abstract records of ancient intraoceanic arc activity now preserved in continental suture zones are commonly used to reconstruct paleogeography and plate motion and to understand how continental crust is formed recycled and maintained through time however interpreting tectonic and sedimentary records from ancient terranes after arc continent collision is complicated by preferential preservation of evidence for some arc processes and loss of evidence for others in this synthesis we examine what is lost and what is preserved in the translation from modern processes to the ancient record of intraoceanic arcs composition of accreted arc terranes differs as a function of arc continent collision geometry forward facing collision can accrete an oceanic arc on to either a passive or an active continental margin with the arc facing the continent and colliding trench and forearc side first in a backward facing collision involving two subduction zones with similar polarity the arc collides backarc first with an active continental margin the preservation of evidence for contemporary sedimentary and tectonic arc processes in the geologic record depends greatly on how well the various parts of the arc survive collision and orogeny in each case preservation of arc terranes likely is biased towards those that were in a state of tectonic accretion for tens of millions of years before collision rather than tectonic erosion the prevalence of tectonic erosion in modern intraoceanic arcs implies that valuable records of arc processes are commonly destroyed even before the arc collides with a continent arc systems are most likely to undergo tectonic accretion shortly before forward facing collision with a continent and thus most forearc and accretionary prism material in ancient arc terranes likely is temporally biased toward the final stages of arc activity when sediment flux to the trench was greatest and tectonic accretion prevailed collision geometry and tectonic erosion vs accretion are important controls on the ultimate survival of material from the trench forearc arc massif intra arc basins and backarc basins and thus on how well an ancient arc terrane preserves evidence for tectonic processes such as subduction of aseismic ridges and seamounts oblique plate convergence and arc rifting forward facing collision involves substantial recycling melting and fractionation of continent derived material during and after collision and so produces melts rich in silica and incompatible trace elements as a result forward facing collision can drive the composition of accreted arc crust toward that of average continental crust

geomorphic analysis of the central range fault the second major active structure of the longitudinal valley suture eastern taiwan numerous landforms along the longitudinal valley suture of eastern taiwan indicate that two opposing reverse faults currently dominate the suturing process between the luzon volcanic arc and the central range of taiwan the east dipping longitudinal valley fault on the eastern flank of the valley is well known the west dipping central range reverse fault on the western flank of the valley is more obscure nonetheless it has produced many uplifted lateritic fluvial terraces along the eastern flank of the central range in the central reach of the valley from just north of the wuhe tableland to near chihshang the fault appears to be active but blind south of chihshang and inactive along the northern part of the longitudinal valley the late quaternary slip rate of the fault is less than 12 8 mm yr this constraint means that the fault is absorbing far less than half of the horizontal shortening across the longitudinal valley suture however the late quaternary slip rate along the fault may be comparable to the uplift and exhumation rate of the central range this suggests that localized brittle slip along the central range fault is an important component of crustal thickening and uplift of the range even though additional shortening and crustal thickening may be occurring because of pervasive deformation beneath the range

seismic cycle and plate margin deformation in costa rica gps observations from 1994 to 1997 global positioning system gps observations in costa rica from 1994 to 1997 reveal a complex pattern of motion consistent with the superposition of seismic cycle and secular plate margin deformation in the south velocity vectors are consistent with motion of the panama block plus postseismic deformation following the 1991 limon earthquake and interseismic strain due to partial locking of the middle america trench mat thrust in the northwest sites west of the volcanic arc are moving to the nw as a forearc sliver superimposed on this sliver motion are vertical and horizontal interseismic deformations from the adjacent nicoya segment of the mat we apply two different inverse methods to understand the source of the seismic strain in nw costa rica we compare fault locking models derived using a singular value decomposition inversion with that of a simulated annealing global optimization approach both methods yield similar models for partial locking of the thrust interface beneath the nicoya peninsula our results define an area of nearly fully locked fault beneath the outer coast of the southern portion of the peninsula with somewhat lower coupling beneath the northern half and with low coupling elsewhere these initial results show the promise for detailed imaging of the locked portion of a thrust interface responsible for future large subduction zone earthquakes

