late paleozoic paleomagnetism of south mongolia exploring relationships between siberia mongolia and north china abstract the tectonic evolution of a large region that includes south siberia mongolia and north china is still a matter of fierce debate this is due at least in part to the contentious proposals detailing the kinematics of the major tectonic units in that area one solution to the problem is the acquisition of better defined paleomagnetic data for the various players in the system we carried out a paleomagnetic study of early carboniferous subduction related volcanic rocks of the trans altai zone in south mongolia and successfully isolated a pre folding and presumably primary component from 22 sites a critical analysis of late paleozoic paleomagnetic data from mongolia revealed several problematic data sets while the paleolatitudes derived from more reliable studies agreed well with the expected values for siberia and were consistently different from those for the north china block we also found that late paleozoic declinations from mongolia are rotated counterclockwise through various angles with respect to siberia in addition those rotations are widespread in both inner mongolia and the north china block in this paper we put forward some testable hypotheses and propose a preliminary paleogeographic scheme of the region 1 consistent difference in late paleozoic paleolatitudes between siberia and mongolia on the one hand and the north china block on the other is largely due to strong inclination shallowing in paleomagnetic data for the latter area where most data are from sedimentary rocks 2 widespread counterclockwise rotations in mongolia inner mongolia and north china may be accounted for by a wide zone of sinistral transpression that was active in the early mesozoic 3 in the early mesozoic the mongol okhotsk ocean was much narrower than is usually assumed 4 there was no large scale oroclinal bending between siberia and north china in the permian early mesozoic

rheological stratification of the lithosphere abstract the rheology of the lithosphere is estimated on the basis of pore pressure dependent frictional failure in the brittle regime and power law steady state creep in the ductile regime different petrological models of the continental and oceanic lithosphere combined with different geotherms and thicknesses are used to generate models covering a variety of geodynamic situations results show that the depth dependence of lithospheric rheology varies with the tectonic province and that in many instances the continental lithosphere has one at the bottom of the crust or two the previous one plus another at mid crustal levels soft ductile layers sandwiched between brittle layers the depth distribution of geophysical parameters seismicity seismic wave velocity and electrical conductivity matches the theological predictions well the lithospheric rheological profiles are used to analyze observed variations in the structural style of deformed continental margin and oceanic terranes in the southeastern canadian cordillera the large scale characteristics of the system show a satisfactory agreement with the inferred rheological structure sub horizontal decollements should be the rule rather than the exception where the lithospheric rheology is strongly stratified

the marinduque intra arc basin philippines basin genesis and in situ ophiolite development in a strike slip setting the marinduque basin is a marine intra arc basin in the north central philippine volcanic arc system a suite of marine geophysical observations show the basin as a whole to be rhombic in shape with its long axis trending north northwest a conspicuous volcanic ridge trends east northeast across the basin dividing it into two smaller active depocenters symmetric linear magnetic anomalies striking parallel to and centered over the central volcanic ridge indicate that it formed by extension in a north south direction by a process analogous to sea floor spreading geometric and temporal relations between the central volcanic ridge and several en echelon sets of north trending left stepping faults indicate that sea floor spreading occurred in the extensional stepover zone of a left lateral strike slip fault system and that the marinduque basin as a whole is a composite pull apart basin whose floor is in part composed of oceanic type crust the evolution of the central volcanic ridge presents an actualistic model for in situ development and emplacement of ophiolites in an island arc setting the presently active left lateral philippine fault zone truncated the eastern flank of the basin as well as the central volcanic ridge following their initial formation shortening associated with a probable restraining bend in the philippine fault has resulted in uplift of basin flank sediments which have been thrust back toward the basin interior the philippine fault has also displaced fragments of the ridge and basin floor toward the north the overall history of the marinduque basin further suggests that strike slip processes may play an important role in the origin and growth of intra arc basins such basins may ultimately become isolated in a forearc or backarc setting

a geophysical study of the manila trench luzon philippines 2 fore arc basin structural and stratigraphic evolution the pattern of sediment distribution within the manila trench subduction zone west of luzon exerts a strong influence on the tectonic evolution of the accretionary prism and the fore arc basins of this convergent plate margin multichannel common depth point seismic reflection data reveal up to 2600 m of turbidite sediments overlying a thin pelagic hemipelagic sediment layer and oceanic basement in the northern part of the manila trench these turbidites were transported from the uplifted collision zone of taiwan southward along the manila trench by gravity controlled processes similarly thinner turbidite deposits in the southern end of the trench were transported northward from the collision zone involving the north palawan block and the central philippines sedimentation within the manila trench is dominated by sources at the ends of the trench with longitudinal transport along the trench axis in contrast the fore arc basin system of the manila trench receives sediment through submarine canyons incised into the west luzon continental slope the rate of sediment supply to the fore arc basin system is not directly related to the rate of sediment supply to the trench the size of the accretionary prism varies significantly along strike of the trench and it strongly correlated with the local thickness of turbidite sediments within the trench a thickness of more than 200 m of trench turbidites is probably required before accretion and uplift become the dominant process affecting the fore arc region this generalization may apply to other convergent plate margins as well the uplift rate of the accretionary prism probably varies dramatically along the trench and is largely independent of variations in the local sediment accumulation rates in the fore arc basins this variability results in the formation of several different types of fore arc morphology along the manila trench the overall seismic stratigraphy of the west luzon trough fore arc basin is remarkably uniform with seismic sequences continuous for up to 110 km along strike the unconformities that bound these seismic sequences are probably the result of a complex interplay of relatively long term approximately 10 ma uplift of the accretionary prism and relatively short term approximately 1 2 ma variations in sediment supply to the fore arc basin modulated by eustatic sea level changes

