use of nmr logging to obtain estimates of hydraulic conductivity in the high plains aquifer nebraska usa 1 hydraulic conductivity k is one of the most important parameters of interest in groundwater applications because it quantifies the ease with which water can flow through an aquifer material hydraulic conductivity is typically measured by conducting aquifer tests or wellbore flow wbf logging of interest in our research is the use of proton nuclear magnetic resonance nmr logging to obtain information about water filled porosity and pore space geometry the combination of which can be used to estimate k in this study we acquired a suite of advanced geophysical logs aquifer tests wbf logs and sidewall cores at the field site in lexington nebraska which is underlain by the high plains aquifer we first used two empirical equations developed for petroleum applications to predict k from nmr logging data the schlumberger doll research equation ksdr and the timur coates equation kt c with the standard empirical constants determined for consolidated materials we upscaled our nmr derived k estimates to the scale of the wbf logging k kwbf logging estimates for comparison all the upscaled kt c estimates were within an order of magnitude of kwbf logging and all of the upscaled ksdr estimates were within 2 orders of magnitude of kwbf logging we optimized the fit between the upscaled nmr derived k and kwbf logging estimates to determine a set of site specific empirical constants for the unconsolidated materials at our field site we conclude that reliable estimates of k can be obtained from nmr logging data thus providing an alternate method for obtaining estimates of k at high levels of vertical resolution

kinematics of the philippine sea plate in this study new instantaneous rotation vectors describing the motion of the philippine sea plate relative to surrounding plates are determined that most nearly satisfy all available geological geophysical and seismological observations along the plate margins pacific philippine and caroline pacific poles are generated from a set of trial eurasia philippine poles of rotation using published determinations of the eurasia pacific and caroline philippine poles the pacific philippine caroline pacific and eurasia philippine poles are then checked for agreement with 1 slip vectors determined from focal mechanisms of earthquakes along the ryukyu trench and the nankai trough 2 convergence at the sagami trough 3 extension in the mariana trough 4 extension behind the bonin trench 5 extension in the western sorol trough and 6 convergence at the mussau trench trial eurasia philippine poles are eliminated if predicted plate motions do not match observed plate motions the locations and rotation rates of the three best fitting poles are eurasia philippine 37 0 n 141 0 e 1 60 ma pacific philippine 3 8 n 130 4 e 1 68 ma caroline pacific 16 9 n 141 4 e 0 54 ma the plate motions predicted by these new poles are compared with the spatial distribution of earthquakes and new focal mechanism solutions in the complex region where the philippine pacific and caroline plates intersect intermediate and deep focus earthquake hypocenters along the southern mariana arc show a southward decrease in the length of the seismic zone which qualitatively agrees with the predicted convergence rate as the pole of rotation is approached and as the effect of back arc spreading in the mariana trough disappears the absence of volcanism and intermediate depth seismicity along the southernmost mariana yap and palau sections of the plate boundary is attributed to very low convergence rates a new focal mechanism solution along the trench south of guam is the first clear underthrusting solution found along the mariana trench and agrees with the convergence direction implied by the pacific philippine pole of rotation no clear underthrusting focal mechanism solutions were found along the yap and palau trenches although three high angle thrust solutions demonstrate that compression is occurring in the upper plate perpendicular to the plate boundary

quantitative assessment of in situ salt karstification using shear wave velocity dead sea abstract the dead sea ds coastal areas have been dramatically affected by sinkhole formation since around 1990 such sinkholes along both israeli and jordanian shores are linked to karst cavities that form through slow salt dissolution a quantitative estimate of such in situ salt karstification would be an important indicator of sinkhole hazard one of the indications of salt karstification is its increased hydraulic conductivity caused by the development of dissolution cavities forming conducting channels within the salt layer we measured the hydraulic conductivity k versus shear wave velocity vs of ds salt in situ for estimating the actual salt karstification in areas of sinkhole development these parameters were measured with the magnetic resonance sounding mrs and multichannel analysis of surface waves masw methods respectively understanding of the field relationships was augmented by similar inter relations obtained in the laboratory on samples of ds salt in situ salt velocities vs vary from 750 m s to over 1650 m s while hydraulic conductivity k in the same zones varies between about 10 4 m s to slightly over 10 8 m s both field and laboratory k and vs values fit the exponential function ln k 0 0045 vs 5 416 with a determination coefficient r2 of 0 88 a classification based on vs and k was generated for salt conditions and the corresponding degrees of sinkhole hazard which was verified in the mineral beach sinkhole development area the mapping of sinkhole sites shows that they form within highly conductive zones with k 5 5 10 5 it is suggested that this methodology with some modification can be used for evaluating the conductive properties of karstified rock and associated sinkhole hazards

late palaeogene quaternary geology of halmahera eastern indonesia initiation of a volcanic island arc the late palaeogene quaternary stratigraphy of halmahera is described and new formation names are proposed based on recent field investigations of the ne and central part of the island this stratigraphic information provides new insights into the neogene history of halmahera and the development of the present island arc the late palaeogene and younger rocks rest unconformably on an ophiolitic basement complex which formed part of a late cretaceous early tertiary fore arc after volcanic arc activity ceased in the eocene the former fore arc terrane was uplifted and deeply eroded in the late palaeogene some of the late palaeogene early miocene river valleys are currently being re excavated by the present rivers slow subsidence began in the mid late oligocene and by the end of the miocene all eastern halmahera was the site of shallow water carbonate deposition there is no evidence for arc volcanism in central halmahera at this time and the reported oligo miocene volcanism in nearby regions is interpreted as volcanism related to the sorong fault system the miocene shallow water region subsided rapidly in the early pliocene and the sedimentary basin formed was filled with marls succeeded by siliciclastic turbidites with increasing amounts of calc alkaline volcanic debris from a pliocene volcanic arc built on the western arms of halmahera probably on the eroded early tertiary arc this phase of rapid subsidence in the pliocene back arc region resulted from the initiation of subduction of the molucca sea lithosphere eastwards beneath halmahera differential subsidence on nw se and ne sw sets of faults in the region immediately behind the active arc led to the formation of deep sediment filled basins adjacent to the eastern arms a major deformation event in the pleistocene resulted in folding and local thrusting at the junction between eastern and western halmahera and volcanism ceased in the pliocene arc the third halmahera arc the quaternary arc currently active in the northern part of the islands began activity within the last 1 ma and is built upon the deformed and partly eroded pliocene arc the pleistocene deformation event and shift in position of the arc are interpreted as the result of the interaction of the eastward dipping molucca sea plate with adjacent plates either with a fragment of the australian continent in the sorong fault zone and or with the philippine sea plate beneath northern halmahera