joint inversion of insar gps teleseismic and strong motion data for the spatial and temporal distribution of earthquake slip application to the 1999 izmit mainshock the space time distribution of slip of the 17 august 1999 izmit earthquake is investigated by inverting synthetic aperture radar sar interferometry and global positioning system gps data together with teleseismic broadband and near field strong motion records surface offsets are used as an added constraint special emphasis is given to analysis of the resolution of the different data sets we use a four segment finite fault model and a nonlinear inversion scheme allowing slip to vary in amplitude direction and duration as well as variable rupture velocity from the inversion of synthetic data we find that the best spatial resolution can be expected in the upper half of the fault model above 12 km where coverage of the interferometric sar data is good western half of the rupture and near the gps and strong motion stations teleseismic data are found to have a lower resolution that is more evenly distributed over the fault model the joint inversion of all the data sets has an increased resolving power compared with the separate inversions and gives a more robust description of the space and time distribution of slip our study shows the importance of resolution tests in evaluating the reliability of earthquake kinematic models and it confirms that an excellent fit of a single kind of data does not necessarily imply a good retrieval of the kinematic properties of an earthquake the izmit rupture which is almost pure right lateral strike slip faulting is dominated by the bilateral breaking of a central asperity located between 29 7 e about 10 km west of the city of golcuk and 30 4 e eastern margin of sapanka lake with slip reaching 6 8 m in the depth range 6 12 km the western termination of the rupture is found near the city of yalova but large slip ends around 29 7 e about 10 km east of hersek delta a second area of large slip is required by all the data sets further east toward the city of duzce between 30 7 e and 31 1 e karadere and duzce faults this eastern slip zone which is separated from the main central asperity by an area of greatly reduced slip is less well constrained by the data however a strong motion station near the city of duzce helps to locate a high slip patch near 31 1 e in the depth range 6 12 km the total seismic moment resulting from the joint inversion is 2 4 1027 dyne cm most of the energy release occurred in a short time less than 15 sec corresponding to the bilateral breaking of the central asperity rupture propagation is relatively uniform and fast toward the west with a rupture velocity close to 3 5 km sec propagation of large slip toward the east is initially slower but it accelerates during a short time interval about 10 sec after rupture nucleation eastward progression then slows down to less than 2 km sec after 15 sec and rupture almost vanishes in amplitude ca 20 sec after initiation rupture propagation then proceeds on the easternmost karadere and duzce fault segments east of 30 7 e from 22 to ca 50 sec supershear rupture propagation is not required for modeling the waveforms considered in this study the hypocenter of the duzce earthquake which occurred 3 months later 12 november 1999 m w 7 2 is located in the immediate vicinity of the easternmost slip patch of the izmit earthquake manuscript received 28 july 2000

measurement of tectonic surface uplift rate in a young collisional mountain belt measurement of the rate of tectonically driven surface uplift is crucial to a complete understanding of mountain building dynamics the lack of a suitable rock record typically prevents determination of this quantity but the unusual geology of papua new guinea s finisterre mountains makes measurement of this rate possible the tectonic surface uplift rate at the finisterre range is 0 8 2 1 mmyr 1 approximately that expected to arise from crustal thickening

plate boundary strain partitioning along the sinistral collision suture of the philippine and eurasian plates analysis of geodetic data and geological observation in southeastern taiwan crustal deformation and strain partitioning of oblique convergence between the philippine sea plate and the eurasian plate in the southern longitudinal valley of eastern taiwan were characterized based on geodetic analysis of trilateration network and geological field investigation the longitudinal valley fault one of the most active faults on taiwan branches into two individual faults in the southern longitudinal valley these two active faults bound the plio pleistocene pinanshan conglomerate massif between the coastal range the luzon island arc belonging to the philippine sea plate and the central range the metamorphic basement of the eurasian plate a geodetic trilateration network near the southern end of the valley shows a stable rate of the annual length changes during 1983 1990 the strain tensors for polygonal regions including triangular regions of the taitung trilateration network reveal that there are two distinct zones of deformation a zone of shortening thrusting between the pinanshan massif and the central range on the west and a strike slip movement between the pinanshan massif and the coastal range on the east the analysis of a discontinuity model consisting of three rigid blocks separated by two discontinuities has been carried out the results show that the deformation in this region can be characterized by two major faults a reverse fault is located between the plio pleistocene pinanshan massif and the metamorphic basement of the central range with a shortening rate of about 12 mm yr in the direction n280 e a strike slip fault is located principally along the river between the pinanshan massif and island arc system of the coastal range with a purely strike slip component of about 22 mm yr in the direction n353 e the analysis of the geodetic data further suggests that substantial deformation probably strike slip in type occurs within the pinanshan massif geological evidence of deformation in the plio pliestocene pinanshan conglomerate includes regional folding a conjugate set of strike slip fractures at the outcrop scale and morphological lineaments related to fracturing all indicating that the pinanshan massif is being deformed within a transpressive stress regime regional kinematic data indicate that a significant portion of the 82 mm yr of motion between the eurasian plate and the philippine sea plate is absorbed in the southern longitudinal valley by the decoupling of two distinct major faults the geometry of the oblique convergence and the rheology of the rock units the well consolidated plio pleistocene conglomerate and the sheared melange formation play the two important roles in the partitioning of crust deformation