fluid pressure and effective stress in sandbox models abstract we have done a series of simple tests on sandpacks involving upward flow of compressed air through the pores and its effect on the yield strength the ultimate objective is to model deformation coupled with fluid flow in sedimentary basins for all tests we used a single batch of fontainebleau sand sieved to a grain size between 0 200 and 0 315 mm poured into a cylindrical container and then fluidized the density of this sand was 1 585 g cm3 irrespective of sand thickness the lithostatic pressure was proportional to the thickness of the sandpack a yield envelope was obtained by shearing the sandpack horizontally compressed air entered the base of the sandpack and flowed upwards through the pore spaces for 69 measurements on air flow without shearing a plot of discharge velocity versus gradient of fluid pressure is close to a straight line verifying darcy s law and yielding an intrinsic permeability of about 1 7 darcy for the sand for 72 tests on simultaneous shearing and fluid flow the estimated effective stress lithostatic stress minus estimated pore fluid pressure ranged from 0 to 1600 pa and the pore fluid ratio between air pressure and lithostatic pressure from 0 0 to 1 0 a plot of shear stress versus effective stress at failure is almost linear verifying terzaghi s principle of effective stress the best fit slope coefficient of internal friction is about 0 55 and the intercept on the shear stress axis cohesion is less than 85 pa the tests show that it is feasible to use compressed air within sandpacks as a means of modelling deformation coupled with fluid flow the next step will be to build sandbox models of layered sequences and to investigate detachments caused by abnormal fluid pressures

magnetic model of the subduction zone in the northeast japan arc abstract magnetic modeling of the forearc side of the tohoku northeastern japan arc was carried out using spectral analysis of shipborne magnetic data the original data show anomalies caused by volcanic rocks on the western shallow side and linear marine magnetic anomalies on the eastern deeper side throughout the forearc region as calculated from temperature gradient data of heat flow measurements and deep drilling the curie depth is deeper than the moho on the eastern deeper side this suggests that the moho may correspond to the base of the magnetic layer because analysis of upper mantle xenolith suites indicates that the upper mantle should be non magnetic in addition the conrad discontinuity may correspond to the apparent top of magnetic layer because the lower crustal layer would be more mafic than the upper layer it was estimated by 2d spectral analysis that the magnetic base and surface lie at about 20 km and 10 km respectively this suggests that the moho and the conrad may roughly correspond to the magnetic base and surface the magnetic anomalies in the pacific basin are traced towards and over the inner side of the trench and show a gradual decrease in amplitude disappearing eventually about 100 km west of the trench this reflects the descent of the oceanic slab the 1d spectral analysis indicates that the marine magnetic layer is descending at an angle of about 4 in the japan trench this angle increasing to 20 degrees below the coast where the marine magnetic layer reaches a depth of more than 30 km this agrees with the seismic reflection data from the trench and because the magnetic layer can be traced down to a depth of 30 km it may be suggested that the curie isotherm deepens to more than 30 km just east of the coast

terrane amalgamation in the philippine sea margin abstract the philippine sea plate includes plateaus of thickened crust interpreted as imbricated ophiolite and arc related terranes of late mesozoic early tertiary age separated by thinner oceanic crust the arrival of plateaus at the subducting southwest margin of the philippine sea plate has caused the philippine trench to propagate southward in increments and caused transfer of terranes to the philippine margin new data from the halmahera region indicate that the position nature and evolution of plate boundaries have been strongly influenced by the heterogeneous character of the philippine sea plate at present the philippine trench terminates at an oceanic plateau which is structurally continuous with an old forearc and ophiolite terrane on halmahera the position of this terrane has caused philippine sea plate eurasia convergence to be transferred from subduction at the philippine trench to the molucca sea collision zone through a broad ne sw dextral transpressional zone across halmahera this plate boundary configuration is unstable and requires the future development of a new subduction zone to the east of halmahera which will result in amalgamation of the halmahera ophiolite terrane to the philippine margin in the halmahera region amalgamation of terranes to the evolving philippine microcontinent is currently in process

structural elements of the sulu sea philippines the structure and tectonic history of the sulu sea are described on the basis of seismic reflection data combined with the findings of onshore and offshore geological studies and the results of odp leg 124 drilling closing of a hypothetical mesozoic proto south china sea associated with the formation of oceanic crustal splinters in the late eocene followed by southward subduction and in turn progressive collision of the north palawan continental terrane with the micro continental borneo plate since the middle miocene resulted in the formation of the structurally complex sulu borneo collision belt the latter comprises north sabah southern and central palawan and the northwest sulu basin fracturing of the borneo micro continental plate into the sulu and cagayan ridges initiated the opening of the southeast sulu basin during the late oligocene through the early miocene collision of the north palawan continental terrane with cagayan ridge in the late early miocene and oblique collision of these blocks with the central philippines resulted in the still ongoing closing of the southeast sulu basin since the middle or late miocene closing of the southeast sulu basin began with the formation of an oceanic crustal slab

physical modelling of deformation in the tasman orogenic zone abstract structural vergence within the western subprovince of the lachlan fold belt is towards the hinterland rather than the foreland in contrast to many well known orogenic belts high angle reverse faults and upright folds verge eastwards away from the australian craton towards the inferred centre of orogenic and magmatic activity we designed a series of analogue models to test the anomalous vergence in the western lachlan fold belt particularly the interaction of a stable australian craton with tasman line geometry interacting with weaker oceanic or transitional lithospheric material we found consistently that vergence direction in the models was towards the hinterland not the foreland as in the western lachlan fold belt irrespective of the way the model was deformed strength gradients between the oceanic and cratonic lithosphere control the deformation patterns an important result of the models is that they demonstrate that fold belts with different vergences can be generated without the requirement of subducting oceanic lithosphere

evolution of a forearc basin luzon central valley philippines the cenozoic history of the 14 km thick luzon central valley sequence illustrates the development of a forearc basin forearc basins are important both as major sediment traps and as sites of hydrocarbon accumulations the luzon basin is floored by oceanic crust on the seaward western side and older accreted terranes on the arc eastern side initial sedimentation on this oceanic crust occurred during early tertiary northward translation and emplacement of the crust as an ophiolite along a strike slip or oblique slip zone the basal sediments consist of pelagic limestones and thin ash layers overlain by sandy turbidites derived from uplift and progressive dissection of the ophiolite a sequence of arc derived sediments at least 26 000 ft 8 km thick was shed into the astern arc side of the basin during late paleogene to quaternary convergence along the western margin of luzon by the middle miocene the central valley became a continuous elongate basin fringed by extensive shelf deposits along both the uplifted seaward and arc sides of the basin detritus shed from both flanks filled the subsiding basin and resulted in progressively shallower depths nonmarine deposition began in central portions of the basin in the pliocene and migrated with time both north and south along the basin axis late miocene to holocene movement along the philippine fault zone caused uplift and folding of adjacent parts of the basin exploration models for the central valley predict gas prone hydrocarbon generation in central portions of the basin at times that coincide with and postdate the formation of both structural and stratigraphic traps previous drilling in the basin has either been in areas with thermally immature source rocks or has failed to reach prospective intervals where thermal maturation is inferred the hydrocarbon potential of the central valley has not been determined adequately