subduction of seamounts at the java trench a view with long range sidescan sonar abstract we describe here a 1300 km by 45 km gloria long range sidescan sonar swath along the eastern java trench the swath images the trench axis a narrow strip of oceanic crust to the south and the toe of the accretionary wedge to the north between 108 and 120 e sonar images of the ocean crust show a pattern of normal faults typical of the outer wall of trenches these result from tension related to the bending of the oceanic lithosphere into the subduction zone a number of sub circular seamounts are also seen some of which are currently being subducted isolated ponds of flat lying sediments occur in the trench axis although along much of its length the trench is devoid of such sediment accumulations on the inner trench wall the accretionary wedge is recognised by its distinctive grainy texture with the grain aligned sub parallel to the deformation front where subducting seamounts are colliding with the wedge large crescentic areas of very high backscattering correlate with re entrants in the deformation front and large indentations in the wedge the high backscattering within these collision scars is interpreted to arise from outcropping strata and talus covered slopes resulting from erosion of an inner trench wall shortened and oversteepened by collision with a seamount all of the sediment ponds in the trench axis occur in close association with collision scars strongly suggesting localised erosion of the accretionary wedge in the scar areas however the volume of the indentations appears to be an order of magnitude greater than the volume of eroded sediments deposited in the trench indicating that erosion cannot be the primary mechanism by which indentations are created most of the missing material must be displaced landward by thrusting and folding ahead of the seamount our observations at the java trench are broadly comparable with those made at the japan trench by french and japanese workers however some differences in the detail of the collision process are evident in particular oversteepening and erosion occur from the earliest stage of collision at the java trench while uplift and understeepening initially dominate at the japan trench differences in the effects of subducting seamounts in the two areas are attributed to contrasts between the relatively unlithified strata of the java trench inner wall and the more lithified strata of the non accreting japan trench inner wall

active slivering of oceanic crust along the molucca ridge indonesia philippine implication for ophiolite incorporation in a subduction wedge a recent marine geophysical survey in the northern molucca sea revealed the structure to be that of a classical active convergent margin we observe from west to east a volcanic arc sangihe a forearc basin resting on an outer ridge the molucca ridge which serves as a buttress for an accretionary wedge and a composite downgoing plate snellius ridge and philippine sea basin gravity modeling indicates a strong negative anomaly above the wedge which cannot be explained with reasonable density values modeling imposes a basement deepening and a rupture of the 700 km long subducting lithosphere this process individualized the lithospheric slab from the snellius ridge which in turn was separated recently from the south philippine basin by the incipient philippine trench this induces a deformation of the forearc region with backthrusting of the outer ridge and forearc basin visible on bathymetry and seismic data we extrapolate the tectonic emplacement of such oceanic blocks to the oligocene times in order to explain the origin of the pujada miangas outer ridge as a sliver previously incorporated to the margin and we discuss the possibility of this deformation process being fabric for terrane accretion

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

three dimensional p wave velocity structure beneath the indonesian region abstract we present the p wave seismic tomography image of the mantle to a depth of 1200 km beneath the indonesian region the artb inversion method is applied to a dataset of 118 203 p wave travel times of local and teleseismic events taken from isc bulletins although the resolution is sufficient for detailed discussion in only a limited part of the study region the results clarify the general tectonic framework in this region and indicate a possible remnant seismic slab in the lower mantle structures beneath the philippine islands and the molucca sea region are well resolved and high velocity zones corresponding to the slabs of the molucca sea and philippine sea plates are well delineated seismic zones beneath the manila negros and cotabato trenches are characterized by high velocity anomalies although shallow structures were not resolved the molucca sea collision zone and volcanic zones of the sangihe and philippine arcs are dominated by low velocity anomalies the philippine sea slab subducts beneath the philippine islands at least to a depth of 200 km and may reach depths of 450 km the southern end of the slab extends at least to about 6 n near southern mindanao in the south the two opposing subducting slabs of the molucca sea plate are clearly defined by the two opposing high velocity zones the eastward dipping slab can be traced about 400 km beneath the halmahera arc and may extend as far north as about 5 n unfortunately resolution is not sufficient to reveal detailed structures at the boundary region between the halmahera and philippine sea slabs the westward dipping slab may subduct to the lower mantle although its extent at depth is not well resolved this slab trends n s from about 10 n in the philippine islands to northern sulawesi a ne sw trending high velocity zone is found in the lower mantle beneath the molucca sea region this high velocity zone may represent a remnant of the former subduction zone which formed the sulawesi arc during the miocene the blocks along the sunda and banda arcs are less well resolved than those in the philippine islands and the molucca sea region nevertheless overall structures can be inferred the bowl shaped distribution of the seismicity of the banda arc is clearly defined by a horseshoe shaped high velocity zone the tomographic image shows that the indian oceanic slab subducts to a depth deeper than 300 km i e deeper than its seismicity beneath andaman islands and sumatra and may be discontinuous in northern sumatra along southern sumatra java and the islands to the east the slab appears to be continuous and can be traced down to at least a depth of the deepest seismicity where it appears to penetrate into the lower mantle