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

sedimentology of coarse grained alluvial fans in the markham valley papua new guinea abstract in alluvial sediment sequences recognition of a hierarchy of bounding surfaces and the shapes and associated lithofacies of the sediment bodies they define provide an appropriate framework for understanding associations among depositional forms the processes responsible for them and their controls on system development this methodology architectural element analysis integrates principles from geomorphology and sedimentology miall 1985 1988 it is used here to analyse the evolution of the modern umi fan and the alluvial fan part of the pleistocene leron formation in the markham valley papua new guinea a series of terraces has developed as the umi river has incised into its fan detailed stratigraphic analysis of the lowest terrace in exposures up to 25 m high and kilometres long reveals that the fan is dominated by sheetflood deposits with minimal preservation of either debris flow or hyperconcentrated flood flow sediments channel fill elements make up a larger proportion of exposures in the proximal fan than elsewhere while 95 of distal fan exposures are composed of sheetflood sequences these depositional features likely result from massive sediment dispersal associated with the rapidly uplifting upland terrain abundant sediment availability and flashy discharge regime channel fill units along with slope related deposits from debris and hyperconcentrated flood flows are only likely to be preserved in the trench backfilling phase following fan entrenchment reworking of deposits plays a dominant role in preservation of sheetflood deposits at the expense of slope related deposits an hierarchical framework of lithosomes is developed which characterizes various scales and bounding surfaces of depositional units which make up the umi fan cf miall 1988 decelles et al 1991 smaller elements observed in terrace exposures are described as first to fourth order lithosomes they reflect geomorphic processes which are evident in and adjacent to the modern umi river deposits which infill the trench are interpreted as a fifth order lithosome constrained by trench geometry proximal distal relationships are remarkably similar in the umi fan and leron formation fan sequences in both instances debris flow deposits are seldom observed the largest sixth and seventh order lithosomes which were deposited within a time frame of approximately one hundred thousand years are interpreted by analysis of the leron formation in relation to its tectonic setting the umi and pleistocene leron formation fans exemplify fan development in a post collisional molasse basin under a tropical monsoonal climate carbonate concretions observed in distal fan facies may provide a possible diagnostic feature of these tropical savanna fans

active fault creep variations at chihshang taiwan revealed by creep meter monitoring 1998 2001 1 the daily creep meter data recorded at chihshang in 1998 2001 are presented the chihshang creep meter experiment was set up across the chihshang thrust fault the most active segment of the longitudinal valley fault which is the present day plate suture between the eurasian and the philippine sea plates in eastern taiwan near continuous data recording at two sites revealed different surface fault motions yet similar annual shortening rates 16 2 mm at the tapo site comprising two connected creep meters and 15 0 mm at the chinyuan site three creep meters straddling parallel fault branches four of the five creep meters showed a seasonal variation with the fault moving steadily during the rainy season from april to october and remaining quiescent during the rest of the year the only exception was recorded by the creep meter located on a melange composed hillslope where local gravitational landsliding played an additional role other than tectonic faulting through comparison with daily precipitation data we inferred that moderate rainfall suffices to trigger or facilitate slippage on the surface fault during the transition period of the dry wet season during the observation period from 1998 to 2001 the subsurface seismicity exhibited clusters of microearthquakes on the chihshang fault at depths of 10 25 km recurrent earthquakes occurred regardless of whether the season was wet or dry indicating that the stress relaxation associated with seismicity in the seismogenic zone did not transfer immediately up to the surface the accumulated strain on the chihshang fault at shallow surface levels was released through creep during the wet season in addition to these short term seasonal variations an apparent decrease in the annual slipping rate on the chihshang fault during the last few years deserves further investigation in order to mitigate against seismic hazard

collision propagation in papua new guinea and the solomon sea the mountain system in papua new guinea png is growing by accretion of australian margin strata to the front of the papuan fold and thrust belt in the south and by accretion of exotic terranes along its ne margin presently the finisterre terrane is being accreted along the ne margin of png and the collision point is migrating eastward within the solomon sea east of the collision point the new britain trench marks the tectonic front between the solomon sea and south bismarck sea plates whereas to the west the tectonic front lies in the southern part of the papuan fold and thrust belt present day sedimentary environments within the western solomon sea are inferred from interpretation of seismic reflection data similar facies are observed accreted to the front of the finisterre terrane in the markham valley the frontal leron formation is largely fluvial and shallow marine and the erap complex contains elements of turbidites and pelagic deposits there appears to be a smaller volume of coarse proximal marine strata accreted to the front of the finisterre terrane than is expected from interpretation of reflection profiles in the western solomon sea this difference may be attributed to a combination of submarine erosion in the markham canyon partial incorporation of these deposits into the erap complex and a difference in depositional rates and products carried to the solomon sea in the past relative to what is observed today based on plate motions alternative collision geometries and the youngest dates available for the erap complex we determine a rate of progression of the collision point of about 212 km m y during the past 1 m y

