measuring the sea ice floe size distribution sea ice is broken into floes whose diameters range from meters to a hundred kilometers this fragmentation affects the resistance of the ice cover to deformation and the melting at floe sidewalls in summer floes are broken by waves and swell near the ice edge and throughout the pack by isostatic imbalances thermal cracking winds and currents in winter they are welded together by freezing floe size can be measured by several properties p for instance area or mean caliper diameter two definitions of floe size distribution seem useful f p the fraction of area covered by floes no smaller than p and n p the number of floes per unit area no smaller than p f p can be measured by sets other than areas such as the fraction of a line or of a point set covered by floes no smaller than p if n behaves like for small where is mean caliper diameter must be greater than 2 so that the small floes occupy finite area if 2 1 the perimeter of small floes is infinite several summertime distributions have been measured on a log log graph their slopes local values of range from 1 7 to 2 5 one distribution follows a power law the others have steeper slopes for larger floes and more gradual slopes for smaller floes another sampling strategy is to measure the lengths of line segments on floes the distribution of these chord lengths is equivalent to the distribution of floe diameters the variance of an estimate of the fraction g of area covered by floes in any size range is g 1 g k 1 where k is the equivalent number of independent samples k can be found from the autocovariance of the indicator function for the chosen size range for line sampling of a narrow range of floe diameters k is the ratio of the sample length to the floe diameter

present day crustal motion in the solomon islands from gps observations site velocities in the solomon islands from global positioning system measurements spanning two years provide direct evidence of active deformation between the pacic plate and the solomon arc block convergence is occurring at the san cristobal trench at a rate of524 mm yr with no apparent local deformation occurring in the australian plate at a distance of100 km from the trench the islands of guadalcanal and makira are in a rst ap proximation moving with the pacic plate although there is evidence of small but signicant decoupling from the pacic plate of 14 23 mm yr in a direction of 75 85

opal accumulation rates in the equatorial pacific and mechanisms of deglaciation 1 a possible imprint on equatorial pacific sediments of a deglacial reinvigoration of the southern ocean overturning is increased opal accumulation rate this would arise from the transmission of silica rich deep water to the equatorial thermocline via subantarctic mode water and an associated increase in diatom productivity in search of this imprint sediment cores from the central tt013 pc72 and eastern v19 30 equatorial pacific have been analyzed for 230th normalized opal accumulation rates over the past five and three glacial terminations respectively equatorial opal accumulation rates sustained relatively low values over much of the records and were punctuated by large increases centered on some terminations but not all furthermore two periods of increased opal flux were observed that do not coincide with terminations sources other than the southern ocean may need to be considered in the silica budget of the equatorial pacific but the 13c of neogloboquadrina dutertrei can be used to support the presence of a deepwater nutrient signal in each case although a common deglacial mechanism or a common imprint thereof for each of the late pleistocene glaciations remains elusive the combination of opal flux and 13c of n dutertrei provides a diagnostic for past injection of deepwater nutrients into the equatorial undercurrent

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

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

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

polarity reversal in the solomon islands arc mckenzie1 proposed that the direction of subduction beneath an island arc will reverse following arc continent collision dewey and bird2 suggested that such an arc polarity reversal occurred in northern new guinea johnson and molnar3 however proposed two alternative models of plate convergence that could explain the observed seismicity we present here a spatial seismicity study of the solomon islands region which has revealed the existence of two juxtaposed wadati benioff w b zones of opposite polarity shallow and intermediate foci define a north east dipping w b zone associated with active subduction along the new britain and san cristobal trenches and a south west dipping w b zone associated with the inactive north solomon trench nst these data provide the first direct seismic evidence of a reversal in subduction polarity at an island arc

late miocene pacific plate kinematic change explained with coupled global models of mantle and lithosphere dynamics the timing and magnitude of a pacific plate motion change within the past 10 ma remains enigmatic due to the noise associated with finite rotation data nonetheless it has been hypothesized that this change was driven by the arrival of the ontong java plateau ojp at the melanesian arc and the consequent subduction polarity reversal the uncertainties associated with the timing of this event however makes it difficult to quantitatively demonstrate a dynamical association here we first reconstruct the pacific plate s absolute motion since the mid miocene 15 ma at high temporal resolution building on previous efforts to mitigate the impact of finite rotation data noise we find that the largest change in pacific plate motion direction occurred between 10 and 5 ma with the plate rotating clockwise we subsequently develop and use coupled global numerical models of the mantle lithosphere system to test hypotheses on the dynamics driving this change these indicate that the arrival of the ojp at the melanesian arc between 10 and 5 ma followed by a subduction polarity reversal that marked the initiation of subduction of the australian plate underneath the pacific realm were the key drivers of this kinematic change