continental extension of the pacific plate at the southern termination of the hikurangi subduction zone the north mernoo fault zone offshore new zealand a complex transition from subduction at the hikurangi margin to transpressive continental collision across the southern alps occurs in the ne south island region of new zealand offshore the southern limit of oceanic subduction is constrained by the chatham rise a submerged continental plateau on the pacific plate which is presumably too buoyant for normal subduction in the nw corner of the rise is the north mernoo fault zone nmfz a 100 by 300 km region of basement involved active normal faulting of late miocene circa 8 6 ma to recent age the nmfz evolved contemporaneously with the rotation and development of the adjacent plate boundary zone growth faulting is characteristic within the sedimentary cover which is generally less than 2 km thick but the distribution of faulting has varied temporally the amount of mid pliocene recent extension is estimated to be not more than a few percent the nmfz consists of an array of overlapping south dipping normal faults which are typically 2 5 km apart and which trend roughly e w at a high angle to the plate boundary zone late quaternary surface traces are widely distributed on the mid to upper continental slope but many surface scarps are poorly preserved due to extensive erosion of the seafloor the western end of the fault zone crosses submarine canyons at the southern end of the hikurangi trough and almost abuts two fold and thrust fault systems on the ne south island continental margin although few data constrain the kinematics of extension a single mw 6 1 earth quake at the western end of the nmfz suggests planar rotating faults with a component of left lateral oblique slip two possible models of regional extension are considered 1 lateral buckling of the upper crust in which faulted slivers of crust are peeled off as the chatham rise slides past the transpressive marlborough fault system toward the southern alps and 2 flexure of the edge of continental pacific plate as the chatham rise is bent downward into the southern end of the hikurangi subduction zone improved seismological data and deep crustal seismic reflection profiles may help to constrain the kinematics of deformation

plate convergence measured by gps across the sundaland philippine sea plate deformed boundary the philippines and eastern indonesia summary the western boundary of the philippine sea ph plate in the philippines and eastern indonesia corresponds to a wide deformation zone that includes the stretched continental margin of sundaland the philippine mobile belt pmb extending from luzon to the molucca sea and a mosaic of continental blocks around the ph australia sunda triple junction the gps geodyssea data are used to decipher the present kinematics of this complex area in the philippines the overall scheme is quite simple two opposing rotations on either side of the left lateral philippine fault clockwise to the southwest and counterclockwise to the northeast transfer 55 per cent of the ph sundaland convergence from the manila trench to the northwest to the philippine trench to the southeast further south 80 per cent of the ph sunda convergence is absorbed in the double subduction system of the molucca sea and less than 20 per cent along both continental margins of northern borneo finally within the triple junction area between the sundaland ph and australia plates from sulawesi to irian jaya preferential subduction of the celebes sea induces clockwise rotation of the sulu block which is escaping toward the diminishing celebes sea oceanic space from the eastwardadvancing ph plate to the south we identify an undeformed banda block that rotates counterclockwise with respect to australia and clockwise with respect to sundaland the kinematics of this block can be defined and enable us to compute the rates of southward subduction of the banda block within the flores trench and of eastward convergence of the makassar straits with the banda block the analysis made in this paper confirms that this deformation is compatible with the eastward motion of sundaland with respect to eurasia determined by the geodyssea programme but is not compatible with the assumption that sundaland belongs to eurasia as was often assumed prior to this study

field experiment provides ground truth for surface nuclear magnetic resonance measurement a field experiment was conducted in the high plains aquifer central united states to explore the mechanisms governing the non invasive surface nmr snmr technology we acquired both snmr data and logging nmr data at a field site along with lithology information from drill cuttings this allowed us to directly compare the nmr relaxation parameter measured during logging t2 to the relaxation parameter t2 measured using the snmr method the latter can be affected by inhomogeneity in the magnetic field thus obscuring the link between the nmr relaxation parameter and the hydraulic conductivity of the geologic material when the logging t 2 data were transformed to pseudo t2 data by accounting for inhomogeneity in the magnetic field and instrument dead time we found good agreement with t 2 obtained from the snmr measurement these results combined with the additional information about lithology at the site allowed us to delineate the physical mechanisms governing the snmr measurement such understanding is a critical step in developing snmr as a reliable geophysical method for the assessment of groundwater resources citation knight r e grunewald t irons k dlubac y song h n bachman b grau d walsh j d abraham and j cannia 2012 field experiment provides ground truth for surface nuclear magnetic resonance measurement geophys res lett 39 l03304

the southern termination of the philippine trench abstract recent studies of the southern end of the philippine trench using gloria sidescan sonar and single channel seismic data and geological studies on the island of halmahera suggest that the trench is in the process of propagating south and that some of the ese wnw convergence is transferred via a broad ne sw zone of dextral strike slip across northern halmahera into the molucca sea collision zone ne sw ridges and lineaments on the seafloor can be traced into major faults and structural trends on land the bathymetric expression of the philippine trench is lost where it meets an oceanic plateau on the philippine sea plate this elevated and probably thickened plateau appears to have inhibited any further propagation of the trench southwards to the north of the plateau there is a well developed accretionary prism but to the south deformation of sediments on the seafloor is less intense a prominent ridge with sediments passively banked up against it marks the southern limit of deformation associated with the philippine trench to the south lies the eastern halmahera waigeo ophiolite terrane an area of shallow water and islands underlain by ophiolitic basement between halmahera and the sorong fault zone there are no bathymetric or structural features to indicate any form of active link between the philippine trench and the sorong fault through or along the northern side of this ophiolite terrane there is no earthquake evidence for any form of fault linking the philippine trench to either the palau trench or the sorong fault but there are numerous earthquake epicentres reported between the southern end of the trench and the molucca sea the island of halmahera is considered to lie in a diffuse boundary zone at the margin of the philippine sea plate continued collision of the opposing arcs in the molucca sea will ultimately prevent further e w convergence between halmahera and sulawesi it is predicted that either the philippine trench will propagate south along its present line to meet the sorong fault or that another trench will develop further to the east in either case the arc and ophiolite terrane of halmahera might then be accreted onto the eurasian margin