active low angle normal faulting in the woodlark extensional province papua new guinea a physical model 1 in eastern papua new guinea the seismicity geomorphology and results of seismic reflection profiling drilling and structural geological analysis reveal a zone of active low angle normal faulting the only clear known active example of this process worldwide this woodlark extensional province is 250 km long in the east west direction located within the continental crust to the south of the solomon sea ocean basin it adjoins the young oceanic spreading center farther east in the woodlark basin and since 4 ma has accommodated northward extension at rates of in excess of 10 mm yr 1 on low angle normal faults that are rooted to the north this geometry and the region s earlier history are assessed in the light of modern ideas for the mechanics of low angle normal faulting to derive a working hypothesis for the existence of this process in this region the extension in the woodlark extensional province and oceanic spreading in the woodlark basin are thus interpreted as consequences of the slab pull force due to northward subduction of the solomon sea floor which has caused the small solomon sea plate to separate from the australian plate it is suggested that this plate geometry developed by 6 5 ma following the ending of southward subduction of the solomon sea floor beneath what is now the woodlark extensional province the double thickness of lithosphere that formerly existed locally cooled the continental crust making it brittle to the moho following the end of this subduction the base of the continental lithosphere became progressively exposed to the asthenosphere causing dramatic heating of the overlying continental crust this resulted in transient thermal disequilibrium inducing transient horizontal lower crustal flow the shear tractions imparted on the base of the brittle layer by such flow combined with the stress components that develop after some of the lower crust has flowed from beneath a locality requiring it to be supported laterally rather than from below will rotate the principal axes of the stress tensor sufficiently to enable low angle normal faults to form and it is estimated that this sense of deformation was initiated around 4 ma the geometry of this study region means that this process will create north dipping low angle normal faults as are observed this scheme can also account for a great deal of other observational evidence including the available thermochronologic evidence and the observed vertical crustal motions to explain this instance of active low angle normal faulting thus requires knowledge of its relationship to earlier subduction and of effects of temperature dependent changes to continental crustal rheology in crust that was initially cold

rupture behavior of a moderate earthquake mw 5 9 april 2006 and its close relation with the 2003 chengkung earthquake mw 6 8 at the southern termination of the plate boundary southeast taiwan the taiwanese orogen is the result of the active and vigorous collision between the eurasian and the philippine sea plates in this strong convergence context a one month long earthquake sequence composed of two mainshocks of magnitude 6 occurred in april 2006 in the taitung area the first mainshock t1 mobilized a structure just west to the plate boundary while the second one t2 is located on the other side in order to retrieve the exact fault geometry of t2 we inverted waveforms from stations located at local and teleseismic distances we performed a grid search on strike and dip each strike dip couple defining a geometry considering the source as a point source then as an extended source in the last case a simple average over stations misfit failed to isolate a precise geometry to overcome this problem we used a more statistical approach on the distribution of misfits to define the best geometry comparing the best model to local structures it appears the generative fault was the plate boundary that rotates from a strike pointing at n20 e north of the event to n0 e in t2 area with an identical eastward dip of 35 for this model the fault slip inversion provides a critical slip of 3 5 cm above which slip rake and rupture time are constrained with uncertainties of 29 14 and 0 47 s respectively the average slip along the rupture was 20 cm with a maximum of 46 13 cm the movement was inverse with a minor left lateral component similar to the faulting behavior of the plate boundary in addition the slip pattern of t2 is contained within the southern portion of the deepest segment of the plate boundary and at the edge of the rupture area of the 2003 chengkung earthquake mw 6 8 a large event also generated by the plate boundary but 2 5 years earlier after 1 s of aseismic spreading the rupture propagated seismically and circularly outward before being stopped by the fault bending

out of sequence thrusts out of sequence thrusts oost are those thrusts which do not obey the foreland propagating or in sequence deformation style they include both isolated thrusts which develop hindward of the thrust front and sequences of break back thrusts which propagate from the foreland to the hinterland two end members of a series of oost types are recognized 1 older in sequence thrusts which are reactivated along their entire length and 2 completely new thrusts which propagate through already deformed thrust sheets between the two end members are thrusts composed partially of reactivated in sequence thrust sequences and partially of new entirely out of sequence segments oosts can be initiated for a variety of reasons including 1 keeping the orogenic wedge at critical taper 2 break back sequences from the suture zone in the overriding plate 3 ramping to overcome a sticking point which inhibits in sequence thrust propagation and 4 during simultaneous displacement along two stacked thrusts culminations which bow up segment of the upper thrust may be chopped through to permit continued displacement on the upper thrust many different types of thrust behavior including gravity sliding plucking and derivation of isolated horses from ramps may mimic some of the characteristics of oosts consequently it may be difficult to conclusively prove an oost origin for a complex thrust geometry

