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

magnetic fabric evidence for conduit fed emplacement of a tabular intrusion dinkey creek pluton central sierra nevada batholith california anisotropy of magnetic susceptibility measurements in the cretaceous dinkey creek pluton dcp central sierra nevada batholith reveal a well defined structural pattern and zonation in bulk susceptibility k outward decrease in k within the pluton is due to a decrease in ferrimagnetic contribution corresponding to a compositional zoning from tonalite to granite magnetic fabrics in the pluton formed during supersolidus flow of a crystal rich magma except in the eastern domain where subvertical magnetic lineations and nw striking magnetic foliations are attributed to deformation associated with a regional shear zone magnetic foliations in the central domain of the dcp define a nnw trending synformal structure passing structurally upward into a dome beneath a small roof pendant interpreted to represent different structural levels in a zone of magma upwelling arcuate foliation trajectories in the pluton s sw lobe resemble deformed passive markers in two dimensional nw to sw horizontal channel flow of non newtonian magma analysis of the fabric pattern in cross section estimates the preerosion thickness of the lobe to between 915 and 3660 m the dcp was emplaced as a tabular body with vertical sides and a gently inclined roof magma with increasingly more mafic compositions entered the chamber continuously or episodically via a nnw trending conduit space for the pluton was created by vertical inflation probably accommodated by depression of its floor these results suggest that the mid crust to upper crust of magmatic arcs may be constructed by vertical stacking of tabular granitic plutons with high width thickness ratios like the dcp

rapid thermal rejuvenation of high crystallinity magma linked to porphyry copper deposit formation evidence from the koloula porphyry prospect solomon islands magmas containing the components needed to form porphyry copper deposits are relatively common within arcs yet mineralising events are uncommon within the long lived magmatic systems that host them understanding what causes the transition from barren to productive intrusions is critical to the development of conceptual deposit models we have constrained the tempo of pre and syn mineralisation magmatic events in relationship to the thermal evolution of the plutonic body that underlies one of the world s youngest exposed plutonic porphyry systems the inamumu zoned pluton koloula porphyry prospect solomon islands high precision id tims u pb dates of texturally and chemically characterised zircons indicate pluton emplacement over 50 crystals state past the point of rheological lock up we estimate that thermal rejuvenation of the deeper high crystallinity magma and generation of a mobile melt fraction may have occurred 10 kyr before its transport and emplacement within the porphyry environment the underlying pluton likely cooled and returned to high crystallinity states prior to subsequent remobilisation emplacement events titanium in zircon geothermometry and whole rock geochemistry suggest pre mineralisation intrusions were remobilised by mixing of a silicic magma with a high temperature less evolved melt in contrast syn mineralisation melts were most likely remobilised by the percolation of hot volatiles exsolved from contemporaneous less evolved intrusions cooling beneath the crystalline silicic magma the evidence for the rapid thermal rejuvenation and long term storage of highly crystalline silicic magmas is consistent with previous studies that indicate two components of exsolved volatiles contribute to ore forming fluids we conclude that the liberation of crystal rich porphyry copper deposit forming magmas and the addition of the chemical components required for ore formation are intrinsically linked to the volatiles released during the recharge of less evolved melt into a highly crystalline silicic magma

geochronology and geochemistry of the frieda river prospect area papua new guinea the frieda river prospect is located between the frieda and lagaip fault zones two major structural features of the new guinea mobile belt in the west sepik district papua new guinea the prospect can be subdivided into three spatially and geologically different areas frieda complex mianmin area and nena diorite area the frieda complex is the remnant volcanic edifice of an island stratovolcano interstratified in the mid miocene wogamush formation intrusive and volcanic units in the complex are texturally similar andesitic hornblende plagioclase porphyries and are probably comagmatic pyritic replacement copper gold and porphyry copper deposits are associated with early and late phases respectively of extensive district scale advanced argillic alteration along the central axis of the frieda complex the mianmin area is a separate unmineralized volcanic center that contains igneous rocks similar to those of the frieda complex the nena diorite a composite holocrystalline intermediate to mafic body intrudes upper cretaceous to eocene age basement rocks it is located north of and separated from the frieda complex by the frieda fault igneous activity in the frieda river prospect extended over an interval of at least 4 m y on the basis of field relations and k ar ages this igneous activity is subdivided into early intrusion 17 3 to 16 8 m y ago main intrusion 16 3 to 13 1 m y ago and late alteration about 12 8 to 11 2 m y ago events k ar ages on alunite appear to be too young to be those of formation of this mineral and the oldest 13 0 or 0 4 m y is interpreted as a minimum age for the nena pyritic replacement copper gold deposit geothermal activity which caused district scale alteration extended from earlier than 14 0 or 0 4 m y until about 11 2 or 0 2 m y ago the discordance of about 2 m y between k ar ages of coexisting hornblende and biotite in two diorites is thought to register the cooling history of the hydrothermal system a k ar age of 13 6 or 0 4 m y was obtained on hornblende from a sample of horse microdiorite regarded as the progenitor of porphyry copper mineralization this age is interpreted as near that of emplacement and provides an older limit for the age of vein controlled transitional alteration associated with most porphyry copper mineralization the primary biotite apparent k ar age of 11 5 or 0 2 m y from the same sample is accepted as a younger limit for the age of porphyry copper mineralization k ar ages from the mianmin area and the nena diorite area are very similar to those found for dioritic intrusions in the frieda complex intrusive and volcanic rocks of the frieda river prospect exhibit limited compositional variation they are all of andesitic composition and belong to a normal k calc alkaline suite the three geologically distinct areas of the prospect are distinguishable on the basis of rock and mineral chemical differences textural and chemical features of the freida complex and mianmin area igneous rocks are compatible with much of their diversity in lithology and dispersion in chemistry being due to their representing cumulate melt mixtures they are thought to have crystallized from hot 900 degrees c oxidized magmas which were emplaced at a shallow level in the crust p total

incipient subduction of the ontong java plateau along the north solomon trench abstract the ontong java plateau ojp in the western central pacific is the largest and thickest oceanic plateau and one of a few oceanic plateaus converging on an island arc solomon island arc sia to better understand the evolution of the north solomon trench nst active oblique convergence between the ojp and sia and late neogene development of malaita accretionary prism map we present 850 km of multichannel seismic reflection data integrated with 7832 km 2 of izanagi side scan sonar coverage we have focussed the study at the transition area between the well defined northwestern end of the north solomon trench and a diffusely deformed area where the trench is actively propagating in a northwestward direction the deeper structure beneath the survey area is discussed by phinney et al oceanic plateau accretion in the malaita accretionary prism inferred from multi channel seismic reflection data this issue using deeper penetration multichannel seismic reflection lines the serial cross sections provided by multichannel seismic profiling combined with the izanagi backscattering imagery provides a time series evolution for the development of the north solomon trench the main evolutionary stages include 1 the incipient trench in the northern area marked by a diffuse zone of deformation above a broad arch in the crust deeper penetration profiles by phinney et al show the bulge is related to a deeper decollement fault that is propagating upward and seaward through the crust 2 the formation of a continuous thrust front in the central area deeper penetration profiles by phinney et al show this thrust front is surface expression of the same decollement present at depth to the north the boundary between the surface trace of the thrust and the diffuse area of deformation in the northern area is inferred as a vertical high angle tear fault with left lateral offset 3 the formation of a deep elongate trench which controls gravitationally related slumping and sedimentation around the steep edges of the trench fill basin the areas to the southeast are those that have undergone convergence for the longest period of time and therefore show better developed trench structures and a reduced width of the map areas to the northwest have undergone convergence for a shorter period of time and show less developed trench structures and a wide area of the map

derivation of isotope ratios errors and error correlations for u pb geochronology using 205pb 235u 233u spiked isotope dilution thermal ionization mass spectrometric data a comprehensive treatment of the derivation of u pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation tracer subtraction blank pb and u subtraction and initial common pb correction derivations include errors and error correlations for total sample u pb and pb isotope ratios including radiogenic and initial common pb for two and three dimensional isochron calculations radiogenic u pb and pb isotope ratios for concordia and radiogenic model age calculations and the propagation of model age errors from radiogenic isotope ratios

unusual mixed silica carbonate deposits from magmatic hydrothermal hot springs savo solomon islands abstract the volcanic island of savo solomon islands hosts an active hydrothermal system discharging unusual alkaline ph 7 8 sulphate rich chloride poor fluid with variable admixtures of ca mg hco 3 rich fluid hot springs and their outflow streams precipitate a variety of deposits including travertine silica sinter and unusual mixed silica carbonate rocks travertine fabrics are dominated by ray crystal calcite associated with rapid abiotic precipitation from a supersaturated solution sinter is produced by evaporation of thermal waters and downstream samples contain preserved traces of micro organisms which potentially acted as templates for precipitation trace element chemistry of sinters and travertines includes anomalously high levels of te indicating a magmatic origin for a component fluid in the hydrothermal system springs are close to or at saturation with both calcite and amorphous silica increased contributions from the ca mg hco 3 end member favours calcite formation this fluid is of low temperature origin and as such is favoured by high rainfall mixed samples show cyclical changes between silica and carbonate precipitation potentially as a result of seasonal variation in rainfall supplementary material a spreadsheet of water and deposit chemical analyses is available at www geolsoc org uk sup18495