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

the eruptive history and volcanic hazards of savo solomon islands savo island is the 6 km diameter emergent summit of an andesitic dacitic stratovolcano rising from the iron bottom sound 35 km nw of honiara solomon islands savo has erupted at least three times within recorded history and the 3 000 inhabitants maintain extensive oral traditions of past events through description and interpretation of the volcaniclastic sequences on the island in conjunction with historical accounts and oral traditions we reconstruct the eruptive processes on savo block and ash flow baf deposits are volumetrically dominant on the island within three main depositional environments near vent sequences thick medial channel sequences and distal fan sequences the deposits comprise universally non vesicular and highly porphyritic 40 70 phenocrysts high silica andesite and dacite clasts these appear to have been derived from collapsing lava domes during an 1560 1570 a d eruption however eyewitness descriptions and crater morphology suggest that similar deposits formed from dome explosions or collapses of eruption columns during later eruptions 1830 1840 a d the high sodium magmas ca 5 7 wt na2o apparently crystallised and strongly degassed prior to eruption shallow explosions were possibly caused by entrapment of magmatic gases beneath a dome or conduit plug of highly crystalline near solid magma repeated sealing of the vent may have been due to inward collapse of the highly altered rocks of the surrounding hydrothermal system these rocks probably were saturated due to contemporaneous high intensity rainfall events bafs were hot enough to char vegetation and attain aligned clast trm thermal remnant magnetism up to 3 km from the vent many being accompanied by ash cloud surges changes with distance in the baf deposits appear mostly dependent on flow confinement and are limited to an overall decrease in thickness and maximum clast size and an increased definition of weak planar fabrics in distal fan sequences there is strong evidence for syn and post eruptive redeposition of primary deposits since the savo population is concentrated on coastal volcaniclastic fans we consider the greatest volcanic risk to life is from bafs associated ash cloud surges and lahars hence the main channels and fans are designated as the highest of three relative hazard zones on a simple map prepared to aid local education and planning initiatives on savo

blueschists eclogites and subduction zone tectonics insights from a review of late miocene blueschists and eclogites and related young high pressure metamorphic rocks abstract in this paper late miocene blueschist and eclogite belts including the world s youngest blueschist belt in timor tanimbar island chain eastern indonesia and the world s youngest coesite bearing eclogite papua new guinea together with selected cenozoic high pressure and ultrahigh pressure metamorphic rocks are reviewed from a synthesis of the geology metamorphism and chronology of these rocks the formation and exhumation process are evaluated and the significance on tectonics at convergent plate boundaries is discussed