coulomb wedge model applied to the subduction of seamounts in the japan trench results from the french japanese kaiko exploration program of the japan trench and complementary data are used to propose a model of subduction of seamounts examples in two seamount chains one at the junction between the japan and kuril trenches and the other in the southern part of the japan trench off inubo cape illustrate the stages of subduction the japan trench margin evolved as a homogeneous wedge of deformable noncohesive coulomb material during seamount subduction this is illustrated by the subduction of the daiichi kashima seamount in the southern japan trench our model predicts that the first consequence of seamount subduction is a compressive thickening of the toe of the wedge above the advancing flank of the seamount because of the oversteepening of the rigid base of the wedge this agrees with our observations in the daiichi kashima area this thickening shifts landward across the wedge with the advancing flank of the seamount erosion dominates the wedge above the trailing flank of the seamount because of the corresponding understeepening of the rigid base of the wedge a reentrant of the toe of the wedge is thus created it is largest when the oceanic flank of the seamount coincides with the base of the landward slope at this time the trench axis will probably have an abnormally large amount of sediments as illustrated at the junction between the japan and kuril trenches after the subduction of the seamount the edge of the wedge should progressively move back to its initial location frontal accretion is absent or very limited

the nature of seismic reflections from deep crustal fault zones deep seismic reflection profiles have recorded reflections from ductile shear zones within the crystalline basement perhaps the best example is the consortium for continental reflection profiling cocorp wind river line in wyoming which shows the wind river thrust fault to be a strong reflector from the surface to depths of about 30 km to identify the physical properties responsible for the seismic reflections from fault zones we report measurements of compressional wave velocity and velocity anisotropy in mylonites recovered from exhumed ductile shear zones these rocks are characterized by extensive ductile deformation of plastic minerals brittle deformation of the more rigid minerals grain size reduction and development of a strong fabric average velocity and density were not found to change significantly and systematically between the mylonites and the adjacent undeformed rock seismic anisotropy of 7 or greater is present in several mylonites dependent upon their composition and fabric using our data on mylonite properties and models for the crustal structure near the wind river line we compute true amplitude synthetic reflection seismograms and compare them to true amplitude plots of the cocorp wind river reflection data when anisotropy and the finely laminated structure of the fault zone are considered our modeling indicates fairly strong reflections from mylonite zones in most cases we also show that elevated pore pressure in shear zones may produce strong reflections however a permeability of the order of 10 16 darcy is required to maintain sufficient pore pressure to produce a velocity anomaly in a fault zone which has long been inactive

orogen scale decollements the continental lithosphere responds to stress by deforming as a generally layered medium deep seismic reflection data coupled with a variety of ancillary geological and geophysical data are interpreted to provide images of fault zones that tend to form moderately dipping ramp structures in mechanically rigid layers and flat detachments in mechanically weak layers this geometry is similar to ramp and flat structures observed at smaller scales in thrust and fold belts and leads to the interpretation that most orogens are underlain by orogen scale decollements in a manner that is analogous to so called thin skin deformation in sedimentary rocks decollements may occur within the crust for example near the base of a sedimentary section or in the middle crust near the moho in the subcrustal lithosphere or in the asthenosphere even intracratonic basement cored thick skin uplifts that occasionally occur in foreland regions such as the wyoming province are probably large crustal scale versions of ramp flat features observed in supracrustal rocks and are thus likely caused by the same fundamental tectonic processes

groundwater nmr in conductive water a surface method of groundwater prospecting using nuclear magnetic resonance nmr in the earth s magnetic field is under study the technique is employed for hydrogeological surveys down to a depth of about 100 m the advantage of this method is that an nmr signal can be observed only in the presence of groundwater a circular wire loop with a diameter of 100 m is laid out on the ground to excite and receive the nmr signal an oscillating current with a rectangular pulse shape is passed through the loop with the carrier frequency being equal to the proton resonance frequency in the earth s field the excitation pulse is followed by a nuclear induction emf caused by the free larmor precession in the earth s field of practical importance is the effect of the electrical conductivity of the ground on a groundwater nmr survey finite ground conductivity can result in induced currents that can screen the nmr signal the calculations of nmr signals are based on the transformation of maxwell s equations in terms of magnetic hertz potentials through use of the reciprocity principle groundwater nmr is measured with an instrument designed at the institute of chemical kinetics and combustion russian academy of science novosibirsk experiments were conducted in the altay region of russia both nmr signal amplitude and phase were measured and compared with the calculated results for horizontally stratified media borehole logs and vertical resistivity profiles were also used for evaluation of results the conductivity is shown to affect both phase and amplitude of the nmr signal at resistivities of a few to a few tens of ohm m depending on the depth of the water saturated layers there is good agreement between calculated and experimental data it is also established that the measurements of only nmr amplitude and phase are not sufficient for determining groundwater salinity

determining multiple length scales in rocks carbonate reservoirs in the middle east are believed to contain about half of the world s oil1 the processes of sedimentation and diagenesis produce in carbonate rocks microporous grains and a wide range of pore sizes resulting in a complex spatial distribution of pores and pore connectivity2 this heterogeneity makes it difficult to determine by conventional techniques the characteristic pore length scales which control fluid transport properties here we present a bulk measurement technique that is non destructive and capable of extracting multiple length scales from carbonate rocks the technique uses nuclear magnetic resonance to exploit the spatially varying magnetic field inside the pore space itself a fingerprint of the pore structure we found three primary length scales 1 100 m in the middle east carbonate rocks and determined that the pores are well connected and spatially mixed such information is critical for reliably estimating the amount of capillary bound water in the rock which is important for efficient oil production this method might also be used to complement other techniques3 4 5 for the study of shaly sand reservoirs and compartmentalization in cells and tissues

basin formation and neogene sedimentation in a backarc setting halmahera eastern indonesia abstract it has been proposed that basins in backarc settings form in association with subduction by thinning of continental crust by backarc spreading in oceanic crust by compression retro arc basins or by trapping of pieces of oceanic plate behind an arc the halmahera basin in eastern indonesia developed in a backarc setting but does not fall into any of these categories it formed by subsidence of thickened crust made up of imbricated mesozoic palaeogene arc and ophiolite rocks halmahera lies at the western edge of the philippine sea plate in a complex zone of convergence between the eurasian margin the oceanic plates of the west pacific and the australian indian plate to the south the basement is an imbricated complex of mesozoic to palaeogene ophiolite arc and arc related rocks during the miocene this basement complex formed an area of thickened crust on which carbonate reef and reef associated sediments were deposited we interpret this shallow marine region of thickened crust to be similar to many of the oceanic plateaux and ridges found within the philippine sea plate to day in the late miocene convergence between the philippine sea plate and the eurasian margin resulted in the formation of the halmahera trench to the west of this region of thickened crust subduction of the molucca sea plate at the trench caused the development of a volcanic island arc subsidence in the backarc area produced a broad sedimentary basin filled by clastics eroded from the arc and from uplifted basement and cover rocks the basin was asymmetric with the thickest sedimentary fill on the western side against the volcanic arc the halmahera basin was modified in the plio pleistocene by east west compression as the molucca sea plate was eliminated by subduction