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

experiments on shortening of a 4 layer model of the continental lithosphere davy ph and cobbold p r 1991 experiments on shortening of a 4layer model of the continental lithosphere in p r cobbold editor experimental and numerical modelling of continental deformation tecronophysics 188 l 25 we present an experimental method for modelhng lithospheric deformation the method is based on a simplified profile of lithosphere rheology where only four layers are considered brittle crust ductile crust brittle mantle and ductile mantle the strengths of the four layers calculated from laboratory determined flow laws depend strongly on geothenns and slightly on strain rates and deformation regimes the rheological profiles of the lithosphere have been recreated in experimental models built with analog materials sand silicone putty and golden syrup which flow under their own weight during lithospheric shortening the style of deformation depends on the relative strengths of the four layers especially those in the mantle of special importance is the existence or non existence of a brittle mantle the initial stages of compression often produce flexures or buckles without local isostatic compensation buckling exerts a strong control on the localisation of thrust faults in later stages the thrusts tend to form at the inflexion points of buckles the style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers in 2 layer lithospheric models without resistant mantle shortening was concentrated upon a small number of thrusts widely spaced in 3 layer models shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before in clayer models a single thrust formed in the brittle mantle motion on this thrust dragged the crustal layers down forming an asymmetric crustal root with steep imbricate slices

role of strike slip faulting in the evolution of allochthonous terranes in the philippines concepts of allochthonous terrane transport and emplacement are dominated by the assumption that most terranes originate on the subducting plate collide with the upper plate and are emplaced there movement of terranes along the convergent margin is recognized but is generally attributed to postcollision slip in the northern philippines allochthonous terranes originate primarily within the arc system have been translated along it by strike slip faults and were emplaced by cessation of that slip the authors suggest that in the philippines some originally vertical strike slip boundaries may have evolved into shallow dipping sutures marked by fold and thrust systems this mode of terrane evolution may be more common than generally appreciated particularly in orogenic belts developed in response to oblique convergence

distribution timing and causes of andean deformation across south america the andean orogeny in south america has lasted over 100 ma it comprises the peruvian incaic and quechuan phases the nazca and south american plates have been converging at varying rates since the palaeocene the active tectonics of south america are relatively clear from seismological and global positioning system gps data horizontal shortening is responsible for a thick crust and high topography in the andes as well as in se brazil and patagonia we have integrated available data and have compiled four fault maps at the scale of south america for the mid cretaceous late cretaceous palaeogene and neogene periods andean compression has been widespread since the aptian the continental margins have registered more deformation than the interior for the peruvian phase not enough information is available to establish a tectonic context during the incaic phase strike slip faulting was common during the quechuan phase crustal thickening has been the dominant mode of deformation to investigate the mechanics of deformation we have carried out 10 properly scaled experiments on physical models of the lithosphere containing various plates the dominant response to plate motion was subduction of oceanic lithosphere beneath continental south america however the model continent also deformed internally especially at the margins and initial weaknesses

oblique plate convergence slip vectors and forearc deformation slip vectors from thrust earthquakes at subduction zones where convergence is oblique to the trench often point between the directions of relative plate convergence and normal to the trench axis suggesting that oblique convergence is taken up by partial decoupling decoupling means that a component of arc parallel motion of the leading edge of the upper plate results in less oblique thrusting at the trench partial decoupling is modeled by partitioning of oblique convergence into slip on thrust and strike slip faults that are parallel to the trench and to each other and starting with a force equilibrium condition a relationship between the obliquity and the earthquake slip vector orientation is derived assuming that either fault slips when shear stress on it reaches a yield stress oblique slip parallel to the plate vector should occur on the thrust fault when obliquity is smaller than a critical angle for obliquity at or greater than this angle the stress on the strike slip fault is large enough to start it slipping and when both faults are active the arc parallel motion of the forearc deflects the slip vector back toward the trench normal if we assume that continued slip on either fault occurs at constant stress but the two faults can be at different stresses the slip vector will maintain a constant angle relative to the trench normal even when obliquity is larger than the critical angle this limiting angle of the slip vector called max measured relative to the trench normal is simply the arcsine of the ratio of the shear forces resisting slip on the strike slip and thrust faults a consequence is that when the obliquity exceeds max the slip vectors on the thrust fault are sensitive only to the thrust fault orientation and contain no information about the convergence direction between the plates slip vectors at the java trench southwest of sumatra show the relationship clearly with max 20 5 while slip vectors at the aleutian trench show the relationship less clearly with max 25 to 45 the greater angle at the aleutian trench suggests that the upper plate is stronger in the aleutian arc relative to the thrust fault than in the sumatran arc consistent with the sumatran arc being continental and having a well developed strike slip fault while the aleutian arc is oceanic and without a clear transcurrent fault slip vectors at the philippine trench which like sumatra has a large strike slip fault inboard of it tend to stay within 25 of the trench normal when obliquity is as large as 50 if obliquity exceeds max and continues to increase along a subduction zone the rate of motion of the forearc relative to the upper plate will vary with obliquity in which case the forearc sliver should extend or contract parallel to the arc from the geometry of modern island arcs arc parallel extension should be the more common and has been hypothesized for both sumatra and the aleutians on the basis of earthquake slip vectors and for these and other arcs from geological observations from estimates of max and the arc parallel gradients in obliquity arc parallel strain rates are estimated to be 1 to 3 10 8 yr for the sumatran forearc 2 to 6 10 8 yr for the aleutian forearc and 0 3 to 3 10 8 yr for the philippine forearc oblique convergence and subsequent arc parallel extension if accompanied by crustal thinning may provide an important yet little appreciated mechanism for bringing high grade metamorphic rocks to the surface of subduction complexes