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

rapid change in drift of the australian plate records collision with ontong java plateau earth s largest and thickest oceanic plateau the ontong java plateau is currently colliding with the australian plate but it has been difficult to date the start of this momentous event with much accuracy now a team from the university of queensland presents strong evidence for a collision starting about 26 million years ago the dating comes from geochronological data on hotspot volcanoes in the tasman sea east of australia which reveal a link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain suggesting that immense oceanic plateaus like ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale geochronological data on hotspot volcanoes in eastern australia are presented which reveal a strong link between collision of the plateau with the melanesian arc and motion of the australian plate the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks in the pacific including the archetypal hawaiian chain and thus provide strong evidence that immense oceanic plateaus can contribute to initiating rapid change in plate boundaries and motions on a global scale the subduction of oceanic plateaux which contain extraordinarily thick basaltic crust and are the marine counterparts of continental flood basalt provinces is an important factor in many current models of plate motion1 2 3 4 and provides a potential mechanism for triggering plate reorganization5 to evaluate such models it is essential to decipher the history of the collision between the largest and thickest of the world s oceanic plateaux the ontong java plateau and the australian plate but this has been hindered by poor constraints for the arrival of the plateau at the melanesian trench here we present 40ar 39ar geochronological data on hotspot volcanoes in eastern australian that reveal a strong link between collision of the greenland sized ontong java plateau with the melanesian arc and motion of the australian plate the new ages define a short lived period of reduced northward plate motion between 26 and 23 myr ago coincident with an eastward offset in the contemporaneous tracks of seamount chains in the tasman sea east of australia these features record a brief westward deflection of the australian plate as the plateau entered and choked the melanesian trench 26 myr ago from 23 myr ago australia returned to a rapid northerly trajectory at roughly the same time that southwest directed subduction began along the trobriand trough6 the timing and brevity of this collisional event correlate well with offsets in hotspot seamount tracks on the pacific plate including the archetypal hawaiian chain7 and thus provide strong evidence that immense oceanic plateaux like the ontong java can contribute to initiating rapid change in plate boundaries and motions on a global scale

from continent to intra oceanic arc zircon xenocrysts record the crustal evolution of the solomon island arc the first u pb ages from a ca 26 24 ma pluton on guadalcanal in the intra oceanic solomon island arc southwest pacific ocean reveal eocene to archean aged zircon xenocrysts xenocryst populations at ca 39 33 ma and ca 71 63 ma correlate with previously obtained ages of supra subduction magmatism within the arc a ca 96 ma zircon population may be derived from cretaceous ophiolite basement crust or region wide continental rift related magmatism xenocryst age populations alternate with periods of oceanic basin formation that fragmented the east gondwana margin early cretaceous to archean zircon xenocryst ages imply continental origins and a cryptic source within the arc crust they may have been introduced by eocene interaction of a continental fragment with the arc and concealed by ophiolite obduction the data demonstrate that continentally derived zircons may be transported thousands of kilometers from their source and added to intra oceanic arc magmas a process likely facilitated by cyclical subduction zone advance and retreat the findings highlight the continuum of arcs that occurs between continental and oceanic end members and the caution with which zircons should be used to determine the provenance and setting of ancient arc terranes accreted to the continental crust

australian plate motion and topography linked to fossil new guinea slab below lake eyre abstract unravelling causes for absolute plate velocity change and continental dynamic topography change is challenging because of the interdependence of large scale geodynamic driving processes here we unravel a clear spatio temporal relation between latest cretaceous early cenozoic subduction at the northern edge of the australian plate early cenozoic australian plate motion changes and cenozoic topography evolution of the australian continent we present evidence for a 4000 km wide subduction zone which culminated in ophiolite obduction and arc continent collision in the new guinea pocklington trough region during subduction termination coinciding with cessation of spreading in the coral sea a 5 cm yr decrease in northward australian plate velocity and slab detachment renewed northward motion caused the australian plate to override the sinking subduction remnant which we detect with seismic tomography at 800 1200 km depth in the mantle under central southeast australia at a position predicted by our absolute plate reconstructions with a numerical model of slab sinking and mantle flow we predict a long wavelength subsidence negative dynamic topography migrating southward from 50 ma to present explaining eocene oligocene subsidence of the queensland plateau 330 m of late eocene early oligocene subsidence in the gulf of carpentaria oligocene miocene subsidence of the marion plateau and providing a first order fit to the present day 200 m deep topographic depression of the lake eyre basin and murray darling basin we propound that dynamic topography evolution provides an independent means to couple geological processes to a mantle reference frame this is complementary to and can be integrated with other approaches such as hotspot and slab reference frames

geochronology of the porgera gold deposit papua new guinea resolving the effects of excess argon on k ar and age estimates for magmatism and mineralization abstract mesothermal epithermal gold mineralization at porgera in the highlands of papua new guinea png occurs in structurally controlled veins and disseminations which overprint and cross cut a suite of shallow level comagmatic mafic alkaline stocks and dykes the porgera intrusive complex pic and their sedimentary host rocks conventional k ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 ma but four biotite separates are concordant at 6 02 0 29 ma 2 40 ar 39 ar step heating experiments on six of the hornblende separates reveal saddleshaped age spectra which indicate the presence of excess 40ar one of these samples yields a well defined plateau with an apparent age of 5 96 0 25 ma 2 this age is identical to that of the biotites and suggests an age of 6 0 0 3 ma 2 for emplacement of the pic older apparent ages obtained from conventional k ar and 40 ar 39 ar analyses are explained by excess 40ar contamination conventional k ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5 1 and 6 1 ma and indicate that ore deposition occurred within 1 ma of magmatism at porgera evidence for the evolution of a magmatic volatile phase and the presence of excess 40ar both in the intrusives and in hydrothermal fluids associated with the orebody suggest that magmatic fluids may have had some involvement in metallogenesis but the exact nature of this involvement is not yet clear late miocene magmatism and mineralization at porgera are thought to have occurred shortly prior to or during the initiation of continent arc collision and to pre date associated pliocene uplift and foreland deformation in the highlands

global tectonic significance of the solomon islands and ontong java plateau convergent zone abstract oceanic plateaus areas of anomalously thick oceanic crust cover about 3 of the earth s seafloor and are thought to mark the surface location of mantle plume heads hotspot tracks represent continuing magmatism associated with the remaining plume conduit or tail it is presently controversial whether voluminous and mafic oceanic plateau lithosphere is eventually accreted at subduction zones and therefore 1 influences the eventual composition of continental crust and 2 is responsible for significantly higher rates of continental growth than growth only by accretion of island arcs the ontong java plateau ojp of the southwestern pacific ocean is the largest and thickest oceanic plateau on earth and the largest plateau currently converging on an island arc solomon islands for this reason this convergent zone is a key area for understanding the fate of large and thick plateaus on reaching subduction zones this volume consists of a series of four papers that summarize the results of joint us japan marine geophysical studies in 1995 and 1998 of the solomon islands ontong java plateau convergent zone marine geophysical data include single and multi channel seismic reflection ocean bottom seismometer obs refraction gravity magnetic sidescan sonar and earthquake studies objectives of this introductory paper include 1 review of the significance of oceanic plateaus as potential contributors to continental crust 2 review of the current theories on the fate of oceanic plateaus at subduction zones 3 establish the present day and neogene tectonic setting of the solomon islands ontong java plateau convergent zone 4 discuss the controversial sequence and timing of tectonic events surrounding ontong java plateau solomon arc convergence 5 present a series of tectonic reconstructions for the period 20 ma early miocene to the present day in support of our proposed timing of major tectonic events affecting the ontong java plateau solomon islands convergent zone and 6 compare the structural and deformational pattern observed in the solomon islands to ancient oceanic plateaus preserved in precambrian and phanerozoic orogenic belts our main conclusion of this study is that 80 of the crustal thickness of the ontong java plateau is subducted beneath the solomon island arc only the uppermost basaltic and sedimentary part of the crust 7 km is preserved on the overriding plate by subduction accretion processes this observation is consistent with the observed imbricate structural style of plateaus and seamount chains preserved in both precambrian and phanerozoic orogenic belts

arc lavas on both sides of a trench slab window effects at the solomon islands triple junction sw pacific abstract the woodlark spreading center wsc is subducted at the san cristobal trench forming a triple junction at the new georgia group ngg arc in the solomon islands wsc lavas are n morb at 100 km from the trench but with decreasing distance they have increasingly arc like sr nd pb isotopic ratios enrichments in rb k pb sr and depletions in hfse and y within 50 km of the trench on the simbo and ghizo ridges many recovered samples are island arc tholeiites to medium k calc alkaline andesites and dacites and many have the same or similar major and trace element and isotopic characteristics as true arc lavas in the ngg on the other side of the trench previous investigations have concluded that these wsc lavas are the result of relic back arc mantle enrichments resulting from subduction of the pacific plate prior to the late miocene at the north solomon trench 200 km to the north however the high silica wsc lavas are more arc like than those recovered from other distal back arcs and are more voluminous forming large submarine ridges and stratovolcanoes we suggest that true arc mantle migrates across the plate boundary from the adjacent ngg arc through slab windows created by the subduction of the wsc this leads to variable mixing between ngg arc and wsc n morb end members forming the transitional lavas recovered from the wsc lavas with similar arc like characteristics have previously been recovered on the chile rise near where it is subducted at the chile trench raising the possibility that such mantle transfer is a common phenomenon where active spreading centers are subducted the presence of slab windows may also be responsible for the unusual forearc volcanism in the ngg and melting of slab window margins may account for the presence of high silica adakite like lavas on the wsc