tracing surface features to great depths a powerful means for exploring the deep crust abstract seismic reflection profiling of the continental basement provides a means for tracing known features from the surface to great depths and hence for exploring and identifying deep and inaccessible subsurface geology some examples based on cocorp data illustrate the power and potential of this method which has not commonly been applied on a crustal scale

geodynamic evolution of the northern molucca sea area eastern indonesia constrained by 3 d gravity field inversion in order to better understand the tectonic framework of the northern molucca sea area we inverted satellite and sea surface gravity data into an iterative scheme including a priori seismological and geological data the resulting 3 d density model images the various tectonic units from the surface down to 40 km we proceed to various tests to assess the stability and robustness of our inversion in particular we performed an offset and average smoothing method to properly refine our results the resulting model shows a striking vertical regularity of the structures through the different layers whereas the density contrasts appear strongly uneven in the horizontal direction the density model emphasizes the complexity of the upper lithospheric structure in the northern molucca sea which is clearly dominated by the interaction between ophiolitic ridges sedimentary wedges and rigid blocks of the philippine sea plate it also provides new hard information that can be used in discussion of the evolution of the region large density variations are concentrated in the central part of northern molucca sea and dominate the upper lithospheric north south trending density structures along the central ridge and west dipping thrust faults on the western side of the region are clearly imaged in the eastern part of the region we distinguish several blocks especially the snellius plateau which seems to be split into two parts we interpret this as an oceanic plateau associated with thicker crust that previously belonged to the

deep crustal structure beneath the central and inner west carpathians abstract the 150 km long deep seismic transect 2t crossing the central part of the slovakian west carpathians has revealed an important whole crustal seismic suture zone dipping to the south this zone is dominated by six large and several minor reflection events because of our good knowledge of the surface geology and a large number of comparative studies of the eastern alps it is clear that the upper reflections are due to overthrust sheets that originated during compression of the apulian passive margin tectonically deeper reflections are interpreted as being due to ligurian oceanic igneous and sedimentary rocks sheared in ductile domain during continental collision with the brianconnais passive margin and continental masses north of the central apulian west carpathians cwc behind a significant strike slip fault are the cenozoic structures of the pieniny klippen belt and the magura rhenodanubian flysch south of the cwc the inner tethyan west carpathians iwc are thrust over the cwc the iwc originated during the late jurassic early cretaceous closure of the tethyan ocean and the seismic image of the iwc is different from the highly reflective cwc suture these three pre neogene nappe systems amalgamated as one overthrust system during the neogene subduction of the moldavian sea as the moldavian accretionary wedge the vepor gemer block of the west carpathians where the cretaceous suture is observed has been untouched by any major cenozoic deformation and has remained relatively rigid this unique structure therefore enables us to observe seismically the cretaceous suture zone which elsewhere in the european alpine belt has been almost completely destroyed more general data about the beginning of subduction and subsequent collision processes are presented significant compression and partial subduction of the apulian upper plate passive margin lasted from the hauterivian to the beginning of the turonian for about 25 30 ma subduction of the ligurian ocean occurred from turonian to campanian times after the early campanian the collision of the apulian and brianconnais plates took place

cenozoic motion of the philippine sea plate palaeomagnetic evidence from eastern indonesia the history of motion of the philippine sea plate is poorly known because it is isolated from the oceanic ridge system interpretation of palaeomagnetic results from the plate has been controversial because declination data have been obtained only from the eastern margin where subduction related tectonic processes may have caused local rather than plate wide rotations new palaeomagnetic data relevant to the problem have been obtained from 34 sites north of the sorong fault and 29 sites within the sorong fault system these sites record south ward movement during the eocene and northward movement during the neogene sites within the sorong fault system record both counterclockwise and clockwise rotations interpreted as the result of neogene block movements at the southern boundary of the philippine sea plate north of the sorong fault all sites record clockwise declinations neogene rocks have small deflections consistent with rotation about the present day eurasia philippine sea plate pole oligocene middle eocene rocks show consistent clockwise declination deflections of 40 o declinations of lower eocene rocks indicate 90 o of clockwise rotation we propose that the entire area north of the sorong fault in east indonesia has always been part of the philippine sea plate and that the whole plate has rotated clockwise in a discontinuous manner by approximately 90 o since the early eocene the new data from north of the sorong fault provide a basis for determining rotation poles which satisfy all the palaeomagnetic data from the philippine sea plate and permit its reconstruction

remarks on the derivation of the paleostress system from inferred faults on deep seismic reflection records abstract geological dip slip faults have become a common feature of interpreted deep crustal seismic reflection records the applicability of anderson s theory of faulting to these cases as argued by previous workers would allow the derivation of the paleostress system from the dip angle of the fault knowledge of paleostress would obviously be important in constraining tectonic reconstructions two well constrained examples of crustal dip slip faults inferred from deep seismic reflection data from the wind river mountains wyoming and the riddleville buried triassic jurassic basin georgia u s a suggest that in certain cases the approach can yield reasonable estimates of the coefficient of static friction and the paleostress system from deep reflection defined faults this study however also notes that the sensitivity of the calculation of the coefficient of static friction and thus stress to the fault dip angle together with ambiguities often involved in measuring angles of deep seismic reflectors may make such estimates meaningless in cases where true dip angles cannot be precisely determined