slip distribution on a thrust fault at a plate boundary the 2003 chengkung earthquake taiwan summary the 2003 chengkung earthquake mw 6 8 occurred on the east dipping chihshang fault of eastern taiwan a listric fault with dominant reverse motion a joint inversion taking into account geodetic gps and seismological accelerometric and teleseismic data is performed this modelling highlights the rupture process behaviour on the fault plane at depth the coseismic rupture developed essentially between 12 and 26 km depth expanding laterally by about 30 km towards the ssw two slip patches dominated with slip values exceeding 1 5 m during the 30 s of the whole process the rupture propagation decelerated with time starting at 3 4 0 3 km s 1 in the two main slip patches and ending with velocities below 1 6 0 1 km s 1 the 48 hr of aftershocks that followed the main shock are distributed essentially inside the coseismic slip patches on the fault surface except in the deepest southern part of the fault where slip is less resolved the results of our modelling illustrate the particular behaviour of the 2003 chengkung earthquake where the coseismic slip rapidly diminished towards the surface in good agreement with the observation of fast post seismic slip followed by interseismic creep along the chihshang fault

grain size control of mineralogy and geochemistry in modern river sediment new guinea collision papua new guinea the new guinea collision includes diverse sediment source areas associated with arc continent collision to the west of the solomon sea triple junction and with subduction of oceanic crust to the southeast and northeast of the triple junction twenty seven sediment suites each comprising up to seven gravel sand and mud samples from the same locality were analysed to determine mineralogical and geochemical variation with changing grain size this paper deals primarily with the mineralogical variation compositional variation from coarse to fine grained fractions is generally 50 of each variable s abundance with many variables showing greater than 100 variation in contrast variation due to analytical error is generally less than 10 between year variation generally ranges from 15 to 25 and downstream variation is generally less than 50 petrographic results indicate that sediment composition is fundamentally dependent on grain size the principal control on sediment composition is mechanical weathering of rock fragments and compositional sorting of the constituent mineral phases while the gazzi dickinson method for point counting sediment reduces modal variability and in most cases provides robust provenance discrimination it is unsuitable for discerning faint source rock signatures often revealed by grain size dependent compositional variation only the upper plate magmatic arc provenance signature is evident from recalculated modal and geochemical data and that signature does not reflect the modern upper plate magmatic arc detailed petrographic analysis of grain types as opposed to recalculated modal or geochemical data is required to elucidate both upper and lower plate provenances and discriminate between ancient and contemporary arc provenances

paleomagnetism and tectonics of malaita solomon islands malaita in the solomon islands is the emergent expression of the pacific province a geologically distinct suspect terrane which is commonly held to be the margin of the ontong java plateau a thickened portion of pacific plate overthrust onto the rest of the solomon islands arc during a late miocene reversal of the polarity of the arc subduction of the australian plate replaced pacific plate subduction during this event and the solomon islands arc was transferred from the australian plate to the pacific plate a paleomagnetic survey of the eastern solomon islands included sampling in southern malaita magnetostratigraphic dating gives an age of 5 8 ma for the haruta calcisiltite hada calcisiltite facies change on malaita this facies change resulted from renewed volcanic activity following the initiation of australian plate subduction paleomagnetic poles from two pre pliocene units do not lie near the pacific apparent polar wander path apwp as the conventional model would suggest but rather fall on the australian apwp suggesting that the pacific province developed as a part of the australian plate correction for 6 5 ma of pacific australia relative motion produces an improved very close fit to the australian apwp consistent with the transfer of malaita along with the rest of the arc to the pacific plate at about that time magnetic paleolatitudes from a dsdp hole on the ontong java plateau which disagree with usually accepted pacific tertiary motion also fail to match the malaitan data the possibility that malaita could be a part of an ontong java sub plate somehow detached from the rest of the pacific plate can be discounted malaita and the rest of the pacific province arose in the fore arc of the originally north east facing arc collision of the ontong java plateau with the trench slowed subduction of the pacific plate and began subduction of the australian plate below the arc both subduction zones may have been active for a period beginning at about 10 ma and ending at about 3 ma until pacific plate subduction broke down and the pacific province was subsequently uplifted and deformed

source tomography by simulated annealing using broad band surface waves and geodetic data application to the mw 8 1 chile 1995 event summary we invert surface wave and geodetic data for the spatio temporal complexity of slip during the mw 8 1 chile 1995 event by simulated annealing this quasi global inversion method allows for a wide exploration of model space and retains the non linearity of the source tomography problem complex source spectra are obtained from 5 to 45 mhz from first and second orbit fundamental mode rayleigh waves using an empirical green s function cross correlation technique coseismic displacement vectors were measured at 10 gps sites near antofagasta they are part of a french chilean experiment which monitors the northern chile seismic gap the spectra together with the geodetic data are inverted for the moment distribution on a 2 d dipping fault under the physical constraints of slip positivity and causality marginal a posteriori distributions of the model parameters are obtained from several independently inverted solutions in general features of the slip model are well resolved data are well fitted by a purely unilateral southward rupture with a nearly uniform velocity around 2 5 3 0 km s 1 and a total duration of 65 s several regions of moment release were imaged one near the hypocentre a major one 80 km south of it and a minor one 160 km south of it the major patch of moment release seemed to have propagated to relatively shallow depths near the trench 100 km ssw of the epicentre the region of major slip is located updip of the 1987 mw 7 5 earthquake suggesting a causal relationship most of the slip occurred updip of the hypocentre 36 km but the entire coupled plate interface 20 40 km ruptured during the chile 1995 event