processes of allochthonous terrane evolution mindoro island philippines on mindoro two tectonostratigraphic terranes the north palawan block and the mindoro block are separated by a steeply dipping shear zone the north palawan block was rifted off southern china in the oligocene and comprises most of the shallow southern end of the south china sea the eastern part of this terrane exposed on mindoro includes jurassic and paleogene strata probable rift basalts of oligocene age and oligo miocene graben fill deposits in the late oligocene and early miocene the eastern north palawan block underwent crustal stretching with grabens oriented at a high angle to synchronous spreading anomalies in the south china basin we hypothesize that grabens formed in an overall transtensional environment near a transform boundary at the eastern margin of the opening south china basin the mindoro block consists of a pre upper cretaceous low grade metamorphic basement overlain by upper cretaceous and upper eocene strata the mindoro block records several episodes of intense deformation of mesozoic and tertiary age and contrasts with the north palawan block both in terms of stratigraphy and structural evolution the mindoro suture which separates the mindoro and north palawan blocks is a complex structural boundary comprising slices of diverse lithologies separated by steeply dipping anastomosing faults deformed tectonic slices in the suture zone include serpentinized ultramafic rock amphibolite and rocks derived from both of the bounding terranes we interpret this suture as a zone of mostly transcurrent faulting that was active through much of mid tertiary time paleogeographic considerations for early and middle miocene time support the premise that the west side of the central philippine archipelago was a left lateral transform boundary the mindoro and north palawan blocks were initially juxtaposed along the transcurrent mindoro suture by late miocene time subsequent deformation on mindoro includes crustal shortening associated with the southern end of the manila trench which projects onshore on southwest mindoro the onset of shortening here probably reflects a late miocene change in philippine eurasian plate motion the mindoro suture zone is now locked and it acts as a rigid buttress against which strata of the north palawan block have been structurally stacked the mindoro suture zone is similar to other terrane boundaries in the northern philippines the history of terrane transport in this region was apparently dominated by strike parallel shuffling of terranes along major transcurrent boundaries within the philippine arc transcurrent motion has taken place in extensional and compressional regimes and it is ongoing today this may be a dominant process by which complex island arc systems are disrupted prior to accretion onto a continental margin

constraining the source regions of lunar meteorites using orbital geochemical data lunar meteorites provide important new samples of the moon remote from regions visited by the apollo and luna sample return missions petrologic and geochemical analysis of these meteorites combined with orbital remote sensing measurements have enabled additional discoveries about the composition and age of the lunar surface on a global scale however the interpretation of these samples is limited by the fact that we do not know the source region of any individual lunar meteorite here we investigate the link between meteorite and source region on the moon using the lunar prospector gamma ray spectrometer remote sensing data set for the elements fe ti and th the approach has been validated using apollo and luna bulk regolith samples and we have applied it to 48 meteorites excluding paired stones our approach is able broadly to differentiate the best compositional matches as potential regions of origin for the various classes of lunar meteorites basaltic and intermediate fe regolith breccia meteorites are found to have the best constrained potential launch sites with some impact breccias and pristine mare basalts also having reasonably well defined potential source regions launch areas for highland feldspathic meteorites are much less well constrained and the addition of another element such as mg will probably be required to identify potential source regions for these

analogue models for the transpressional docking of volcanic arcs in the western pacific abstract many convergent zones involve oblique subduction of oceanic crust the subduction often ends with oblique collision of a microcontinent island arc or oceanic plateau against a larger continental area we have conducted a series of experiments in which scaled multilayered models of the lithosphere made of sand and silicone were subjected to horizontal shortening oblique to lithospheric boundaries the aim was to investigate tectonic phenomena occurring at such boundaries for example subduction reversal partitioning of dip slip and strike slip motions lateral translation of continental fragments block rotations and entrapment of oceanic material the physical models ranged in complexity from a simple uniform oceanic lithosphere to configurations involving a volcanic arc against a continental margin our models explore the importance of rheology and width of continental indenters rather than the obliquity of convergence the models confirm that intra oceanic deformation is likely prior to complete subduction beneath both the arc and the continental margin they further explain the presence of extensional features in a convergent setting subduction zones resulting from oblique convergence may be associated with large strike slip faults by partitioning of the convergence vector when a model volcanic arc collides against a model continent a subduction reversal occurs after suturing and the strike slip fault which accompanies the subduction reversal initiates with a horse tail pattern the resulting plate boundary is somewhat different from the one that existed prior to the docking of the allochthonous belt these experiments tested many of the mechanisms proposed for terrane formation and accretion the models have been used to interpret the late neogene geodynamic setting of the western pacific with emphasis on the southern philippines and the island of new guinea

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

bivergent thrust wedges surrounding oceanic island arcs insight from observations and sandbox models of the northeastern caribbean plate at several localities around the world thrust belts have developed on both sides of oceanic island arcs e g java timor panama vanuatu and the northeastern caribbean in these localities the overall vergence of the backarc thrust belt is opposite to that of the forearc thrust belt for example in the northeastern caribbean a north verging accretionary prism lies to the north of the eastern greater antilles arc hispaniola and puerto rico whereas a south verging thrust belt called the muertos thrust belt lies to the south researchers have attributed such bivergent geometry to several processes including reversal of subduction polarity subduction driven mantle fl ow stress transmission across the arc gravitational spreading of the arc and magmatic infl ation within the arc new observations of deformational features in the muertos thrust belt and of fault geometries produced in sandbox kinematic models along with examination of published studies of island arcs lead to the conclusion that the bivergence of thrusting in island arcs can develop without reversal of subduction polarity without subarc mantle fl ow and without magmatic infl ation we suggest that the eastern greater antilles arc and comparable arcs are simply crustalscale bivergent or doubly vergent thrust wedges formed during unidirectional subduction sandbox kinematic modeling suggests in addition that a broad retrowedge containing an imbricate fan of thrusts develops only where the arc behaves relatively rigidly in such cases the arc acts as a backstop that transmits compressive stress into the backarc region further modeling shows that when arcs behave as rigid blocks the strike slip component of oblique convergence is accommodated entirely within the prowedge and the arc the retrowedge hosts only dip slip faulting frontal thrusting the existence of large retrowedges and the distribution of faulting in an island arc may therefore be evidence that the arc is relatively rigid the rigidity of an island arc may arise from its mafi c composition and has implications for seismic hazard analysis

middle oligocene oceanic crust of south china sea jammed into mindoro collision zone philippines mindoro island south of luzon philippines is a complex junction between the manila trench and the collision zone of the north palawan block with the western philippines mobile belt middle oligocene ophiolites recognized in this suture are coeval with the oldest magnetic anomalies of the south china sea basin these ophiolites are part of a complex pile of terranes thrust above the north palawan block at the lower middle miocene boundary these ophiolites are interpreted as fragments of south china sea oceanic crust jammed between two distinct continental blocks during the counterclockwise rotation of luzon