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

geochronology of the central tanzania craton and its southern and eastern orogenic margins geological mapping and zircon u pb hf isotope data from 35 samples from the central tanzania craton and surrounding orogenic belts to the south and east allow a revised model of precambrian crustal evolution of this part of east africa the geochronology of two studied segments of the craton shows them to be essentially the same suggesting that they form a contiguous crustal section dominated by granitoid plutons the oldest orthogneisses are dated at ca 2820 ma dodoma suite and the youngest alkaline syenite plutons at ca 2610 ma singida suite plutonism was interrupted by a period of deposition of volcano sedimentary rocks metamorphosed to greenschist facies directly dated by a pyroclastic metavolcanic rock which gave an age of ca 2725 ma this is supported by detrital zircons from psammitic metasedimentary rocks which indicate a maximum depositional age of ca 2740 ma with additional detrital sources 2820 and 2940 ma thus 200 ma of episodic magmatism in this part of the tanzania craton was punctuated by a period of uplift exhumation erosion and clastic sedimentation volcanism followed by burial and renewed granitic to syenitic magmatism in eastern tanzania handeni block in the heart of the east african orogen all the dated orthogneisses and charnockites apart from those of the overthrust neoproterozoic granulite nappes have neoarchaean protolith ages within a narrow range between 2710 and 2630 ma identical to but more restricted than the ages of the singida suite they show evidence of ediacaran pan african isotopic disturbance but this is poorly defined in contrast granulite samples from the wami complex nappe were dated at ca 605 and ca 675 ma coeval with previous dates of the eastern granulites of eastern tanzania and granulite nappes of adjacent ne mozambique to the south of the tanzania craton samples of orthogneiss from the northern part of the lupa area were dated at ca 2730 ma and clearly belong to the tanzania craton however granitoid samples from the southern part of the lupa block have palaeoproterozoic ubendian intrusive ages of ca 1920 ma outcrops further south at the northern tip of lake malawi mark the se continuation of the ubendian belt albeit with slightly younger ages of igneous rocks ca 1870 1900 ma which provide a link with the ponte messuli complex along strike to the se in northern mozambique in sw tanzania rocks from the mgazini area gave ubendian protolith ages of ca 1980 1800 ma but these rocks underwent late mesoproterozoic high grade metamorphism between 1015 and 1040 ma one granitoid gave a crystallisation age of ca 1080 ma correlating with known mesoproterozoic crust to the east in se tanzania and ne mozambique however while the crust in the mgazini area was clearly one of original ubendian age reworked and intruded by granitoids at ca 1 ga the crust of se tanzania is a mixed mesoproterozoic terrane and a continuation from ne mozambique hence the mgazini area lies at the edge of the ubendian belt which was re worked during the mesoproterozoic orogen south irumide belt providing a further constraint on the distribution of ca 1 ga crust in se africa hf data from near concordant analyses of detrital zircons from a sample from the tanzania craton lie along a pb loss trajectory lu hf 0 extending back to 3 9 ga this probably represents the initial depleted mantle extraction event of the cratonic core furthermore the hf data from all igneous samples regardless of age from the entire study area including the neoproterozoic granulite nappes show a shallow evolution trend lu hf 0 028 extending back to the same mantle extraction age this implies the entire tanzanian crust sampled in this study represents over 3 5 billion years of crustal reworking from a single crustal reservoir and that the innermost core of the tanzanian craton that was subsequently reworked was composed of a very depleted mafic source with a very high lu hf ratio our study helps to define the architecture of the tanzanian craton and its evolution from a single age source in the early eoarchaean

time resolved construction of a bimodal laccolith torres del paine patagonia abstract granitic and mafic magma pulses were sequentially accreted in the spectacularly exposed shallow crustal torres del paine laccolith in southern patagonia this 12 5 ma pluton forms a composite intrusion with a subvertical feeding system in the west and a laccolith in the east a key unknown in the formation of sill complexes is how individual magma pulses are assembled over time and the geometry and localization of their feeding system high resolution zircon ca id tims u pb dating shows that the laccolith grew first by under accretion of granitic sills over 90 30 ka linked to a sheet like feeding system followed by underplating of mafic sills after 20 ka of quiescence in the mafic sills complex individual sills were injected by over accretion during 41 11 ka our data show that successive granitic and mafic magmas emplacement generated a volume of 88 km3 in 162 11 ka

subduction of the nazca ridge and the inca plateau insights into the formation of ore deposits in peru a large number of ore deposits that formed in the peruvian andes during the miocene 15 5 ma are related to the subduction of the nazea plate beneath the south american plate here we show that the spatial and temporal distribution of these deposits correspond with the arrival of relatively buoyant topographic anomalies namely the nazca ridge in central peru and the now consumed inca plateau in northern peru at the subduction zone plate reconstruction shows a rapid metallogenic response to the arrival of the topographic anomalies at the subduction trench this is indicated by clusters of ore deposits situated within the proximity of the laterally migrating zones of ridge subduction it is accordingly suggested that tectonic changes associated with impingement of the aseismic ridge into the subduction zone may trigger the formation of ore deposits in metallogenically fertile suprasubduction environments c 2005 elsevier b v all rights reserved

an algorithm for u pb isotope dilution data reduction and uncertainty propagation this is the publisher s version also available electronically from http onlinelibrary wiley com

the structure of the south fiji basin abstract new magnetic seismic and bathymetric data show that the south fiji basin is a structurally complex marginal basin a gap in the identifiable magnetic anomaly lineations exist over the central part of the basin and prevents the unequivocal linking of the anomaly lineations anomalies 7a to 12 associated with the ridgc ridge ridge triple junction in the north with an apparent single spreading centre of the same age in the south this gap which makes a detailed synthesis of the historical development of the basin difficult may arise from post spreading intraplate tectonics if symmetric spreading is assumed part of the oceanic lithosphere formed during the oligocene episode of seafloor spreading has subsequently been consumed presumably by subduction westward under three kings rise

oligocene to recent tectonic history of the central solomon intra arc basin as determined from marine seismic reflection data and compilation of onland geology systematic analysis of a grid of 3450 km of multichannel seismic reflection lines from the solomon islands constrains the late tertiary sedimentary and tectonic history of the solomon island arc and its convergent interaction with the cretaceous ontong java oceanic plateau ojp the ojp the largest oceanic plateau on earth subducted beneath the northern edge of the solomon arc in the late neogene but the timing and consequences of this obliquely convergent event and its role in the subduction polarity reversal process remain poorly constrained the central solomon intra arc basin csb which developed in oligocene to recent time above the solomon arc provides a valuable record of the tectonic environment prior to and accompanying the ojp convergent event and the subsequent arc polarity reversal recognition of regionally extensive stratigraphic sequences whose ages can be inferred from marine sedimentary sections exposed onland in the solomon islands indicate four distinct tectonic phases affecting the solomon island arc phase 1 late oligocene late miocene rifting of the northeast facing solomon island arc produced basal normal fault controlled asymmetrical sequences of the csb the proto north solomon trench was probably much closer to the csb and is inferred to coincide with the trace of the present day kia kaipito korigole kkk fault zone this protracted period of intra arc extension shows no evidence for interruption by an early miocene period of convergent soft docking of the ontong java plateau as proposed by previous workers phase 2 late miocene pliocene oblique convergence of the ontong java plateau at the proto north solomon trench kkk fault zone and folding of the csb and formation of the malaita accretionary prism map the highly oblique and diachronous convergence between the ontong java plateau and the solomon arc terminates intra arc extension first in the southeast russell subbasin of the csb during the late miocene and later during the pliocene in the northwest shortland subbasin of the csb folds in the csb form by inversion of normal faults formed during phase 1 phinney et al sequence stratigraphy structural style and age of deformation of the malaita accretionary prism solomon arc ontong java plateau convergent zone show a coeval pattern of southeast to northwest younging in folding and faulting of the map phase 3 late pliocene early pleistocene arc polarity reversal and subduction initiation at the san cristobal trench effects of this event in the csb include the formation of a chain of volcanoes above the subducting australia plate at the san cristobal trench the formation of the broad synclinal structure of the csb with evidence for truncation at the uplifted flanks and widespread occurrence of slides and seismites deposits formed by seismic shaking phase 4 pleistocene to recent continued shortening and synclinal subsidence of the csb continued australia pacific oblique plate convergence has led to deepening of the submarine elongate basin axis of the synclinal csb and uplift of the dual chain of the islands on its flanks

ocean basin evolution and global scale plate reorganization events since pangea breakup we present a revised global plate motion model with continuously closing plate boundaries ranging from the triassic at 230 ma to the present day assess differences among alternative absolute plate motion models and review global tectonic events relatively high mean absolute plate motion rates of approximately 9 10 cm yr 1 between 140 and 120 ma may be related to transient plate motion accelerations driven by the successive emplacement of a sequence of large igneous provinces during that time an event at 100 ma is most clearly expressed in the indian ocean and may reflect the initiation of andean style subduction along southern continental eurasia whereas an acceleration at 80 ma of mean rates from 6 to 8 cm yr 1 reflects the initial northward acceleration of india and simultaneous speedups of plates in the pacific an event at 50 ma expressed in relative and some absolute plate motion changes around the globe and in a reduction of global mean plate speeds from about 6 to 4 5 cm yr 1 indicates that an increase in collisional forces such as the india eurasia collision and ridge subduction events in the pacific such as the izanagi pacific ridge play a significant role in modulating plate velocities

zircon growth and recycling during the assembly of large composite arc plutons abstract concordant u pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth advances in zircon tims analysis and geochronological studies by sims on zircon from young volcanic rocks along with evolving views of the evolution of magmatic systems makes it necessary to revaluate this interpretation assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton distinctions must be made between true inherited zircon xenocrystic zircon derived from host rocks during magma ascent and or emplacement and multi stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons these complications are exemplified by recent high precision u pb zircon age dating from the cretaceous tuolumne and mt stuart batholiths which clearly show that the timeframe of pluton assembly was long 5 ma for mt stuart and 8 ma for tuolumne zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon several 10 5 yr up to 2 10 6 yr and assignment of statistically valid rock solidification ages is not possible from these data low zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages and is compatible with progressive growth of a large long lived crystal mush body because eruption of magma requires low to moderate crystal fraction and high temperatures relative to stored crystal mushes zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions