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

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

sequence stratigraphy structure and tectonic history of the southwestern ontong java plateau adjacent to the north solomon trench and solomon islands arc the ontong java plateau ojp is the largest and thickest oceanic plateau on earth and one of the few oceanic plateaus actively converging on an island arc we present velocity determinations and geologic interpretation of 2000 km of two dimensional multi channel seismic data from the southwestern ontong java plateau north solomon trench and northern solomon islands we recognize three megasequences ranging in age from early cretaceous to quaternary on the basis of distinct interval velocities and seismic stratigraphic facies megasequence oj1 is early cretaceous upper igneous crust of the ojp and correlates with basalt outcrops dated at 122 125 ma on the island of malaita the top of the overlying megasequence oj2 a late cretaceous mudstone unit had been identified by previous workers as the top of igneous basement seismic facies and correlation to distant deep sea drilling project ocean drilling program sites indicate that oj2 was deposited in a moderately low energy marine environment near a fluctuating carbonate compensation depth that resulted in multiple periods of dissolution oj2 thins south of the stewart arch onto the solomon islands where it is correlated with the kwaraae mudstone formation megasequence oj3 is late cretaceous through quaternary pelagic cover which caps the ontong java plateau it thickens into the north solomon trench and seismic facies suggest that oj3 was deposited in a low energy marine environment we use seismic facies analysis sediment thickness structural observations and quantitative plate reconstructions of the position of the ojp and solomon islands to propose a tectonic magmatic and sedimentary history of the southwestern ontong java plateau prior to 125 ma late jurassic and early cretaceous oceanic crust formed from 125 to 122 ma the first mantle plume formed igneous crust oj1 between 122 and 92 ma marine mudstone oj2 and kwaraae mudstone of malaita solomon islands was deposited on ontong java plateau at 92 ma a second mantle plume caused widespread volcanism on the plateau from 92 to 15 ma pelagic carbonate sediment oj3 was deposited at 15 ma the southern ontong java plateau was deformed by normal faults during its approach toward the north solomon trench finally from 4 to 0 ma the malaita accretionary prism formed during collision between a substantially thicker portion of the ontong java plateau and the solomon islands arc flexure of the ontong java plateau near the trench caused coeval normal faulting

switching between alternative responses of the lithosphere to continental collision summary we study possible responses to arc continent or continent continent collision using numerical models our short term integration models show that the initial stage of deformation following continental collision is governed by the competition between three potential weakness zones 1 mantle wedge 2 plate interface and 3 lower continental crust depending on which of these is the weakest zone in the system three different responses can be recognized 1 subduction polarity reversal 2 continuation of subduction and 3 delamination and back stepping subduction polarity reversal occurs if the mantle wedge is the weakest zone in the system this happens only if the viscosity of the mantle wedge is at least one order of magnitudelowerthantheaverageviscosityofthelithosphere incontinent continentcollision one additional condition needs to be satisfied for subduction polarity reversal to occur for the subducting lithosphere the ratio of the viscosity of the lower continental crust to the viscosity of the upper lithospheric mantle must be equal to or higher than 0 006 the time required for polarity reversal depends on several parameters the convergence rate the sinking velocity of the detached slab and the relative strength of the mantle wedge arc and backarc the response to collision is continued subduction if the plate interface is the weakest zone and is delamination and back stepping if the lower continental crust is the weakest area in the system our finding that a low viscosity wedge is a prerequisite for a reversal of subduction polarity agrees with inferences about regions for which subduction polarity reversal has been proposed

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

growth of the maritime continent and its possible contribution to recurring ice ages the areal extent of the maritime continent the islands of indonesia and surrounding region has grown larger by 60 since 5 ma we argue that this growth might have altered global climate in two ways that would have contributed to making recurring ice ages possible first because rainfall over the islands of the maritime continent not only is heavier than that over the adjacent ocean but also correlates with the strength of the walker circulation the growth of the maritime continent since 5 ma may have contributed to the cooling of the eastern tropical pacific since that time scaling relationships between the strength of the walker circulation and rainfall over the islands of the maritime continent and between sea surface temperature sst of the eastern tropical pacific and the strength of easterly wind stress suggest that the increase in areal extent of islands would lead to a drop in that sst of 0 75 c although only a fraction of the 3 4 c decrease in ssts between the eastern and western tropical pacific the growth of the maritime continent may have strengthened the walker circulation increased the east west temperature gradient across the pacific and thereby enabled ice sheets to wax and wane over canada since 3 ma second because the weathering of basaltic rock under warm moist conditions extracts co2 from the atmosphere more rapidly than weathering of other rock or of basalt under cooler or drier conditions the increase in weathering due to increasing area of basalt in the maritime continent may have drawn down enough co2 from the atmosphere to affect global temperatures simple calculations suggest that increased weathering of basalt might have lowered global temperatures by 0 25 c possibly important for the overall cooling