plate boundary evolution in the halmahera region indonesia abstract halmahera is situated in eastern indonesia at the southwest corner of the philippine sea plate active arc arc collision is in process in the molucca sea to the west of halmahera new stratigraphic observations from halmahera link this island and the east philippines and record the history of subduction of the molucca sea lithosphere the halmahera basement complex and the basement of east mindanao were part of an arc and forearc of late cretaceous early tertiary age and have formed part of a single plate since the late eocene early oligocene there is no evidence that halmahera formed part of an oligo miocene arc but arc volcanism associated with eastwards subduction of the molucca sea beneath halmahera began in the pliocene and the pliocene arc is built on a basement of the early tertiary arc arc volcanism ceased briefly during the pleistocene and the arc shifted westwards after an episode of deformation the present active arc is built upon deformed rocks of the pliocene arc the combination of new stratigraphic information from the halmahera islands and models of the present day tectonic structure of the region deduced from seismic and other geophysical studies is used to constrain the tectonic evolution of the region since the miocene diachronous collision at the western edge of the philippine sea plate which began in mindanao in the late miocene impeded the movement of the philippine sea plate and further motion has been achieved by a combination of strike slip motion along the philippine fault subduction at the philippine trench and subduction of the molucca sea lithosphere beneath halmahera

phanerozoic growth of asia geodynamic processes and evolution abstract accretion processes often obscured in mountain belts can be documented with great detail in se asia where these have taken place during the tertiary the resulting configuration showing accreted continental strips and tectonised wedges is illustrated by the tethysides jammed between the northern laurasian cratons baltica and siberia and gondwanian cratons africa arabia india and australia eurasia increased progressively in size due to the amalgamation of crustal and sedimentary belts at places where the processes are documented in the recent times they can be included within a collision factory which displays the opening of basins by rifting and sea floor spreading within the upper plate until they undergo a process of shortening both stages being subduction controlled in se asia the early stages are illustrated in the eastern sunda arc where the subduction of the sunda trench is blocked in sumba and timor region and flipped into the flores trough in less than 2 my the incipient shortening is at present taking place in the pliocene damar basins another stage where half of the upper plate basin has disappeared is documented in the celebes sea the examples of deformation being transferred further inland exist in the northern celebes sea and the makassar basin the next important stage is the complete consumption of the marginal basin where both margins collide and the accretionary wedge is thrust over the margin as illustrated in nw borneo and palawan each of these stages is responsible for a single short lived tectonic event the succession of several events composes an orogen which may last for over 10 my these events predate the arrival of the conjugate margin of the large ocean which marks the beginning of continental subduction as observed in the himalaya tibet region these examples show that the closure is generally diachronous through time as illustrated in the philippines we observe that the ophiolite obducted in such context is generally of back arc origin upper plate rather than the relict of the vanishing large ocean which is rarely preserved in the philippines once the crust is accreted the subduction zone progressively moved southward until its present position we propose that the lithospheric mantle of the accreted block is delaminated and rolls back in a continuous manner whereas the crust is deformed and accreted

measurements of friction coefficients and cohesion for faulting and fault reactivation in laboratory models using sand and sand mixtures abstract simple laboratory tests have been made on dry quartz sand and on mixtures of sand and clay and sand and cement tests were made on both low and high density specimens of each mixture as determined by physical handling techniques the tests included measurements of density estimates of the coefficient of internal friction based on fault orientation and estimates of the friction coefficients and cohesion values for fault initiation and reactivation in a shear frame apparatus which was also used to test artificial fault zones made from pure clay glass microspheres and wheat flour the results suggest that density is determined more by handling technique than by composition and that friction coefficients and perhaps cohesion both increase with increasing density the shear testing revealed a consistent drop in cohesion for fault reactivation in all densely packed mixtures tested although small this drop may be significant for some types of laboratory models

models of lithospheric scale deformation during plate collision effects of indentor shape and lithospheric thickness abstract analogue models designed to test the effects of lithosphere thickness and indentor shape during continental collision indicate that strength and density differences between lithosphere types control structural vergence during collision models presented here include layers of putty and sand to form four three and two layer lithosphere types craton arc and oceanic equivalent respectively which collide during unidirectional shortening in most cases the stronger thicker four layer lithosphere does not deform but simply acts to bulldoze weaker material aside deformation concentrated in the thinnest layers consistently verges away from the indenting continent irrespective of indentor shape lateral strike slip component deformation occurs in the early stages of most models even with simple unidirectional shortening and is enhanced by highly oblique indentor shapes the effects of extrusion during collision tested by repeating models with and without a free edge which allows extrusion consistently indicate that with the ability to extrude structures are smaller and more numerous without extrusion fewer larger structures tend to develop two geological examples from the precambrian part of the tasman orogenic belt and the modern day collision at the leading edge of australia illustrate aspects of the models

effects of buoyancy and mechanical layering on collisional deformation of continental lithosphere results from physical modeling abstract we use two suites of lithospheric scale physical experiments to investigate the manner in which deformation of the continental lithosphere is affected by both 1 variations of lithospheric density quantified by the net buoyant mass per area in the lithospheric mantle layer m b and 2 the degree of coupling between the crust and lithospheric mantle characterized by a modified ampferer ratio am the dynamics of the experiments can be characterized with a rayleigh taylor type ratio clm models with a positively buoyant lithospheric mantle layer m b 0 and clm 0 result in distributed root formation and a wide deformation belt in contrast models with a negatively buoyant lithospheric mantle layer strongly coupled to the crust m b clm 0 2 and am 10 3 exhibit localized roots and narrow deformation belts syncollisional delamination of the model lithospheric mantle layer and a wide deformation belt is exhibited in models with negatively buoyant lithospheric mantle layers weakly coupled to the crust m b clm am 3 syncollisional delamination of the continental lithosphere may initiate due to buoyancy contrasts within the continental plate instead of resulting from wedging by the opposing plate rayleigh taylor instabilities dominate the style of deformation in models with a negatively buoyant lithospheric mantle layer strongly coupled to the crust and a slow convergence rate m b clm 0 2 the degree of coupling am between the model crust and lithospheric mantle plays a lesser role in both the style of lower lithospheric deformation and the width of the crustal deformed zone with increasing density of the lithospheric mantle layer