drowned carbonate platforms in the huon gulf papua new guinea 1 the western huon gulf papua new guinea is an actively subsiding foreland basin dominated by drowned carbonate platforms we investigated these platforms using new high resolution multibeam side scan sonar and seismic data combined with submersible observations and previously published radiometric and sedimentary facies data the data reveal 14 distinct drowned carbonate platforms and numerous pinnacles banks that increase in age 20 450 kyr and depth 0 1 2 5 km ne toward the ramu markham trench superimposed on this overall downward flexing of the platforms toward the trench is a systematic tilting of the deep platforms 15 m km toward the nw and the shallow platforms 2 m km toward the se this may reflect the encroaching thrust load from the nw finisterre range and spatial variations in the flexural rigidity of the underlying basement the drowned platforms form a complex system of promontories and reentrants with abundant pinnacles and banks preserved at similar depths seaward of the main platforms this configuration closely mimics the present day huon coastline and its seaward islands fringed by modern coral reefs the platforms retain structural morphologic and sedimentary facies evidence of primary platform growth drowning and subsequent backstepping despite some lateral erosion of the platform margins 100 m slope defacement by mass wasting both platforms and pinnacles are composite features containing multiple terrace levels and notches corresponding to multiple phases of growth emergence and drowning in response to rapid climatic and sea level changes during the evolution of each structure on the basis of all observational and numerical modeling data we propose a chronology for the initiation growth and drowning of the 14 platforms over shorter timescales 100 kyr the rate and amplitude of eustatic sea level changes are critical in controlling initiation growth drowning or subaerial exposure subsequent reinitiation and final drowning of the platforms however continued tectonic subsidence and basement substrate morphology influence the overall backstepping geometry and subsequent tilting of the platforms over longer timescales 100 500 kyr

focal mechanism determination and identification of the fault plane of earthquakes using only one or two near source seismic recordings a waveform inversion scheme was developed in order to explore the resolving power of one or two seismic recordings at short epicentral distance for the determination of focal mechanisms and the identification of the fault plane of earthquakes two key features are used to constrain the fault parameters with a reduced number of stations 1 a simple finite dimension source model and 2 the modeling of the complete displacement field including the near field waves the identification of the fault plane should be possible even with a single station as soon as the seismograms produced by the two nodal planes of a same focal mechanism are significantly different which is the general case when waveforms are controlled by source finiteness seven parameters including the strike dip rake and dislocation are explored with a grid search and the minima of the misfit error between the observed and calculated seismograms are mapped with such an approach it is possible to conclude about the uniqueness or nonuniqueness of the solutions the method is tested with three earthquakes of moderate to large size for which the fault plane is well established and for which strong motion records are available at maximum distances of a few tens of kilometers test events are the 1994 northridge m w 6 7 california the 1996 copala m w 7 3 mexico and the 1996 pinotepa nacional m w 5 4 mexico earthquakes in the case of inversions with two stations we find a unique solution or a group of similar solutions with a good estimation of the focal mechanism and the proper selection of the fault plane our results also show that in some cases a single station may be enough to recover the fault parameters the inversion scheme presented here may be systematically applied to future earthquakes especially to those recorded by few stations it should be particularly useful in the case of blind faults for which the fault plane may not be identified with the help of other data

seismotectonics of taiwan abstract high quality seismicity data and focal mechanism solutions obtained during 1973 1983 by the permanent taiwan telemetered seismographic network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the taiwan area seismicity distribution in southern taiwan clearly reveals an east dipping benioff zone which has a thickness of about 30 km and begins to deepen along 121 e at a dip angle of 55 60 the leading edge of this benioff zone reaches a depth of about 180 km between 21 n and 22 n but tapers off to a shallower depth of about 100 km from 22 n to 23 n the presence of this seismic zone implies that subduction of the south china sea plate under the philippine sea plate extends from luzon northward to about 23 n the position of the northern boundary of the south china sea plate as tentatively defined according to the seismicity distribution passes through southern taiwan from the offshore area in the taiwan strait west of kaohsiung in an east northeast direction to the taitung area where a triple junction probably lies seismicity is found to disappear abruptly below a certain depth in many parts of taiwan this phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely developed faults and fractures however the converse is not necessarily true this may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas finally focal mechanism solutions for west central taiwan and the kuangfu fuli area in eastern taiwan predominantly show a maximum horizontal compression in the se nw direction which can be related to collision between the eurasian and philippine sea plates however focal mechanism solutions for both the hualien area in eastern taiwan and the tainan area in southwestern taiwan show remarkable irregularities which may result from local tectonic complexities