collision and strike slip faulting in the northern molucca sea philippines and indonesia preliminary results of a morphotectonic study a swath mapping gravity and single channel seismic survey was carried out in the northern molucca sea with rv l atnlnnfe preliminary interpretation of these data reveals the presence of an almost complete sangihe arc and forearc the mangas pujada talaud ridge in the cenaal part of the molucca sea appears to be a backstop within the sangihe forearc east of the ridge very contrasting terranes arc separated by a major nw se crustal discontinuity interpreted as a left lateral strike slip fault north of 6 n the philippine trench inner wall is dissected by nw se trending left lateral strike slip faults resulting from the dominantly oblique convergence between colliding ans south of 6 n the westward subduction of the buoyant sneliius volcanic plateau a fragment of the halmahera arc terrant has induced the formation of a new plate boundary the philippine trench along what is interpreted as a former strike slip fault zone east of the miangas pujada talaud ridge there is a wide sedimentay wedge separated from the snellius ridge to the south by the major nw se crustal discontinuity the thickness of this wedge cannot be explained by subduction along the very young philippine trench it could be either an accretionary wedge developed at the deformed leading edge of the sangihe forearc or be part of a former intra arc basin which was part ofthe colliding halmahera arc terrane there is no clear evidence for accreted oceanic c s f belonging to the recently subducted molucca sea the mangas pujada talaud ridge is part of the san gihe forearc and the ophiolites could represent its basement u p i i d along the outer arc ridge the dog leg shaped philippine trench is propagating southward a m s s the fragmented halmahera arc tenane and its southern segment could reactivate a former strike slip fault zone

anomalous seismic structure beneath the klyuchevskoy group kamchatka 1 the klyuchevskoy group is among the most active volcanic features on earth yet its position within the kamchatka subduction zone is hard to explain with a simple tectonic mechanism the geochemistry of klyuchevskoy group lavas is typical for volcanic products resulting from flux melting in the mantle wedge yet the depth to the subducting pacific plate beneath it is much larger than the average depth of volatile release in subduction zones we present new seismological constraints on the upper mantle structure beneath the klyuchevskoy group and identify a planar dipping seismic feature in the mantle wedge which is 100 km deep and appears to be sharply bounded we hypothesize that this upper mantle structure may be the true source of melts erupting at the klyuchevskoy group and that the presence of this feature may explain both the extraordinary productivity of the klyuchevskoy group and its unusual location

genetic relations between the central and southern philippine trench and the sangihe trench we surveyed the junction between the central and southern philippine trench and the sangihe trench near 6on using swath bathymetry gravity and magnetics these data along with seismicity allow us to discuss the genetic relations between these trenches and the forces acting on converging plates our final model favors the northern extension of the halmahera arc up to 8on with three segments offset left laterally along nw se transform faults accretion of the northern segment to mindanao island 4 to 5 m y ago resulted in the failure within the philippine sea plate east of the arc initiation of the philippine trench between 7on and 10on agrees with the maximum recorded depth of the philippine trench floor 10 000 m below sea level and philippine sea slab 200 km south of 6on trench junction another segment of the arc is being subducted beneath the sangihe margin while south of 3on the southern segment of the halmahera arc is still active the rapid southward shallowing of the trench floor along the southern philippine trench the type of faulting affecting both sides of the trench the lack of significant interplate seismicity and the concentration of the seismicity beneath the miangas talaud ridge are interpreted as a slowing down of the subduction along this branch of the philippine trench compared with the rest of the subduction zone the sangihe deformation front has been recognized up to 7on but seems active only south of 6on

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

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

three trench endings in eastern indonesia abstract underway surveys using long range sidescan sonar as well as conventional marine geophysical techniques have been made of three trench endings in eastern indonesia in each case the termination is associated with the presence of a block of thickened crust the transition between the sunda trench and the troughs which encircle the banda arc is a consequence of collision with the northern margin of the australian continent whereas the southern termination of the philippine trench is defined by the presence of an oceanic plateau which impedes further southwards propagation both these trenches are currently active and are associated with clearly defined wadati benioff zones the new guinea trench terminates in the west at a n trending seafloor rise known as the mapia ridge the relationship between the two features is not well understood there are no clear indications of present day subduction along the northern margin of new guinea and insufficient data from the flanks of the mapia ridge to show whether this ever had a subduction history the new data obtained within the trench immediately east of the ridge suggests that subduction has ceased in the westernmost part of the new guinea trench and that the oceanic crust of the ayu basin west of the mapia ridge post dates the period of active subduction

seismic studies of crustal structure during the past four years seismology has continued its role as the key technique for determination of the structure of the continental and oceanic crust by and large however the field and analysis methods used bear little resemblance to those of ten years ago a continued push to increase the number of recording channels has led to the routine use of ocean bottom seismometers sonobuoys and multichannel streamers in oceanic studies and to the use of large sets of portable instruments and multichannel seismic reflection strings on land in studies using earthquakes as sources local and regional arrays provide a capability for obtaining geologically useful two and three dimensional information about the crust

application of two dimensional synthetic seismogram modeling to interpretation of wide angle seismic data from wind river wyoming in conjunction with the 1977 cocorp wind river project wide angle reflection refraction data were recorded on a fixed array the goals of the present study were to model these wide angle data to provide independent constraints on the wind river structure a common receiver profile extracted from these data forms a pseudo split spread with maximum offsets of 13 3 km toward the southwest and 19 1 km toward the northeast the first 10 s of these data are modeled by iterative fitting of synthetic seismograms computed by asymptotic ray theory the result is a two dimensional velocity model that extends from the center of the wind river mountains into the wind river basin and from the earth s surface to a depth of 25 km the major features of the model are 1 surficial sediments that thicken toward the center of the wind river basin 2 precambrian basement that underlies the sediments and outcrops in the wind river mountains 3 an intrabasement reflector at 5 km depth 4 a low velocity region from 6 to 15 km depth 5 a local high reflectivity laminar zone possibly an intrusive feature at 15 km depth 6 a second low velocity region from 17 to 23 km depth and 7 a laminar transition zone between 23 and 25 km depth that separates the middle crust average velocity 6 km s from the lower crust average velocity 7 km s this study clearly reveals the major crustal structures the most interesting result is tentative evidence that they may be secondary local intrusion of high velocity material as well as the primary horizontal thrusting in the wind river uplift