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

granite magma formation transport and emplacement in the earth s crust the origin of granites was once a question solely for petrologists and geochemists but in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation transport and emplacement of silica rich melts in the continental crust accepted petrological models are being questioned either because they require unrealistic rheological behaviours of rocks and magmas or because they do not satisfactorily explain the available structural or geophysical data provided flow is continuous mechanical considerations suggest that far from being geologically sluggish granite magmatism is a rapid dynamic process operating at timescales of or 100 000 years irrespective of tectonic setting

alkalic intrusive rocks associated with the mount kare gold deposit papua new guinea comparison with the porgera intrusive complex the mount kare gold deposit is located 18 km southwest of the giant porgera gold mine proven and probable reserves of 51 5 million metric tons at 0 23 oz au t in the highlands of papua new guinea both deposits are spatially associated with late miocene alkalic intrusive complexes emplaced in mesozoic tertiary shelf sedimentary sequences near the edge of the australasian plate close geologic similarities between the two deposits including the presence of quartz roscoelite au veins and breccias have spurred recent exploration efforts at mount kare the mount kare and porgera intrusive complexes consist of hypabyssal suites of comagmatic volatile rich mafic intrusions containing clinopyroxene olivine plagioclase hornblende chromian spinel and apatite in melagabbros and mafic porphyries with plagioclase hornblende clinopyroxene titanian magnetite and apatite in more evolved rocks listed in decreasing order of abundance analcite occurs as an alteration phase of the groundmass and fills miarolitic cavities in evolved leucogabbros at mount kare but has not been observed in the porgera intrusive complex least altered igneous rocks from both suites are nepheline normative and are characterized by high volatile alumina large ion lithophile and high field strength element contents they display light rare earth element enrichments la yb cn 9 28 at mount kare 14 20 at porgera but relatively low concentrations of k and ti on mantle normalized diagrams the rocks are mostly sodic na 2 o 2 or k 2 o according to the total alkali silica classification scheme nd sr and pb isotope compositions and k ar ages of hornblende and biotite from the two suites are virtually indistinguishable epsilon nd 6 7 87 sr 86 sr asymp 0 7036 208 pb 204 pb asymp 38 5 207 pb 204 pb asymp 15 55 206 pb 204 pb asymp 18 65 k ar age of mount kare intrusive complex 6 0 or 0 1 ma 2gma porgera intrusive complex 6 0 or 0 3 ma 2gma these combined characteristics indicate a dominantly alkalic intraplate affinity for the parental magma at both locations although a weak subduction zone signature is also suggested by the relatively high volatile and alumina and low ti abundances it is proposed that magmatic activity was related to changes in geodynamic configuration occurring immediately prior to early pliocene collision between the australasian plate and an island arc system minor differences such as divergent trajectories of pb isotope crustal contamination arrays the occurrence of analcite only at mount kare and the widespread presence of excess 40 ar in igneous amphiboles only at porgera indicate that although related the mount kare and porgera intrusive complexes developed independently of one another disseminated auriferous pyrite from the mount kare au deposit is associated with phyllic alteration of intrusive and sedimentary rocks a single k ar analysis of illitc from altered rock records an age of 5 5 or 0 1 ma 2sigma which falls in the middle of the range of illite and roscoelite ages from the porgera au deposit 5 1 6 1 ma this result combined with slightly older ages of igneous activity suggests a close temporal and genetic association between magmatism and mineralization at both locations in addition the narrow range of pb isotope compositions of galenas from veins at mount kare is identical with that from porgera suggesting that similar ore forming processes were involved at the two locations recognition of closely similar igneous lithologies and ore types at mount kare and porgera supports theories of a genetic relationship between volatile rich mafic alkalic magmatism and meso to epithermal au mineralization as previously proposed at porgera it is suggested that both of these deposits belong to the alkalic epithermal group to which cripple creek colorado emperor fiji and ladolam lihir island papua new guinea also belong igneous centers with these characteristics and located in complex convergent tectonic settings should be considered as prospective targets for precious metal exploration

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

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

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

giant ore deposits formed by optimal alignments and combinations of geological processes giant ore deposits are priority targets for mining companies a review of the characteristics of several giant porphyry and epithermal deposits worldwide suggests that they formed from ordinary processes that were fortuitously operating at maximum efficiency

origin of the oceanic basalt basement of the solomon islands arc and its relationship to the ontong java plateau insights from cenozoic plate motion models abstract cenozoic global plate motion models based on a hotspot reference frame may provide a useful framework for analyzing the tectonic evolution of the solomon islands convergent margin a postulated late miocene collision of the ontong java plateau ojp with a ne facing arc is consistent with the predicted path of the ojp across the pacific basin and its miocene arrival at the trench late stage igneous activity 65 30 ma predicted for the ojp as it rode over the samoan hotspot occurred in correlative stratigraphic sections on malaita the supposed accreted flake of ojp in the solomon islands arc convergence similar to the present velocities between australia and the pacific plates was characteristic of the last 43 million years prior to 43 ma pacific australia plate motions were divergent seemingly at odds with geologic evidence for early tertiary convergence particularly in papua new guinea a postulated south pacific plate may have existed between australia and the pacific plate and would have allowed implied northward subduction along the northeastern australia plate boundary that lasted into the early eocene subsequent reorganization of plate motions in the middle eocene correlates with middle eocene marginal basin formation along ridges oblique to the main plate boundary cessation of spreading on the pacific south pacific ridge and its subsequent subduction beneath asia followed the change in pacific plate motion at 43 ma a trapped remnant of the extinct nw trending ridge may still lie beneath the western philippine sea the terminal deformation metamorphism and ophiolite obduction in the eocene orogen of the southwest pacific also correlates with the major change in pacific plate motion at 43 ma and the subsequent compression of the dying eocene arc against outlying continental and oceanic crustal blocks of the australian plate the solomon islands oceanic basement may represent juxtaposition of oceanic plateaus of the australian plate beneath overthrust dismembered ophiolite derived from adjacent marginal basin crust

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

melanesian back arc basin and arc development constraints from the eastern coral sea abstract the eastern coral sea is a poorly explored area at the north eastern corner of the australian tectonic plate where interaction between the pacific and australian plate boundaries and accretion of the world s largest submarine plateau the ontong java plateau has resulted in a complex assemblage of back arc basins island arcs continental plateaus and volcanic products this study combines new and existing magnetic anomaly profiles seafloor fabric from swath bathymetry data ar ar dating of e morb basalts palaeontological dating of carbonate sediments and plate modelling from the eastern coral sea our results constrain commencement of the opening of the santa cruz basin and south rennell trough to c 48 ma and termination at 25 28 ma simultaneous opening of the melanesian basin solomon sea further north suggests that a single 2000 km long back arc basin with at least one triple junction existed landward of the melanesian subduction zone from eocene oligocene times the cessation of spreading corresponds with a reorganisation of the plate boundaries in the area and the proposed initial soft collision of the ontong java plateau the correlation between back arc basin cessation and a widespread plate reorganisation event suggests that back arc basins may be used as markers for both local and global plate boundary changes

anomalous alkaline sulphate fluids produced in a magmatic hydrothermal system savo solomon islands abstract in magmatic hydrothermal and associated geothermal systems acidic magmatic derived fluids ph 5 geothermal fluids are typically limited to lateral outflows some distance from the main vent here we describe an unusual hydrothermal system associated with savo volcano a recently eruptive 1830 1840 ad trachyte dominated island arc stratovolcano in the solomon islands hot springs 100 c near to the volcanic crater discharge alkaline waters instead of the more commonly recognised acidic fluids the hydrothermal system of savo dominantly discharges sinter and travertine forming alkaline sulphate ph 7 8 waters at hot springs on its upper flanks in addition to a small number of lower discharge acid sulphate springs ph 2 7 alkaline sulphate fluids are dilute chloride poor 600 mg l and silica rich 250 mg l they have restricted 34sso4 5 4 1 5 and 18oh2o values 4 local non thermal groundwater is 8 acid sulphate springs discharge low chloride geochemical modelling shows that water rock reaction and dilution in the presence of secondary anhydrite pyrite and quartz leads to chloride being diluted to low concentrations whilst maintaining high sulphate and silica concentrations in the fluid strontium oxygen and hydrogen isotopes confirm water rock reaction and mixing with groundwater as primary controls on the composition of the alkaline sulphate springs the highly unusual dilute chemistry of all discharges at savo is a consequence of high regional rainfall i e climatic control and results from open system mixing at depth between hydrothermal and meteoric waters