arctic sea ice leads from advanced very high resolution radiometer images a large number of advanced very high resolution radiometer avhrr images from throughout 1989 are analyzed to determine lead characteristics the units of analysis are square 200 km cells and there are 270 such cells in the data set clouds are masked manually leads are determined from images of the potential open water a scaled version of the surface temperature or albedo that weights thin ice by its thermal or brightness impact the lead fraction is determined as the mean the monthly mean lead fraction ranges from 0 02 in winter to 0 06 in summer in the central arctic and is near 0 08 in the winter in the peripheral seas a method of accounting for lead width sampling errors due to the finite sample areas is introduced in the central arctic the observed mean lead width for a threshold of 0 1 ranges from 2 or 3 km near the resolution of the instrument in the winter to 6 km in the summer in the peripheral seas it is about 5 km in the winter width distributions are often more heavily weighted in the tail than exponential distributions and are well approximated by a power law the along track number density power law n aw b has a mean exponent of b 1 60 standard deviation 0 18 and shows some seasonal variability mean floe widths in the central arctic are 40 to 50 km in the winter dropping to about 10 km in the summer for floes the power law has a mean exponent of 0 93 and exhibits a clearer annual cycle lead orientation is determined with a method based on the direction of maximum extent

methods for digital analysis of avhrr sea ice images algorithms for obtaining information on sea ice conditions from advanced very high resolution radiometer avhrr digital imagery are presented these algorithms comprise three steps discrimination of cloud covered areas from ice covered and open water areas estimation of ice concentration and measurement of the sizes of ice floes and open water areas polynyas within the ice field for ice water cloud discrimination a dynamic threshold technique is applied to six combinations of the five spectral channels of the avhrr this results in an image that is useful for ice analyses to derive ice concentration estimates a simple linear relationship between pixel intensity values and amount of ice cover is applied to the visible channel reflectance data the method for obtaining size frequency distributions is based on a size transform of binary realizations of the ice concentration result and is equally applicable to measuring polynyas and ice floes

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

determining the floe size distribution of east antarctic sea ice from digital aerial photographs floe size within the antarctic pack ice is an important parameter that affects both the ocean ice atmosphere energy exchange and the mechanical properties of pack ice in this paper we present a computer based algorithm to extract floe size distribution information from digital aerial photographs of antarctic sea ice the algorithm segments digital images of sea ice into distinct floes from which the size of each floe is calculated and floe size distributions are derived through a recursive application of the morphological erosion operator each floe is eroded towards its centre while each pixel s erosion number is recorded this ensures no information is lost while individual floes are identified the algorithm combines image processing techniques with some manual input to accurately identify the boundaries of individual floes within an image to demonstrate the functionality of the algorithm six images representing regions along a 22 km south to north transect through a transitional zone of pack ice near 65 s 140 e have been processed regional variations in the floe size distributions show an increase in the number of smaller floes relative to larger floes and a decrease in the dominant floe size in terms of areal coverage from south to north these results are consistent with ship based observations

tracking the australian plate motion through the cenozoic constraints from 40ar 39ar geochronology here we use geochronology of australian intraplate volcanoes to construct a high resolution plate velocity record and to explore how tectonic events in the southwest pacific may have influenced plate motion nine samples from five volcanoes yield ages from 33 6 0 5 to 27 3 0 4 ma and when combined with published ages from 30 to 16 ma show that the rate of volcanic migration was not constant instead the results indicate distinct changes in australian plate motion fast northward velocities 61 8 and 57 4 km ma prevailed from 34 to 30 0 5 and from 23 to 16 0 5 ma respectively with distinct reductions to 20 10 and 22 5 km ma from 30 to 29 0 5 ma and from 26 to 23 0 5 ma these velocity reductions are concurrent with tectonic collisions in new guinea and ontong java respectively interspersed between the periods of sluggish motion is a brief 29 26 0 5 ma burst of atypically fast northward plate movement of 100 20 km ma we evaluate potential mechanisms for this atypically fast velocity including catastrophic slab penetration into the lower mantle thermomechanical erosion of the lithosphere and plume push forces none are appropriate this period of fast motion was however coincident with a major southward propagating slab tear that developed along the northeastern plate margin following partial jamming of subduction and ophiolite obduction in new caledonia although it is unclear whether such an event can play a role in driving fast plate motion numerical or analogue models may help address this question key points we determine nine 40ar 39ar ages from five cenozoic volcanoes in australia slow velocities correlate with new guinea and ontong java collisions anomalously fast velocity of 100 20 km ma is identified from 29 26 ma

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