gravity fields in eastern halmahera and the bonin arc implications for ophiolite origin and emplacement classic ophiolites as exemplified by the troodos massif in cyprus and the papuan ultramafic belt in eastern new guinea are large overthrust masses which are generally associated with large positive gravity anomalies however similar rocks occurring in extensive fragmented terranes which have also been described as ophiolitic do not produce large gravity effects the eastern part of the island of halmahera in northeastern indonesia is an ophiolite of this latter type on the two eastern arms of the island a mesozoic ophiolitic basement is overlain by and imbricated with upper cretaceous and paleogene arc volcanic and sedimentary rocks bouguer gravity values are generally in the range 50 to 150 mgal and are characterised by steep local gradients indicative of shallow sources the bouguer gravity average suggests that the crust is at least 20 km thick and it must be even thicker if a significant part of the anomalous gravity field is due to the presence of a cold and therefore dense lithospheric slab within the asthenosphere associated with the present day subduction beneath halmahera the absence of any exposures of continental basement rocks or of quartzose sediments in eastern halmahera suggests that these ophiolites have not been overthrust onto continental crust and that the thickening occurred in an intraoceanic island arc the paleogene arc was evidently characterised by volcanism occurring over an unusually wide area in this it resembles the izu bonin volcanic arc which like halmahera has been situated at the margin of the philippine sea plate throughout its history the gravity field of the halmahera ophiolite is comparable with that of the bonin volcanic arc but there is no halmahera parallel to the very high gravity fields recorded over the bonin islands forearc ridge the equivalents of this part of the paleogene arc may be represented by the ophiolitic complexes now distributed along the northern margin of the orogenic belt in new guinea

southeast asia new views of the geology of the malay archipelago southeast se asia is surrounded by subduction zones causing intense seismicity and volcanic activity subduction has been the principal tectonic driver of collisions that caused the growth of continental se asia and most recently the collision of australia with se asia the western part of se asia sundaland is a heterogeneous and weak region reflecting processes that can be observed today in the east where there are subduction zones in different stages of development a close relationship between subduction rollback and extension has caused dramatic elevation of land exhumation of deep crust and spectacular subsidence of basins observable with remotely acquired images and seismic and multibeam data obtained from oil exploration new dating indicates that subsidence and uplift occurred at high rates during short time intervals laboratory studies modeling and reconstructions provide valuable insights but field based studies continue to present surprises and new discoveries essential for interp

earthquakes and ophiolite emplacement in the molucca sea collision zone indonesia the molucca sea in eastern indonesia is the site of an active arc arc collision and among its interesting features is the exposure of ophiolitic rocks on islands of the talaud mayu ridge tmr a largely submarine ridge formed of highly deformed rocks that bisects the collision zone to explore the relationship between earthquakes and uplift of the ophiolite the centroid depths and fault plane solutions of 18 large earthquakes occurring in the past 20 years beneath the tmr are constrained by inversion of their teleseismic long period p and sh waveforms centroid depths range from 16 to 36 km but uncertainties allow a range of 10 to 45 km all events show thrust faulting nodal planes strike parallel to the nne trending tmr and dip 40 9 to the ese and 53 9 to the wnw published seismic refraction and gravity data support the inference that the wnw dipping steeper nodal planes are the fault planes dips do not change with depth indicating either that the fault flattens below 40 km depth or that the steeply dipping fault penetrates the entire thickness of the lithosphere i conclude that the molucca sea ophiolite is being lifted by high angle thrust faults 45 to 60 that extend at least 15 km into the upper mantle summing seismic moment tensors and assuming uniformly distributed deformation suggest that the closure across the molucca sea collision zone may account for 14 to 59 of the pacific eurasia convergence vector or 15 to 63 of the philippine eurasia convergence moment tensor sums provide estimates of crustal thickening rates beneath the tmr of 7 to 20 mm yr and these imply uplift rates of 2 to 6 mm yr if the increase in crustal thickness is isostatically compensated hence the ophiolite and an enormous volume of melange are being lifted higher than the flanks of the island arcs and will be emplaced on them by gravity i suggest that ophiolites occur in this tectonic setting because the concave down shape of the subducted oceanic lithosphere beneath the molucca sea increases its effective buoyancy preventing it from sinking out of the way of the encroaching island arcs this case suggests a family of convergent margin settings in which normal oceanic lithosphere is buoyed up so that ophiolites can be stripped from it and emplaced on a continental margin or island arc these settings include those where 1 slab geometry restricts asthenospheric flow away from the convergent margin 2 subduction of younger less dense lithosphere decreases its negative buoyancy 3 a passive continental margin resists subduction or 4 the presence of a thick thermal boundary layer beneath a passive continental margin inhibits asthenospheric flow necessary to accommodate the sinking oceanic lithosphere

geology of the talaud islands molucca sea collision zone northeast indonesia abstract the talaud islands lie at the northern margin of the collision zone between the sangihe and halmahera island arc systems rock units on talaud are neogene marine strata basalt and andesite tectonic melange and ophiolite the units are exposed in n s trending belts that are commonly separated by faults the marine strata consist of tuffaceous siltstone sandstone shale and marl they are strongly deformed by west verging folds with wavelengths of 20 500 m volcanic rocks of island arc affinity are exposed on the east coast of karakelang island and appear to be interbedded with the lowermost marine strata tectonic melanges contain blocks of serpentinite gabbro basalt red middle eocene chert and limestone and greywacke turbidites the blocks range in length from a few millimetres to hundreds of metres and are enclosed in a scaly clay matrix several mappable slabs of ophiolite are separated by tertiary strata or melange the dismembered ophiolites consist of serpentized peridotite gabbro spilites and cherts locally the melanges and ophiolites are thrust over the younger sedimentary rocks along east dipping faults the dominant eastward dips of melange foliation the westward vergence of structures in the neogene strata the eocene ages of the cherts and the miocene age of the strata overlying the ophiolite slabs suggest that the ophiolites are pieces of eocene or older oceanic crust derived from a mid ocean ridge or back arc basin and upper mantle that were emplaced as thrust slices into the lower slope of a west facing arc during the miocene and have been uplifted during arc arc collision