ramu basin papua new guinea a record of late miocene terrane collision the ramu basin lies along a plate boundary where the finisterre terrane is colliding with the indo australian plate estimates for the age of initial collision range from early miocene to middle pliocene two unsuccessful wells keram 1 and tsumba 1 drilled to basement and two dimensional seismic data show that folded and faulted early to middle miocene carbonates and clastics the wogamush sequence are overlain by relatively undeformed pliocene marine clastics the wewak sequence along a regional unconformity the pre pliocene section which is at the crux of resolving the age of initial collision has been correlated previously to the finisterre terrane clastics within that section derived from older terranes south of the basin imply an early miocene age for colli ion i propose that miocene sedimentary and volcanic rocks in the two wells are correlative with the wogamush beds of the maramuni arc the ramu basin can then be viewed as having a two stage evolution during the miocene the basin was part of the maramuni arc the polarity of which is unresolved a collisional successor basin developed in the late miocene as the finisterre terrane adelbert block collided with the arc thrust faults on the northeastern side of the basin truncated by a regional unconformity are interpreted to mark the suture of the adelbert block a northern earliest pliocene sediment source for the basal wewak sequence was probably the finisterre terrane but multiple source areas are inferred for the rest of that sequence middle pliocene inversion of the basin s n rtheastern flank characterized by reverse faulting and forced folding is attributed to plate boundary reorganization caused by rifting in the bismarck sea the ramu basin has numerous untested structures related to both collision and basin inversion folded miocene carbonates wogamush sequence pose a considerable risk regarding the presence of reservoir and the wewak sequence is dominated by volcanolithic arkoses with poor reservoir quality on the northeastern side of the basin where they are in structural closure gas prone source rocks are present but are largely immature reservoir and charge considerations place the ramu basin in the very high risk sector for exploration

rotations in the actively colliding finisterre arc terrane paleomagnetic constraints on plio pleistocene evolution of the south bismarck microplate northeastern papua new guinea abstract we report paleomagnetic results from 12 plio pleistocene localities in the actively colliding finisterre arc terrane of northeastern papua new guinea png calcareous hemipelagic cover rocks possess a stable syn collisional remagnetization indicating a clockwise rotation of the colliding terrane through about 40 in post miocene time a decrease in paleomagnetic declination anomalies as a function of along strike distance in the finisterre arc terrane analyzed by our preferred model of a linear remagnetization and a migrating euler pole suggests an average rotation rate of 8 ma 1 in good agreement with the instantaneous rate from global positioning system geodesy thus we propose that this rotation results from a coherent rigid body rotation of the finisterre terrane rather than from sequential docking of independently colliding blocks of the terrane moreover we conclude that these paleomagnetic declinations result mainly from south bismarck plate motion and not decoupled rotation of the crustal terrane independent of the underlying lithosphere we examine models of a syn collisional remagnetization with both fixed and migrating euler poles of south bismarck australia plate relative motion and suggest that the euler pole describing south bismarck plate motion has migrated southwestward to its present location on the collision suture in response to the propagating collision this plate kinematic model agrees with the variability in depth of the seismogenic slab beneath the collision zone our best fit model of pole migration describes south bismarck australia relative motion producing a highly oblique collision in its early stages with the finisterre arc terrane converging along a left lateral ramu markham suture gradually changing to the nearly orthogonal convergence observed today

suturing history of an allochthonous terrane at a modern plate boundary traced by flysch to molasse facies transitions abstract the southern boundary of the south bismarck plate in papua new guinea changes character from west to east as a consequence of oblique collision and suturing of the allochthonous finisterre terrane with the margin of the australian continent and plate plate rearrangements are occurring and a triple junction is migrating eastwards along the plate boundary characterization of modern sedimentary facies leads to the following correlations between facies and tectonic settings tectonic setting sector of plate boundary facies terrane already docked markham valley fluvial molasse terrane now docking huon gulf marine molasse terrane yet to dock western new britain trench conglomeratic flysch these relationships provide the basis for a tectonic depositional model showing the distribution and relationships between facies in time distance space along the plate boundary facies transitions are time transgressive and their trajectories provide a measure of the rate of triple junction migration rapid uplift of the southern margin of the south bismarck plate has exposed older sediments related to this history of terrane suturing their facies characterization provides support for the model but age control is insufficient for conclusive demonstration of its validity