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

experimental insights into the partitioning of motion within zones of oblique subduction abstract in zones of oblique convergence the motion is often partitioned into subduction and an associated strike slip component fitch 1972 in this paper we first describe the geodynamic setting of the philippine region as a case study of deformation by strike slip faulting behind parallel subduction zones then we present six small scale laboratory experiments with analogue materials to investigate how partitioning may be controlled by internal boundaries between oceanic lithosphere and continental or island arc lithosphere during the early stages of a typical experiment deformation is taken up on oblique thrusts subduction zones which root into internal boundaries paired strike slip faults develop at later stages thereafter subduction zones and paired strike slip faults operate simultaneously the partitioning occurs without any need for complex constraints on external boundaries

philippine fault a key for philippine kinematics on the basis of new geologic data and a kinematic analysis we establish a simple kinematic model in which the motion between the philippine sea plate and eurasia is distributed on two boundaries the philippine trench and the philippine fault this model predicts a velocity of 2 to 2 5 cm yr along the fault geologic data from the visayas provide an age of 2 to 4 ma for the fault an age in good agreement with the date of the beginning of subduction in the philippine trench the origin of the philippine fault would thus be the flip of subduction from west to east after the locking of convergence to the west by the collision of the philippine mobile belt with the eurasian margin

implications of paleomagnetic data on the collision related bending of island arcs paleomagnetic studies from the marianas central philippines ryukyus sulawesi fiji new hebrides all show major differences in the declination values observed from individual islands within the same arc comparable differences are not observed in the inclination values a simple plate tectonic model would predict that each island arc should experience the same rotation along its entire length as the plate moves around its rotation axis bathymetry from each of these arcs show that the differentially rotated segments of the upper plate occur in areas where the downgoing plate carries a feature such as a seamount chain rifted continental fragment or an island arc the coincidence of the bathymetric feature and the discordant paleomagnetic declination data support an earlier hypothesis of vogt 1973 that these features are buoyant zones on the downgoing plate which interact with the margin of the upper plate as a result of this interaction the upper plate boundary is locally deformed in addition to the paleomagnetic evidence other geologic and geophysical data from these areas suggest such interactions for example there appears to be a good correlation between the deflections in major structures around the collision zone and the paleomagnetic directions that are reported as this deformation proceeds eventually the stresses resulting from the collision may result in any of the following tectonic processes 1 slip line style strike slip faults allowing for the sideward extrusion of portions of the upper plate tapponnier et al 1983 2 changes in the polarity of the subduction zone 3 development of strike slip faults around the margin of the indenter or 4 reorganization of the entire plate margin in addition data from the marianas southern end of the ryukyu arc and the lau basin suggest that back arc basin development may result from the collision related deformation

geodynamic patterns of ophiolites and marginal basins in the indonesian and new guinea regions analysis of spatial temporal geological and geochemical patterns of ophiolites in the indonesian and new guinea region indicates a strong correlation with marginal basin development and closure the spatial distribution of ophiolites is mostly linked with marginal basin producing zones of oblique convergence and collision strain partitioning in these zones creates a series of ephemeral plate boundaries between several independently moving lithospheric blocks repeated disruption of the diffuse boundaries between the blocks by changes in plate motion and collision induced mantle extrusion creates space that is rapidly filled by new ocean basins in the upper plate of subduction zones suprasubduction zone ssz spreading of these basins is enhanced by episodic extrusion of asthenosphere escaping collisional suture zones various closure events and global plate motion changes are reflected in the temporal distribution of marginal basin and ophiolite ages most ophiolite slabs in the indonesian and new guinea region represent fragments of oceanic lithosphere with a subduction zone component as indicated by the common refractory petrochemistry of the mantle sequence and occurrence of boninite age and compositional heterogeneity may indicate that some ophiolite bodies are composite terranes collisions with buoyant lithosphere transfo parts of these ocean basins into ophiolites the connection between ophiolites and marginal basins is strongest where parts of actively spreading ssz basins are partially represented as ophiolites in collision zones many of the various tectonic models proposed for the origin and tectonic evolution of ophiolites are supported by plate interactions in the indonesian and new guinea region dewey bird 1970 silver smith 1983 moores et al 1984 searle stevens 1984 dilek moores 1990 harris 1992 dickinson et at 1996 however the geolo gical associations of these interactions remain poorly resolved allowing different parts of the region to be used as modern analogues for almost any tectonic scenario simple explanations based on plate kinematics alone fail to account for the increasing levels of complexity observed in the indonesian and new guinea region new ideas and supporting evidence for mantle extrusion mechanisms e g flower et at 2001 also rely heavily on descriptions of plate interactions in this region doglioni et at 1999 and raise new questions about how these processes may relate to the spatial and temporal pattern of ophiolites this paper provides a synthesis of the temporal and spatial patterns and tectonic associations of marginal basins and ophiolites throughout the indonesian and new guinea region it explores the relations between various plate boundary processes and ophiolite genesis and emplacement such as the role of strain partitioning trench rollback subduction polarity asymmetry astheno spheric flow and plate kinematics

from subduction to collision control of deep processes on the evolution of convergent plate boundary 1 using laboratory experiments we investigate the dynamics of the collisional process that follows the closure of an oceanic basin the evolution of these experiments systematically shows four successive episodes of deformation which correspond to 1 the initiation of oceanic subduction 2 a mature period of oceanic subduction 3 an episode of continental subduction during which the trench absorbs all the convergence and the superficial tectonic regime does not change significantly within the continental plates and 4 continental collision that starts when the trench locks and convergence is absorbed by a system of thrust faults and folds we observe that the amount of continental material that subducts before the onset of collision depends on the slab pull exerted by the subducted oceanic lithosphere the slab pull force in turn depends on the amount of subducted oceanic material on the thickness of the convective mantle and also on the rheology of the slab our experiments indeed suggest that parts of the oceanic slab may separate from the superficial slab to sink rapidly in the mantle decreasing the slab pull level and triggering the rapid onset of collision we observe two possible modes of slab deformation slab break off and development of viscous instabilities we define two dimensionless rheological numbers to characterize the possible occurrence of these modes of deformation in all cases oceanic closure is followed by episode of continental subduction during which the continental lithosphere may reach depths varying between 50 and 450 km prior to the onset of continental collision