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

sequence stratigraphy structural style and age of deformation of the malaita accretionary prism solomon arc ontong java plateau convergent zone possibilities for the fate of oceanic plateaus at subduction zones range from complete subduction of the plateau beneath the arc to complete plateau arc accretion and resulting collisional orogenesis deep penetration multi channel seismic reflection mcs data from the northern flank of the solomon islands reveal the sequence stratigraphy structural style and age of deformation of an accretionary prism formed during late neogene 5 0 ma convergence between the 33 km thick crust of the ontong java oceanic plateau and the 15 km thick solomon island arc correlation of mcs data with the satellite derived free air gravity field defines the tectonic boundaries and internal structure of the 800 km long 140 km wide accretionary prism we name this prism the malaita accretionary prism or map after malaita the largest and best studied island exposure of the accretionary prism in the solomon islands mcs data gravity data and stratigraphic correlations to islands and odp sites on the ontong java plateau ojp reveal that the offshore map is composed of folded and thrust faulted sedimentary rocks and upper crystalline crust offscraped from the solomon the subducting ontong java plateau pacific plate and transferred to the solomon arc with the exception of an upper sequence of quaternary island derived terrigenous sediments the deformed stratigraphy of the map is identical to that of the incoming ontong java plateau in the north solomon trench

special paper major gold deposits and belts of the north and south american cordillera distribution tectonomagmatic settings and metallogenic considerations a compilation of economically viable gold concentrations containing 10 moz in the north and south american cordillera reveals the existence of 22 discrete belts in addition to five major isolated deposits most formed over the last 150 m y the gold concentrations are attributed to eight widely recognized deposit types of which porphyry sediment hosted and high sulfidation epithermal are economically the most important individual gold belts are typically several tens to hundreds of kilometers long dominated by single deposit types and metallogenically active for relatively brief periods 5 20 m y many of the gold belts and major isolated deposits were generated under extensional or transtensional tectonic conditions in either arc or back arc settings nevertheless the two main high sulfidation epithermal gold belts were generated in thickening or already thickened crust during low angle subduction eight other gold belts or districts also accompanied compression or transpression with two of them the main orogenic gold belts occupying fore arc sites there is a strong suggestion that the preeminent cordilleran gold concentrations formed either during or immediately following prolonged contraction the major gold deposits and belts occur along the craton edge as well as in adjoining accreted terranes but almost all are of postaccretionary timing many of the gold belts and isolated deposits were localized by crustal scale faults or lineaments which may be either parallel or transverse to the cordilleran margin the gold concentrations accumulated during active subduction commonly in close spatial and temporal association with intermediate to felsic medium to high k calc alkaline igneous rocks by contrast the low sulfidation epithermal gold deposits accompany bimodal volcanic pulses of calc alkaline tholeiitic or alkaline affinity however a gold alkaline rock association is uncommon a genetic link between gold mineralization and coeval magmatism is widely accepted for most of the deposit types the exceptions being the sediment hosted and orogenic gold deposits notwithstanding the small cumulative extent of the gold concentrations relative to the entire cordilleran margin there is a marked tendency for two or more belts or isolated deposits of different ages and genetic types to occur in close proximity within relatively restricted arc including fore and back arc segments in the case of the western united states for example six belts and four isolated major deposits make up a particularly prominent cluster if fortuity is discounted this clustering or pairing of gold concentrations must imply a predisposition of certain arc segments to gold mineralization an analogous situation is evident for other metals particularly copper and tin the reason for the recurrent generation of major deposits and belts dominated by one or more metals remains uncertain although heterogeneously distributed metal preconcentrations favorable redox conditions or other parameters somewhere above the subducted slab between the mantle wedge and upper crust are widely contemplated possibilities elucidation of the reason s for this metallogenic inheritance at the scale of limited arc segments poses an important and challenging series of research questions as well as being critical to the planning of potentially successful greenfield exploration programs

tectonic and magmatic responses to the subduction of high bathymetric relief subduction of high bathymetric relief such as aseismic ridges and magmatic plateaus is considered to be responsible for dramatic changes in the dynamics and kinematics of the subduction zone for example the buoyancy of high bathymetric relief is thought to flatten the dip of the subducting slab modifying the structural and magmatic evolution of the overriding plate and terminating arc volcanism in addition the effect of ridge subduction in retreating plate boundaries can inhibit subduction rollback a process that could locally pin the subduction hinge and lead to the development of cusps and slab tearing here we discuss the tectonic response to subduction of high bathymetric relief using examples from the circum pacific subduction systems we demonstrate that flattening of the subduction dip angle is only significant in the eastern pacific where the average slab dip angle is relatively shallow in the western pacific in contrast the average subduction dip angle is steeper and there is no significant flattening of the dip angle in areas of ridge subduction subduction of high bathymetric relief in the circum pacific is commonly associated with reduced arc volcanism and in many cases the area of ridge subduction coincides with a volcanic gap in the overriding plate ridge subduction is associated with pronounced changes in the style of deformation involving uplift reactivation of basement thrusts development of orogen perpendicular tear faults and block rotations leading to oroclinal bending the discussed characteristic patterns associated with ridge subduction provide important guidelines for reconstructing past plate tectonic processes and could help constraining the geodynamics of ancient subduction systems

the petrology geochronology and tectono magmatic setting of igneous rocks in the suckling dayman metamorphic core complex papua new guinea abstract southeastern papua new guinea has hosted magmatism throughout the cenozoic with the latest phase being concurrent with active extension in the woodlark rift there the suckling dayman metamorphic core complex sdmcc has exhumed middle crustal rocks in the past few million years along a still active low angle normal fault the mai iu fault uplift of the sdmcc has exposed metasedimentary and metaigneous rocks of late cretaceous pleistocene age that record an evolution from oceanic spreading to subduction to collision and finally to subduction inversion and extensional exhumation we present new petrographic whole rock geochemical and geochronologic zircon u pb la icp ms data from the sdmcc to reconstruct its long term evolution the dominant footwall lithology of the sdmcc the goropu metabasalt has a morb composition detrital zircons from metasedimentary beds intercalated with these basalts yield u pb based maximum estimates for deposition of 103 and 72 ma suggesting a late cretaceous spreading age of the oceanic protolith bulk compositions of low grade tholeiitic meta gabbroic and tonalitic rocks yau igneous complex that intrude the goropu metabasalt reveal both enriched and depleted light rare earth element patterns zircon u pb ages from the yau igneous complex range between 60 and 57 ma providing a minimum age for the formation of the goropu metabasalt much younger syn extensional granitoids in the mafic footwall of the sdmcc have calc alkaline to high k compositions and u pb ages on zircon between 3 7 and 2 0 ma our data indicate that the mai iu fault had re activated a paleogene thrust as an extensional detachment fault by 3 7 0 2 ma u pb ages of detrital zircons in modern streams draining the footwall of the sdmcc essentially reflect the ages revealed by the paleocene and plio pleistocene intrusions xenocrystic zircons in the plio pleistocene granitoids imply that the crust underlying the metabasaltic carapace of the sdmcc consists chiefly of australian continent derived sedimentary rocks

continental rifting and initial sea floor spreading in the woodlark basin our understanding of the processes by which continents rift and sea floor spreading initiates is derived primarily from studies either of old passive margins and oceanic crust or of young regions of intra continental extension where spreading has not yet started it has been thought that continental rifting ceases when sea floor spreading begins1 2 that oceanic fracture zones develop from transfer or transform faults within continental rifts3 4 and that linear magnetic anomalies correlate with the onset of sea floor spreading during times of magnetic reversals5 6 here we present a marine geophysical survey of one of the few active examples of continental rifting and spreading initiation the western woodlark basin papuan peninsula region of new guinea which shows that in detail these assumptions do not hold the data confirm models of the rifting to spreading transition that invoke both ridge propagation and nucleation of discrete spreading cells7 10 and provide an unambiguous example of a spreading centre reorienting by synchronous jumping rather than propagation

tectonics of rift propagation into a continental margin western woodlark basin papua new guinea oblique convergence along the irregular boundary between the pacific and indo australian plates in the sw pacific has resulted in a transpressional regime the drag of the overriding pacific plate has produced a breakup of the solomon microplate from the indo australian plate and has induced its rotation within the mega shear zone between the two major plates the microplate is being separated from the indo australian plate by passive rifting and seafloor spreading in the woodlark basin which is propagating westward along the continental margin of papua new guinea the present day pole of the microplate rotation with respect to indo australian plate is estimated to be near the owen stanley fault zone which represents the suture after paleocene eocene arc continent collision the average rate of the seafloor spreading propagation is estimated to be 150 mm yr 1 over the last 3 4 ma the transition between seafloor spreading and continental rifting is characterized by a dramatic reduction in the production of new oceanic crust and by a change from localized deformation within oceanic lithosphere to distributed continental extension variations between the two structural styles are being balanced by an accommodation zone and a newly developing transform fault the style of continental deformation progressively changes due to a gradual decrease in lithospheric thinning along the rift axis toward the pole of opening three accommodation zones are proposed to balance differential extension and slippage between individual series of tilted blocks