climate sea level and tectonic controls on sediment discharge from the sepik river papua new guinea during the mid to late pleistocene abstract amongst the rivers draining the mountainous islands of the indonesian archipelago the sepik river of papua new guinea is the largest contributor of solute and particulate material to the world ocean sites u1484 and u1485 drilled during international ocean discovery program iodp expedition 363 provide a continuous 555 kyr long high resolution record of mainly siliciclastic slope sedimentation on the northern continental margin of png just offshore the mouth of the sepik river sedimentological analysis based on a combination of smear slide petrography particle size analysis high resolution physical properties track data and visual core description offers an unprecedented opportunity to investigate the evolution of this major tropical river throughout changing climate and sea level conditions during the mid to late pleistocene the sites u1484 and u1485 sediment records exhibit a dramatic lithologic change at 370 ka the oldest deposits are dominated by pelagic mud suggesting that the coarser grained terrigenous discharge from rivers draining the new guinea highlands including a proto sepik river was captured before reaching the ocean when the sepik river basin was an epicontinental sea the occurrence of coarser grained mass gravity mainly hybrid flows after 370 ka suggests that the epicontinental sea became a more restricted shallow water to nonmarine basin probably due to both basin infilling and the uplift of local coastal ranges during the last three glacial interglacial cycles 300 kyr this shallow inland basin was strongly affected by global variations in sea level during sea level lowstands the sepik river cut into older sediments and discharged further offshore onto the shallow continental margins promoting mass gravity flows pelagic mud deposition on the continental margin during the most extreme highstands following deglaciations suggests a return to shallow marine conditions in the sepik basin and repositioning of the river mouth further inland away from the shelf and from the location of the iodp sites this also indicates that variations in terrigenous fluxes during these extreme highstands were not solely controlled by the intensity of the hydrological cycle and that global sea level variations also influenced sediment deposition the sequence is interrupted by several massive grain flow deposits occurring diachronously at sites u1484 and u1485 and related to a period of intensified tectonic activity between 280 ka and 140 ka the sedimentologic characteristics of these mass gravity deposits and their lack of correlation between sites are interpreted as resulting from channelized flows along margin parallel fault controlled channels caused by local and regional failures of the continental margins including the area of the yalingi canyon 50 km north of the sites where large tsunamigenic events have also occurred in recent years time series analyses of magnetic susceptibility and natural gamma radiation proxies for sand and clay content respectively indicate that river discharge fluxes were modulated at orbital frequencies including obliquity scale cycles which suggests that precipitation and river discharge were not only linked to precessionally driven shifts in the mean position of the intertropical convergence zone but also to high latitude climate change

arc continent collisional orogenesis in the sw pacific and the nature source and correlation of emplaced ophiolitic nappe components abstract southwest pacific ophiolitic nappes emplaced upon the former margin of eastern australia provide a record of cyclical episodes of arc continent collisional orogenesis that occurred throughout the paleogene cenozoic nappes in papua new guinea new caledonia and northland new zealand consist of two key components 1 a basal and apparently older late cretaceous eocene metamorphosed normal to weakly enriched morb like basaltic ocean floor terrane and 2 an uppermost supra subduction zone ssz ophiolite that represents basement of the associated magmatic forearc a detailed comparison of geochemical characteristics of unequivocal ssz basalts with those considered as representative of the basaltic ocean floor terranes illustrates that formation of the majority of the latter is consistent with melting of a source transitional to normal n and enriched e morb these slight ree enriched or transitional t morb have generally been considered as the result of seafloor spreading in a back arc basin bab or basins to the east of australia in the late cretaceous paleocene 85 55 ma the enrichment recorded by the bab basalts is likely a result of upwelling plumes associated with initial fragmentation of eastern gondwana that left their signature on the mantle source consequently tapped by the bab basalts a subset of slightly more enriched basalts may owe their enrichment to source contamination minor assimilation of thinned gondwana derived continental crust associated lree depleted n mor basalts previously considered as part of the basaltic ocean floor terranes on the other hand are similar to ssz basalts of the northland and papuan ultramafic belt ophiolites refinement of existing tectonic models is hindered by ambiguities in the stratigraphic relationships of the underlying basaltic ocean floor terranes with the overlying ssz ophiolites a paucity of age data and uncertainties in the tectonomagmatic affinities of the underlying basalts nonetheless the geochemical characteristics displayed by lree depleted basalts of the basaltic ocean floor terranes suggest that these may instead represent younger magmatic forearc basement components subsequently overthrust by lower sections of a genetically related ophiolite whether these and for that matter the majority of basalts of the basaltic ocean floor terranes indeed represent complementary forearc crust to the ssz ophiolite as opposed to a slice of late cretaceous eocene bab floor backthrust beneath the ssz ophiolite upon emplacement will have to await the acquisition of corresponding radiometric age data