western irian jaya the end product of oblique plate convergence in the late tertiary abstract the island of new guinea is the surface expression of the deformed northern margin of the australian continental block to the south undeformed continental crust is overlain by flat lying platform sediments that form a conformable sequence from late palaeozoic to late tertiary along the new guinea cordillera these platform sediments have been tightly folded and overthrust to the south as a result of crustal shortening caused by convergence between the australian plate and the pacific plate the earth movements resulting from this convergence started at the end of the miocene and continue to the present day it is postulated that all the major structures of western irian jaya were caused by this orogeny and can be explained by the convergence directed southwest northeast between the australian and pacific plates it is further postulated that the convergence has remained essentially constant since the start of the earth movements this convergence is consistent with that postulated in the circum pacific plate tectonic map aapg 1981 of 12 5 cm yr directed about 15 south of west only if the effects of the sorong yapen fault system are taken into account this complex east west trending fault system forms the southern boundary of the pacific plate and has responded to the convergence by left lateral strike slip displacement of at least 370 km since the late miocene as a consequence south of the sorong yapen fault system the convergence has been directed more to the south i e at least 45 south of west in this region south of the fault zones the north south component of the convergence has been accommodated in the east by folding and overthrusting over a wide zone of interaction called the new guinea mobile belt dow 1977 in the west on the other hand the australian continental crust has been a stable buttress kepala burung fig 2 which following convergence has protruded into the pacific plate its northeastern margin has therefore been exposed to the full brunt of collision with the pacific plate and large segments of continental crust have been broken off and rafted westwards in order to accommodate the difference in crustal shortening between the new guinea mobile belt and kepala burung two fundamental crustal fractures formed in the intervening region the waipoga and wandamen faults these fractures make angles of about 65 with the regional structural trend fig 2 and they were the focus of most of the resulting differential crustal displacement two of the more spectacular examples of this differential movement have been the thrusting of large slabs of crustal rocks tens of kilometres southwards over the australian continental crust weyland overthrust and lengguru fold belt

accreted terranes in the northern part of the philippine archipelago the philippine archipelago consists of a complex array of ophiolites continental fragments and island arc elements that can be identified as accreted terranes similar to those increasingly recognized as components of orogenic zones the northern philippines including luzon mindoro and nearby smaller islands can be divided into at least six of these tectonic elements from west to east across central luzon there are the neogene accretionary prism of the west luzon arc the eocene zambales ophiolite the cretaceous angat ophiolite and a late cretaceous early paleogene volcanic arc built on an older metamorphic basement this arc terrane is repeated by offset along the philippine fault to the south the western luzon terranes are juxtaposed against the metamorphic basement block of mindoro which itself is colliding with the north palawan microcontinental fragment the ophiolites appear to have originated in back arc basins rather than in oceanic plateaus and the metamorphic belts have protoliths indicating deformation along active margins perhaps deep within accretionary prisms the philippine terranes are interpreted as fragments that originated within the complex pacific eurasian plate boundary rather than as intra pacific basin units these terranes have been assembling at least since the oligocene by strike slip and convergent displacements strike slip displacement either along transcurrent faults or as a component of convergence played a very important if not dominant role in this assembly most terranes identified in the northern philippine appear to be plate fragments with attached crust and upper mantle their characteristics contrast with those usually interpreted for terranes in the north american cordillera where near orthogonal convergence and shallow dipping terrane sutures are favored

arc continent collision and subduction reversal in the cenozoic evolution of the northwest pacific an example from kamchatka ne russia abstract the principal new feature in the cenozoic evolution of the northwest pacific is based on the fact that two arc terranes of upper cretaceous paleogene age were incorporated in the kamchatka orogen during the early tertiary and at the end of the miocene respectively these two arc terranes so called achaivayam valaginskaya campanian lower paleocene and kronotskaya coniacian eocene arcs are separated by a suture zone that contains deformed slices of young paleocene lower eocene oceanic crust paleomagnetic data on the original paleolatitudes of the arc positions as well as geochemical data characterized the arcs as intraoceanic originated in the northwest pacific the arcs were separated by the vetlovka oceanic plate suggested to be a trapped fragment of the kula plate in which a new spreading zone developed in the early tertiary the principal event in the reconstructed evolution is the eocene collision of the extended achaivayam valaginskaya arc terrane with the continental margin of asia the margin is interpreted as the southeastern facing the pacific cretaceous accretionary boundary of the okhotsk microplate docked against the asian continent at 55 65 ma the arc continent collision developed progressively northward from the end of the paleocene early eocene in southern kamchatka to the middle eocene in the olutorka region the different stages of the collision occurred simultaneously along strike of the achaivayam valaginskaya arc according to the geodynamic model based on geological data for southern kamchatka collision included 1 the entrance of the continental margin in the subduction zone deformation of the overriding plate and subduction of fore arc block 2 arc obduction on the continental margin blocking of continental margin subduction and 3 subduction reversal with onset of oceanic subduction at the eastern side of the deformed arc this subduction reversal produced deformation and reverse faulting along the eastern side of the arc thus allowing accretion of tectonic slices of young oceanic crust of the vetlovka plate which began to subduct during the subduction reversal the subducted plate dipping to the southeast was broken off and detached at depth under the arc area the subduction polarity reversal the most characteristic feature of the collision model is strongly supported by structural and geologic data of southern kamchatka and explains the main tectonic metamorphic and magmatic events related to and postdating the collision