two stage growth of laccoliths at elba island italy at elba island italy nine shallow level late miocene granite porphyry layers connected by feeder dikes built up three nested christmas tree laccoliths detailed mapping and reconstruction of tectonic history led to restoration of the original 5 km thick sequence and determination of the dimensional parameters of each intrusive layer the laccolith layers were emplaced at depths between 1 9 and 3 7 km exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes the intrusive layers are 50 700 m thick with diameters between 1 6 and 10 km length to thickness relationships for individual laccolith layers show a power law correlation that does not fit the known dimensional distribution for laccoliths but instead fits a line with a slope typical of the theoretical vertical inflation stage of laccolith development this is interpreted as the first reported natural example of the occurrence of a vertical inflation stage during laccolith growth the dimensional data for elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet like bodies while multilayer laccoliths form when coalescence fails possibly owing to the large availability of crustal magma traps

giant porphyry deposits characteristics distribution and tectonic controls more than half of the 25 largest known porphyry copper deposits defined in terms of contained copper metal formed during three time periods the paleocene to eocene eocene to oligocene and middle miocene to pliocene these giant deposits are clustered within three provinces central chile northern chile and southwest arizona northern mexico other giant deposits occur in montana utah panama peru argentina irian jaya mongolia and iran compressive tectonic environments thickened continental crust and active uplift and erosion were associated with the formation of many of these deposits calc alkalic magmas are most favorable for the formation of giant porphyry copper deposits although several of the largest systems are associated with high k calc alkalic intrusions the 25 largest gold rich porphyry deposits are concentrated in the southwest pacific and south america with other occurrences in eurasia british columbia alaska and new south wales many of the deposits formed in the last 13 m y the largest of the deposits are associated with high k calc alkalic intrusions many calc alkalic porphyritic intrusions have also produced giant gold rich porphyries in the last 20 m y the formation of giant porphyry copper molybdenum and copper gold deposits in the circum pacific region has been closely associated with subduction of aseismic ridges seamount chains and oceanic plateaus beneath oceanic island and continental arcs in several examples these tectonic perturbations have promoted flat slab subduction crustal thickening uplift and erosion and adakitic magmatism coeval with the formation of well endowed porphyry and or epithermal mineral provinces similar tectonic features are inferred to be associated with the giant porphyry copper molybdenum provinces of northern chile eocene oligocene and southwest united states cretaceous paleocene topographic and thermal anomalies on the downgoing slab appear to act as tectonic triggers for porphyry ore formation other factors such as sutures in the overriding plate permeability architecture of the upper crust efficient processes of ore transport and deposition and in some cases formation and preservation of supergene enrichment blankets are also vital for the development of high grade giant ore deposits a low grade geochemical anomaly may be the final product of mineralization if ore forming processes do not operate efficiently even in the most favorable geodynamic settings

geological tectonic framework of solomon islands sw pacific crustal accretion and growth within an intra oceanic setting the solomon islands are a complex collage of crustal units or terrains herein termed the solomon block which have formed and accreted within an intra oceanic environment since cretaceous times predominantly cretaceous basaltic basement sequences are divided into 1 a plume related ontong java plateau terrain ojpt which includes malaita ulawa and northern santa isabel 2 a normal ocean ridge related south solomon morb terrain ssmt which includes choiseul and guadalcanal and 3 a hybrid makira terrain which has both morb and plume plateau affinities the ojpt formed as an integral part of the massive ontong java plateau ojp at c 122 ma and 90 ma respectively was subsequently affected by eocene oligocene alkaline and alnoitic magmatism and was unaffected by subsequent arc development the ssmt initially formed within a normal ocean ridge environment which produced a morb like basaltic basement through which two stages of arc crustal growth subsequently developed from the eocene onwards the makira terrain records the intermingling of basalts with plume plateau and morb affinities from c 90 ma to c 30 ma and a contribution from late miocene present day arc growth two distinct stages of arc growth occurred within the solomon block from the eocene to the early miocene stage 1 and from the late miocene to the present day stage 2 stage 1 arc growth created the basement of the central part of the solomon block the central solomon terrain cst which includes the shortland florida and south isabel islands stage 2 arc growth led to crustal growth in the west and south the new georgia terrain or ngt which includes savo and the new georgia and russell islands both stages of arc growth also added new material to pre existing crustal units within other terrains the solomon block terrane collage records the collision between the alaska sized ojp and the solomon arc initial contact possibly first occurred some 25 20 ma but it is only since around 4 ma that the ojp has more forcefully collided with the solomon arc and has been actively accreting since that time continuing to the present day we present a number of tectonic models in an attempt to understand the mechanism of plateau accretion one model depicts the ojp as splitting in two with the upper 4 10 km forming an imbricate stack verging to the northeast over which the solomon arc is overthrust whilst deeper portions of the ojp beneath a critical detachment surface are subducted the subduction of young 5 ma hot oceanic lithosphere belonging to the woodlark basin at the ssts has resulted in a sequence of tectonic phenomena including the production of unusual magma compositions e g na ti rich basalts and an abundance of picrites an anomalously small

dehydration melting of a basaltic composition amphibolite at 1 5 and 2 0 gpa implications for the origin of adakites this study presents the results of dehydration melting experiments on a basaltic composition amphibolite under conditions appropriate to a hot slab geotherm 1 5 and 2 0 gpa and temperatures of 850 to 1150 c dehydration melting produces an omphacitic augite and garnet bearing residue coexisting with rhyolitic to andesitic composition melts at 1 5 gpa the amphibolite melts in two stages between 800 and 1025 c the 2 0 gpa data also define two melting stages at 2 0 gpa the first stage involves nearly modal melting of the original amphibolite minerals qtz pl amp to produce melt cpx grt during the second stage the eclogite restite melts non modally 0 86 cpx 0 14 grt 1 melt the experimental results were combined with data from the literature to generate a composite p t phase diagram for basaltic composition amphibolites over the 800 to 1100 c temperature range for pressures up to 2 0 gpa comparison of the major element compositions of the experimentally produced melts with compositions of presumed slab melts adakites shows that partial melting of amphibolite at conditions appropriate to a hot slab geotherm produces melts similar to andesitic and dacitic adakites except for significant mgo and cao depletions trace element modelling of amphibolite dehydration melting using the 2 0 gpa melting reactions produces ree abundances similar to those of adakites at 10 15 wt batch melting but the models do not reproduce the high sr y ratios characteristic of adakites taken together the major and trace element results are not consistent with the derivation of adakites by dehydration melting of the subducted slab with little or no interaction with the mantle wedge or crust if adakites are partial melts of the subducted slab they must undergo significant interaction with the mantle and or crust during which they acquire a number of their distinctive characteristics

mineralization parameters and exploration targeting for gold copper deposits in the baguio luzon and pacific cordillera mindanao mineral districts philippines a review abstract the baguio mineral district in luzon philippines is known to host several world class epithermal gold porphyry copper deposits the interplay of tectonic setting magma composition structural control and hydrothermal systems contributed to the generation of these deposits ridge subduction scarborough seamount resulting to flat subduction and a transpressional regime could also be related to the formation of epithermal gold porphyry copper deposits in baguio subduction processes leading to the formation of calc alkaline rocks associated with high water pressure oxygen fugacity and late sulfur saturation are almost always associated with the gold copper deposits in the district compared to the baguio mineral district less exploration work mine development and production were done in the pacific cordillera mineral district mindanao in southern philippines it is worth noting however that both mineral districts show similarities and overlapping features in terms of geological geophysical and geochemical characteristics this leads one to conclude that the pacific cordillera mineral district has ore deposits waiting to be discovered

the submarine tectono magmatic framework of cu au endowment in the tabar to feni island chain png abstract the southwest pacific region and papua new guinea in particular is spectacularly endowed with mineral resources including some of the youngest and richest porphyry cu mo au deposits in the world among them is the giant porphyry epithermal ladolam au deposit on lihir island in the tabar lihir tanga feni tltf island chain northeast of new ireland its setting within a former forearc basin is very different from most southwest pacific porphyry and epithermal deposits our synthesis of published and previously unreleased data from ship based multibeam and seismic studies satellite gravimetry geochemistry and geochronology reveals a far more complex crustal structure and composition than is presently understood from the geology of the islands alone we show that the unique regional au endowment results from the alignment of various preconditions that are prolific to ore formation i hydrous and metal rich metasomatic veins in the mantle source ii second stage low volume partial melting due to incipient rifting iii high volatile contents and oxygen fugacities of the melts due to preferential melting of hydrous phases in the metasomatic veins and iv in the specific case of lihir unroofing of the volcanic edifice that led to boiling and rapid metal deposition this study shows that the location of the ladolam deposit on lihir is controlled by large scale structures that can be traced offshore and are the site of continuing submarine volcanism and epithermal style au mineralization the observed structural framework is dominated by the emergence of trans lithospheric faults that provided pathways for the melts to the seafloor near surface structural focusing of the ascending melts and fluids and a regional tectonic stress regime that stabilized the conditions over a significant period of time and or repeatedly marine seismic data confirms the complex structure of the tltf island chain each island group sits on tilted blocks that form horst structures separated by half grabens developed due to regional nw se directed extension regional compression perpendicular to the extension continues as a result of the transition from subduction to collision at the leading edge of the ontong java plateau the protracted transtensional motion between distinct crustal blocks controls the location and timing of magmatism and mineralization a kinematic link between volcanism at the location of lihir and the splitting of new ireland by ne directed propagation of seafloor spreading in the manus basin is suspected by combining onshore and offshore geology we propose a new model of the evolution of the new ireland basin magmatism along the tltf island chain and ultimately ore deposit formation this study demonstrates the importance of integrating offshore geology and geophysics into models that aim to explain the structural magmatic and sedimentary evolution of marginal basins that are host to economic mineral deposits