basement rocks of the halmahera region eastern indonesia a late cretaceous early tertiary arc and fore arc halmahera is a k shaped island located at the junction of several major arc trench systems of the western pacific eastern indonesia region western halmahera is an active volcanic arc above a zone of intense seismicity which characterizes the north molucca sea eastern halmahera has a basement of dismembered ophiolitic rocks with slices of mesozoic and eocene sediments overlain unconformably by middle oligocene and younger sedimentary and volcanic rocks the mesozoic and eocene sediments reveal notable stratigraphical and petrological similarities to the marianas fore arc and the eastern halmahera basement complex is interpreted as a pre oligocene fore arc lacking an accretionary complex there is some evidence that the pre oligocene volcanic arc behind this fore arc now forms part of the basement of western halmahera the mesozoic and tertiary sediments were imbricated together with igneous and metamorphic rocks representing the deeper parts of the fore arc during the late eocene plate reorganization event recognized throughout the western pacific margins the eastern halmahera basement complex can be traced into eastern mindanao probably further northwards in the eastern philippines and may be related to similar terranes within and around the present philippine sea plate in contrast the southern part of the island of bacan at the southwestern end of the halmahera group has a basement of continental metamorphic rocks associated with a deformed ophiolitic complex quite different to the basement of eastern halmahera the metamorphic rocks are interpreted to be part of the north australian continental margin basement which is separated from the halmahera basement complex by a splay of the sorong fault system and the deformed ophiolite complex of bacan is suggested to represent magmatism in the fault zone

origin and motion history of the philippine sea plate abstract the philippine sea plate is the one major plate whose tertiary motion is poorly constrained and whose origin is problematical its southern boundary is the sorong fault system which is part of a major left lateral fault system at the northern margin of the australian plate the southern part of the plate in eastern indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the daito ridge province of the northern philippine sea by the west philippine central basin the east indonesian islands of the halmahera waigeo region contain a good mesozoic and tertiary stratigraphic record indicating a long arc history for the southern part of the plate new palaeomagnetic data from these islands define two sub areas an area forming part of the philippine sea plate north of the sorong fault and an area within the sorong fault system the area north of the fault records a long term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge rocks of philippine sea plate origin within both areas record similar latitudinal shifts the rotation of the area north of the sorong fault is considered to represent the motion of the southern part of the philippine sea plate the new data indicate large tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested for the southern part of the plate there was 40 rotation with northward translation between 0 and 25 ma no significant rotation between 25 and 40 ma and there was 50 rotation with southward translation between 40 and 50 ma we show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate the translation history of the southern part of the plate in eastern indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate 15 n 160 e for the interval 5 25 ma and 10 n 150 e for the interval 40 50 ma reconstructions based on these poles predict that at 45 ma the palau kyushu ridge had a wnw ese orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the izu bonin marians forearc at this time the long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the west philippine basin by spreading in a backarc basin

lithospheric deformation within the molucca sea arc arc collision evidence from shallow and intermediate earthquake activity this paper presents the results of a local earthquake survey in the molucca sea arc arc collision zone of eastern indonesia although intermediate depth earthquakes define zones dipping to the west beneath the sangihe arc and to the east beneath the halmahera volcanic arc only shallow activity less than 60 km depth occurred beneath the central part of the collision zone where the majority of activity is observed the concentration of earthquake foci in the 10 to 50 km depth range in a limited region beneath the talaud mayu ridge suggests that convergence between the arcs proceeds by shortening within basement of the intervening molucca sea plate rather than by slip along shallow dipping planes between the arcs and the subducted slabs published focal mechanism solutions suggest that high angle reverse faulting beneath the talaud mayu ridge generates almost all of the seismic energy release within the collision zone the predominance of shallow reverse and strike slip faulting at the axis of the bilaterally subducting molucca sea lithosphere suggests that stresses are not due to bending alone and have a large horizontal regional component perpendicular to the island arcs the geometry and state of stress within the molucca sea lithosphere requires an external driving force for the convergence between the two arcs and a strong degree of coupling between the island arcs and the subducting molucca sea plate at least at the present stage of collision

investigating internal magnetic field gradients in aquifer sediments abstractinternal magnetic field gradients in porous materials if sufficiently large can be a source of error in nuclear magnetic resonance nmr measurements of the transverse relaxation time t2 and the diffusion coefficient d given that these measurements can provide information about the pore fluid and the pore geometry it is important to determine the magnitude of internal gradients and assess their potential impact we estimated the effective internal gradients in aquifer sediment samples using three methods we used a 2d nmr method to map the distribution of internal gradients versus t2 and found gradients up to 1000 g cm with peak gradient values in the range of 20 100 g cm for most of the samples the average effective gradient values calculated from the slope of the mean log relaxation rate versus the squared echo time typically fell above the peak gradient values in the 2d distributions with a range from 12 to 230 g cm the maximum effective gradients calculated from the magnetic suscep

the manokwari trough and the western end of the new guinea trench the new guinea trench is a sparsely surveyed seafloor depression which runs parallel to the north coast of new guinea for a distance of some 700 km its western end lies about 600 km east of the philippine trench the intervening region is occupied by a complex series of north trending ridges and troughs of which the ayu trough and the tobi and mapia ridges are the most prominent it has been suggested and is now widely accepted that the trench marks a site of subduction but the degree of present day activity is disputed the western termination has recently been defined by combined single channel seismic and long range sidescan sonar surveys as lying at the ridge system which culminates in mapia island the trench contains as much as 1 km of flat lying undisturbed sediments within the area surveyed and the southern slopes are extensively channeled suggesting a lack of recent deformation the same survey has also shown that for a distance of some 400 km west of the western end of the trench the north coast of new guinea is flanked at a distance of only a few tens of kilometers by a deep trough the sonar imagery of this manokwari trough suggests recent convergence as well as transcurrent movement both the existence of this trough and the abrupt termination of the new guinea trench are consequences of the seafloor spreading which took place in the ayu trough after subduction had ceased at the trench