pre eocene synmetamorphic structure in the mindoro romblon palawan area west philippines and implications for the history of southeast asia the structure of the pre eocene rocks considered as the basement of the philippines has been investigated in the mindoro lubang romblon tablas sibuyan and north palawan areas in the former two areas the same pre eocene succession of units is recognized from top to bottom 1 a pre eocene olistostrome 2 an ophiolitic nappe 3 a schistose sequence pelites sandstones basic schists and marbles and 4 a gneissic unit the nature of the contact between the olistostrome and the underlying units is unclear but the ophiolite and the schistose sequence form two thrust sheets of oceanic origin thrust upon the gneissic unit that is considered a part of a continental basement called the west philippines block small scale structures show that the early deformation stage is characterized by a submeridian 0 n40 e lineation formed in greenschist to amphibolite facies conditions during the thrusting kinematic analysis show that the thrusting was from north to south in north palawan metamorphic rocks with similar microtectonic and kinematic characteristics are found they are overlain by a late jurassic olistostrome which is correlated with the olistostromes found in calamian north mindoro carabao and buruanga peninsula north panay the microstructural features and the presence of the olistostrome suggest that the north palawan mindoro tablas romblon sibuyan and carabao islands belong to the same north palawan block of hamilton 1979 which is a continental fragment rifted from asia in cenozoic times it is assumed that the western panay and zamboanga areas which are characterized by mesozoic ophiolites and metamorphic rocks also belong to the north palawan block all these islands experienced to some extent the same mesozoic geohistory the south verging thrusting is interpreted as the result of an oblique collision of the west philippines block with asia the contemporaneous left lateral strike slip faulting and the calc alkaline magmatism widespread along the chinese margin are also included in the geodynamic model

active deformation in the new guinea fold and thrust belt seismological evidence for strike slip faulting and basement involved thrusting the new guinea fold and thrust belt forms a 1000 km long mountain chain running the length of new guinea but its trend is oblique to the predicted convergence direction a large component of left lateral shear across the mountain belt is expected but geological evidence for such motion is ambiguous surface structures in the fold and thrust belt show mountain building by folding and thrusting supposedly thin skinned of pre pliocene continental shelf sediments although some evidence indicates that active faulting penetrates the basement we examine the active deformation of this mountain belt by using teleseismic long period p and sh waveforms short period p waveforms and p wave first motions to determine reliable focal mechanisms and source depths for the 18 largest earthquakes in the new guinea fold and thrust belt since 1964 eight thrust events with seismic moments m0 between 1017 and 2 1018 n m are distributed throughout the thrust belt at depths ranging from 0 5 to 45 km and have steeply dipping fault planes that strike parallel to surface structural trends six of the eight events are between 11 and 25 km deep the steep dips of nodal planes the depths of these earthquakes and the known thicknesses of the sedimentary section show that thrust faulting is not exclusively thin skinned i e confined above a shallow detachment surface but penetrates the crystalline basement at high angles most earthquakes in the western half of the thrust belt including the three largest earthquakes examined m0 5 1018 n m show strike slip faulting with the probable fault plane oriented east west so that displacement is left lateral the nine strike slip earthquakes account for almost 90 of the observed seismic moment in the period 1964 1985 and indicate a seismic slip rate of 5 25 mm a all but one of these strike slip earthquakes are shallower than the thrust earthquakes suggesting that east northeast convergence across the fold and thrust belt is accommodated by a decoupled system of strike slip and dip slip faults while the great height of the mountain belt is an obvious indication that crustal thickening is an important consequence of plate motion in new guinea seismological evidence suggests that translation by strike slip faulting may play a much greater role than previously recognized by analogy in older collisional orogens whose motions can be inferred only from remnant structures and not from seismicity strike slip faulting may have been equally important in accommodating convergence we estimate from seismic moments of earthquakes and from the volume of the mountains that deformation of the new guinea highlands may account for 5 20 of the total convergence between the pacific plate and australia in new guinea

paleogeographic reconstruction and origin of the philippine sea abstract our reconstruction of the philippine sea suggests that it formed by two distinct episodes of back arc spreading each of which resulted from seaward retreat of the trench in the first episode the protoizu bonin trench retreated northward and the west philippine basin formed behind the northern half of the palau kyushu ridge in the second episode the izu mariana trench retreated eastward and the shikoku and parece vela basins formed behind it during the last 17 ma the philippine sea basin has been moving northwestward with respect to eurasia shifting the ttt triple junction off central japan westward by about 50 km the motion of the philippine sea with respect to eurasia at the triple junction changed from north northwestward to west northwestward 10 5 ma ago for the period before 17 ma ago we construct two models retreating trench model and anchored slab model the izu bonin trench migrated from south to northeast rotating in a clock wise sense since 48 ma ago in the retreating trench model in the anchored slab model the trench has been fixed with respect to eurasian margin since 43 ma ago we prefer the retreating trench model because the deformation of the plate boundary along the eastern margin of eurasia during 30 17 ma ago is much simpler for this model than for the anchored slab model furthermore rotations of the bonin mariana islands are consistent with those predicted from the retreating trench model the 48 ma ages of the northern part of the palau kyushu ridge and of chichi jima of the bonin islands indicate that there was subduction beneath the northern half of the ridge beginning at least 48 ma ago from this and the subparallelism in trend between the northern part of the palau kyushu ridge and the central basin ridge we propose that the major part of the west philippine basin formed by back arc spreading in a n s direction behind the northern part of the palau kyushu ridge the pacific plate was moving northward with respect to hot spots from 48 to 43 ma ago which implies that the pacific plate is not likely to have been subducting beneath the west philippine basin during this time we speculate that another plate existed south of the pacific plate and thai it was subducting beneath both tile west philippine basin and australia the annihilation of this plate might he a cause for the sudden change of the pacific plate motion at 43 ma ago

multiple strike slip fault sets a case study from the dead sea transform in many strike slip tectonic settings large rotations of crustal blocks about vertical axes have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which presumably accommodate the relative motion between the blocks as regional deformation progresses a mechanical model suggests that rotations greater than c 25 to 45 must be accommodated by more than one set of faults with the angle c between their directions consequently the sum of the angles between sets must be roughly equal to the total tectonic material rotation to test this model we investigated the fault geometry and field relationship of fault sets in the mt hermon area in northern israel where paleomagnetic declination data imply 69 13 counterclockwise ccw block rotation the statistical and field relationship analysis of over 315 faults shows that the faulting is predominantly right lateral strike slip consisting of three distinct sets the oldest set strikes 254 the second oldest set strikes 295 and the youngest strikes 331 this last direction is consistent also with the current north south direction of the maximum principal stress axis the angle c between the first and second sets is 41 and between the second and third sets 36 in good agreement with the c angle predicted from mechanical considerations the sum of the two angles is 77 ccw in good agreement with the 69 13 ccw paleomagnetically derived rotation the results suggest specifically that the sequential development of multiple intersecting fault sets is responsible for the faulting in the mt hermon area and generally that the model of block rotation with multiple faults provides very good simple rules for analyzing very complex fault patterns