subducted slabs beneath the eastern indonesia tonga region insights from tomography tomographic images of mantle structure beneath the region north and northeast of australia show a number of anomalously fast regions these are interpreted using a recent plate tectonic reconstruction in terms of current and former subduction systems several strong anomalies are related to current subduction the inferred slab lengths and positions are consistent with neogene subduction beneath the new britain and halmahera arcs and at the tonga and the new hebrides trenches where there has been rapid rollback of subduction hinges since about 10 ma there are several deeper flat lying anomalies which are not related to present subduction and we interpret them as former subduction zones overridden by australia since 25ma beneath the bird s head and arafura sea is an anomaly interpreted to be due to north dipping subduction beneath the philippines halmahera arc between 45and 25ma a very large anomaly extending from the papuan peninsula to the new hebrides and from the solomon islands to the east australian margin is interpreted to be the remnant of south dipping subduction beneath the melanesian arc between 45and 25ma this interpretation implies that a flat lying slab can survive for many tens of millions of years at the bottom of the upper mantle in the lower mantle there is a huge anomaly beneath the gulf of carpentaria and east papua new guinea this is located above the position where the tectonic model interprets a change in polarity of subduction from north dipping to south dipping between 45and 25ma we suggest this deep anomaly may be a slab subducted beneath eastern australian during the cretaceous or subducted north of australia during the cenozoic before 45ma the tomography also supports the tectonic interpretation which suggests little neogene subduction beneath western new guinea since no slab is imaged south of the new guinea trench however one subduction zone in the tectonic model and many others that associated with the trobriand trough east of papua new guinea and the miocene maramuni arc is not seen in the tomographic images and may require reconsideration of currently accepted tectonic interpretations 4 2002 elsevier science b v all rights reserved

tholeiitic calc alkaline and alkaline igneous rocks of the shortland islands solomon islands abstract the shortland islands lie in a northeast southwest line across the western end of solomon islands immediately adjacent to bougainville three major islands dominate the group fauro and surrounding islands in the northeast have an altered basement suite comprising tholeiite icelandite and tholeiitic dacite this is intruded by a high level calc alkaline assemblage of microdiorite hornblende andesite and rhyodacite and overlain by volcanogenic sandstones derived from an andesitic to dacitic volcano pyroclastics comprising high alumina basalt and pyroxene andesite overlie the volcanogenic sandstones the tholeiitic basement lavas may be of late oligocene to early miocene age and the calc alkaline rocks are probably also pre pliocene in age alu in the centre of the group also has an altered tholeiitic lava basement which is intruded by a quartz diorite body and overlain by hypersthene augite basaltic andesite pliocene siltstone and quaternary shallow marine carbonates cover these igneous rocks over much of the island mono in the southwest has a small basement exposure of altered pillowed hawaiite overlain by miocene pelagic limestone pliocene siltstone and quaternary reef limestone isolated clasts of pyroxene andesite and benmoreite occur in streams and on beaches the younger calc alkaline suites on all islands were formed in an island arc environment possibly related to subduction from the southwest beneath the new britain trench the basement lavas on alu are probably early island arc tholeiites and both these lavas and the calc alkaline rocks of alu share a common trend on variation diagrams the two igneous suites of fauro however have distinctly different trends the basement lavas have some chemical similarities with oceanic tholeiites but an early island arc origin for these lavas cannot be ruled out the altered hawaiite and benmoreite on mono probably originated in an oceanic island environment

high sr y arc magmas and porphyry cu mo au deposits just add water porphyry cu mo au deposits are commonly associated with intermediate composition arc related igneous rocks with high sr y and la yb ratios and such rocks are equated by some researchers with melts derived from subducted oceanic crust undergoing eclogite facies metamorphism adakites however similar geochemical characteristics are readily developed in normal asthenospheric mantle wedge derived magmas by fractionation of amphibole and or garnet and or by interaction with crustal materials during ascent through the upper plate lithosphere while there is widespread evidence for such fractionation and contamination processes in porphyry magmas there is little direct evidence for an origin by slab melting the enhanced fertility of arc magmas relates primarily to their high water content which is required for the formation of magmatic hydrothermal systems upon shallow crustal emplacement magmatic water contents 4 wt percent h 2 o cause abundant amphibole garnet fractionation but suppression of plagioclase crystallization at deep crustal levels resulting in increasing sr y and la yb ratios with differentiation commonly into adakitic ranges but not reflecting slab melting additional factors that affect arc magma fertility are relatively high oxidation states and sulfur content but these are secondary to the requirement for sufficient water thus arc magmatic suites with high sr y ratios and evidence for the presence of hydrous phenocryst phases hornblende and or biotite are indeed prospective for porphyry cu mo au deposits but only because these parameters indicate high magmatic water content

accelerating late quaternary uplift of the new georgia island group solomon island arc in response to subduction of the recently active woodlark spreading center and coleman seamount abstract the new georgia island group of the solomon islands is one of four places where an active or recently active spreading ridge has subducted beneath an island arc we have used coral reef terraces paleobathymetry of neogene sedimentary rocks and existing marine geophysical data to constrain patterns of regional quaternary deformation related to subduction of the recently active woodlark spreading center and its overlying coleman seamount these combined data indicate the following vertical tectonic history for the central part of the new georgia island group 1 subsidence of the forearc region tetepare and rendova islands to water depths of 1500 m and deposition of marine turbidites until after 270 ka 2 late quaternary uplift of the forearc to sea level and erosion of an unconformity 3 subsidence of the forearc to 500 m bsl and deposition of bathyal sediments and 4 uplift of the forearc above sea level with holocene uplift rates up to at least 7 5 mm yr on tetepare and 5 mm yr on rendova in the northeastern part of the new georgia island group our combined data indicate a slightly different tectonic history characterized by lower amplitude vertical motions and a more recent change from subsidence to uplift barrier reefs formed around new georgia and vangunu islands as they subsided 300 m by 50 100 ka subsidence was replaced by uplift that accelerated to holocene rates of 1 mm yr on the volcanic arc compared with rates up to 7 5 mm yr in the forearc area of tetepare and rendova uplift mechanisms such as thermal effects due to subduction of spreading ridges tectonic erosion or underplating of deeply subducted bathymetric features are not likely to function on the 270 ka period that these uplift events have occurred in the new georgia island group a more likely uplift mechanism for the post 270 ka accelerating uplift of the forearc and volcanic arc of the new georgia island group is progressive impingement of the coleman seamount or other topographically prominent features on the subducting plate regional effects we relate to this ongoing subduction related process include 1 late quaternary post 270 ka accelerating uplift of the rendova tetepare forearc area in response to initial impingement of the coleman seamount followed by exponentially increasing collisional contact between the forearc and seamount 2 later quaternary propagation of uplift arcward to include the volcanic arc as the area of collisional contact between the forearc and seamount increased and 3 large wavelength folding that has produced regional variations in late holocene uplift rates observed in both forearc southern rendova tetepare and volcanic arc new georgia island areas we propose that the dominant tectonic effect of coleman seamount impingement is horizontal shortening of the forearc and arc crust that is produced by strong coupling between the subducting seamount and the unsedimented crystalline forearc of the new georgia island group the horizontal forces due to mechanical resistance to subducting rugged ridge and seamount topography may have terminated spreading of the woodlark spreading center entering the trench ghizo ridge and converted it to a presently active strike slip fault zone

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

arc continent collision in papua guinea constraints from fission track thermochronology the papua new guinea png mobile belt adjacent to the finisterre arc was formerly the leading ne corner of the australian plate that converged obliquely with the pacific plate forty new apatite and zircon fission track analyses of mobile belt rocks previously dated by k ar and rb sr analyses constrain neogene time temperature paths and tectonic models the paleogene arc along the southern margin of the caroline plate was juxtaposed against png in the early miocene coeval with locking up of the west dipping solomon subduction zone by the ontong java plateau these events initiated wrenching along the northern png margin and increased westward subduction of the solomon sea plate beneath the eastern margin the mobile belt underwent extension above the downgoing slab with rapid cooling of metamorphic rocks at 17 ma immediately prior to emplacement of the maramuni arc from 17 to 12 ma a change in plate motion at 12 10 ma terminated the arc and caused png caroline plate convergence creating the orogenic belt in new guinea from 12 to 4 ma this resulted in 4 5 km of uplift and 3 km of denudation and cooling of the entire mobile belt in the late miocene propagating westward along the mobile belt at 8 5 ma and southward into the fold belt at 5 4 ma the compression caused thrusting of miocene strata within the mesozoic type section a further change in plate motion at 4 3 ma returned the margin to transpression with local compression along strike slip faults and ongoing collision of the finisterre arc terrane

the early middle miocene subduction complex of the louisiade archipelago southern margin of the woodlark rift field microstructural and 40ar 39ar thermochronologic data from the louisiade archipelago the southern rifted margin of the woodlark basin in se papua new guinea document an accretionary wedge that formed during early middle miocene n dipping subduction of the australian margin and transpression along the australian pacific plate boundary metasedimentary rocks of the calvados schist and the metagabbros that intrude them were metamorphosed at up to greenschist facies conditions three tectonic foliations s1 s3 are present and f1 f3 fold hinges plunge ese or wnw parallel to mineral stretching and intersection lineations fold vergence is dominantly to the sw and top to the sw thrusting of ultramafic rocks over the calvados schist is documented locally on rossel island the data suggest progressive deformation associated with nne ssw shortening and ese wnw extension via dissolution precipitation creep and more locally dislocation creep 40ar 39ar step heating analyses of three white mica separates yield middle miocene plateau or plateau like segments that are affected by variable pliocene argon loss and are interpreted as syntectonic mica growth during metamorphism and deformation followed by partial resetting just prior to the onset of seafloor spreading in the woodlark basin a 12 ma 40ar 39ar age from a dacite sill of the panarora volcanics provides a minimum age constraint for the termination of northward subduction and a maximum age for crosscutting brittle strike slip faults these data are critical to constraining the subduction exhumation history of the world s youngest high ultrahigh pressure terrane and further support analogies between se papua new guinea and the early oligocene western alps

tectonic environments of south american porphyry copper magmatism through time revealed by spatiotemporal data mining porphyry ore deposits are known to be associated with arc magmatism on the overriding plate at subduction zones while general mechanisms for driving magmatism are well established specific subduction related parameters linking episodes of ore deposit formation to specific tectonic environments have only been qualitatively inferred and have not been formally tested we develop a four dimensional approach to reconstruct age dated ore deposits with the aim of isolating the tectono magmatic parameters leading to the formation of copper deposits during subduction we use a plate tectonic model with continuously closing plate boundaries combined with reconstructions of the spatio temporal distribution of the ocean floor including subducted portions of the nazca farallon plates the models compute convergence rates and directions as well as the age of the downgoing plate through time to identify and quantify tectonic parameters that are robust predictors of andean porphyry copper magmatism and ore deposit formation we test two alternative supervised machine learning methods the random forest rf ensemble and support vector machines svm we find that a combination of rapid convergence rates 100 km myr subduction obliquity of 15 a subducting plate age between 25 70 myr old and a location far from the subducting trench boundary 2000 km represent favorable conditions for porphyry magmatism and related ore deposits to occur these parameters are linked to the availability of oceanic sediments the changing small scale convection around the subduction zone and the availability of the partial melt in the mantle wedge when coupled these parameters could influence the genesis and exhumation of porphyry copper deposits

the role of preexisting geologic architecture in the formation of giant porphyry related cu au deposits examples from new guinea and chile development of giant porphyry related copper and or gold deposits in new guinea and northern and central chile occurred during tertiary magmatic events that overprinted earlier extensional tectonic settings the earlier tectonic settings consisted of a mesozoic passive margin in new guinea a jurassic cretaceous back arc basin in northern chile and an oligocene intra arc basin in central chile the preexisting architecture of the basement rocks the fault systems and the stratigraphic packages associated with these settings played a strong role in controlling development of the giant deposits although the extensional tectonic settings in these terranes were different they share a number of elements that were used during formation of the tertiary ore deposits these were the presence of coupled systems of deeply detached listric synsedimentary faults and steep transverse faults and the presence of a relatively flat lying heterogeneous volcano sedimentary rock package during tertiary collision the deeply detached listric faults were inverted and strongly focused uplift exhumation and associated overpressuring induced failure and fluid flow in addition steep transverse faults were activated to form wrench systems with attendant steep deeply tapping pathways for magma and or fluid associated with dilatant jogs flexures or fault intersections ore deposits are commonly located in the hanging wall of the thrust faults under compression associated with collision the competent units of flat lying stratigraphic packages formed plates overlying the folded weaker units underneath and separated by a detachment or otherwise faulted contact examples of the competent upper units include the darai mendi limestone or equivalents in new guinea and the lavas of the farellones formation in central chile these plates appear to have impeded magma ascent and formed a cap on a magma and or fluid system pressurized by collision and orogenesis this geometry provided an ideal location for focusing of magmas and magmatic hydrothermal fluids from these three example terranes a set of exploration indicators have been constructed to highlight belts that are prospective for giant porphyry cu systems these indicators include 1 the presence of magmatic arcs that migrated into preexisting extensional architectures 2 coupled systems of deeply detached listric synsedimentary faults and steep transverse faults 3 rigid basement blocks that could form regional stress field perturbations and 4 fold belts with a major plate to 50 km wide of competent relatively undeformed or gently folded stratigraphy overlying more complexly folded and faulted parts of the sequence

cenozoic tectonic and depth age evolution of the indonesian gateway and associated back arc basins abstract we reconstruct the tectonic and paleo age and depth history of the indonesian seaway and associated southeast asian back arc basins to create a comprehensive paleogeographic framework for the region during the cenozoic we combine published tectonic models with revised models of the evolution of back arc basins based on available marine magnetic anomaly data and a recently published high resolution 1 min marine gravity anomaly grid which reveals many fine scale tectonic features previously not resolved reconstructions of the now subducted ceno tethys ocean floor are integrated with a revised absolute plate motion reference frame for the indian ocean based on moving hotspots all marginal seas north of australia formed in a back arc setting with the caroline and celebes seas opening north of a northward dipping subduction zone and the solomon sea south of a southward dipping subduction zone north of australia we suggest that the evolution of the two sub basins of the caroline sea from 36 to 25 ma followed different seafloor spreading patterns with the western sub basin being influenced by the manus hotspot our model suggests that collision between the melanesian and the caroline arcs led to cessation of oceanic crustal production in the caroline basin shortly before 25 ma combined geophysical and geological observations suggest that several major tectonic events occurred north of australia at around 45 ma roughly at the time when australia started moving northwards at fast rates similar to the present we suggest that this event is related to a relocation of the subduction zone nw of australia under the philippine sea plate due to a collision and subsequent accretion of old pacific plate material to the northward subducting australian plate our gridded oceanic paleo depth maps capture the history of marginal seas north of australia and the stepwise destruction of the ceno tethys ocean floor in the northern wharton basin and the west pacific providing boundary conditions for both paleoceanograpic and geodynamic models a regional analysis of basement depth anomalies reveals that median negative anomalous depth of several back arc basins is of the order of 650 800 m with a range of 300 1100 m accompanied by negative regional heatflow anomalies suggesting that mantle driven dynamic topography plays a larger role in modulating regional basement depth than previously suggested regional shear wave mantle tomography cross sections highlight that all marginal basins that exhibit large negative dynamic topography are underlain by massive buried slab material in the lower portion of the upper mantle and or below the transition zone supporting the notion that the observed paired negative dynamic topography and heatflow anomalies are due to basin formation above slab burial grounds

adakite like volcanism of ecuador lower crust magmatic evolution and recycling in the northern andes of ecuador a broad quaternary volcanic arc with significant across arc geochemical changes sits upon continental crust consisting of accreted oceanic and continental terranes quaternary volcanic centers occur from west to east along the western cordillera frontal arc in the inter andean depression and along the eastern cordillera main arc and in the sub andean zone back arc the adakite like signatures of the frontal and main arc volcanoes have been interpreted either as the result of slab melting plus subsequent slab melt mantle interactions or of lower crustal melting fractional crystallization and assimilation processes in this paper we present petrographic geochemical and isotopic sr nd pb data on dominantly andesitic to dacitic volcanic rocks as well as crustal xenolith and cumulate samples from five volcanic centers pululagua pichincha ilalo chacana sumaco forming a nw se transect at about 0 latitude and encompassing the frontal pululagua pichincha main ilalo chacana and back arc sumaco chains all rocks display typical subduction related geochemical signatures such as nb and ta negative anomalies and lile enrichment they show a relative depletion of fluid mobile elements and a general increase in incompatible elements from the front to the back arc suggesting derivation from progressively lower degrees of partial melting of the mantle wedge induced by decreasing amounts of fluids released from the slab we observe widespread petrographic evidence of interaction of primary melts with mafic xenoliths as well as with clinopyroxene and or amphibole bearing cumulates and of magma mixing at all frontal and main arc volcanic centers within each volcanic center rocks display correlations between evolution indices and radiogenic isotopes although absolute variations of radiogenic isotopes are small and their values are overall rather primitive e g end 1 5 to 6 87sr 86sr 0 7040 0 70435 rare earth element patterns are characterized by variably fractionated light to heavy ree la ybn 5 7 34 and by the absence of eu negative anomalies suggesting evolution of these rocks with limited plagioclase fractionation we interpret the petrographic geochemical and isotopic data as indicating open system evolution at all volcanic centers characterized by fractional crystallization and magma mixing processes at different lower to mid crustal levels as well as by assimilation of mafic lower crust and or its partial melts thus we propose that the adakite like signatures of ecuadorian rocks e g high sr y and la yb values are primarily the result of lower to mid crustal processing of mantle derived melts rather than of slab melts and slab melt mantle interactions the isotopic signatures of the least evolved adakite like rocks of the active and recent volcanoes are the same as those of tertiary normal calc alkaline magmatic rocks of ecuador suggesting that the source of the magma did not change through time what changed was the depth of magmatic evolution probably as a consequence of increased compression induced by the stronger coupling between the subducting and overriding plates associated with subduction of the aseismic carnegie ridge

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

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