new age constraints for the salamanca formation and lower rio chico group in the western san jorge basin patagonia argentina implications for cretaceous paleogene extinction recovery and land mammal age correlations the salamanca formation of the san jorge basin patagonia argentina preserves critical records of southern hemisphere paleocene biotas but its age remains poorly resolved with estimates ranging from late cretaceous to middle paleocene we report a multi disciplinary geochronologic study of the salamanca formation and overlying rio chico group in the western part of the basin new constraints include 1 an 40ar 39ar age determination of 67 31 0 55 ma from a basalt flow underlying the salamanca formation 2 micropaleontological results indicating an early danian age for the base of the salamanca formation 3 laser ablation hr mc icp ms high resolution multi collector inductively coupled plasma mass spectrometry u pb ages and a high resolution tims thermal ionization mass spectrometry age of 61 984 0 041 0 074 0 100 ma for zircons from volcanic ash beds in the penas coloradas formation rio chico group and 4 paleomagnetic results indicating that the salamanca formation in this area is entirely of normal polarity with reversals occurring in the rio chico group placing these new age constraints in the context of a sequence stratigraphic model for the basin we correlate the salamanca formation in the study area to chrons c29n and c28n with the banco negro inferior bni a mature widespread fossiliferous paleosol unit at the top of the salamanca formation corresponding to the top of chron c28n the diverse paleobotanical assemblages from this area are here assigned to c28n 64 67 63 49 ma 2 3 million years older than previously thought adding to growing evidence for rapid southern hemisphere floral recovery after the cretaceous paleogene extinction important peligran and carodnia zone vertebrate fossil assemblages from coastal bni and penas coloradas exposures are likely older than previously thought and correlate to the early torrejonian and early tiffanian north american land mammal ages respectively

the depth of trenches as a function of the subduction rate and age of the lithosphere abstract the trench is one of the most characteristic features of the subduction zones for a given arc or for all the subduction zones put together relationships between their depth variations the variations of the subduction rate and the age of the sinking lithosphere can be established certain other parameters have been examined with those mentioned above but without success it will be demonstrated that the observed relationships can be interpreted in terms of an elastic model and of a viscous model

relations among subduction parameters clues to the dynamics of the subduction process are found in the many measurable parameters of modern subduction zones based on a critical appraisal of the geophysical and geological literature 26 parameters are estimated for each of 39 modern subduction zones to isolate causal relationships among these parameters multivariate analysis is applied to this data set this analysis yields empirical quantitative relations that predict strain regime and strike slip faulting in the overriding plate maximum earthquake magnitude benioff zone length slab dip arc trench gap and maximum trench depth excellent correlation is found between length of the benioff zone and the product of convergence rate and age of the downgoing slab this relationship is consistent with the conductive heating model of molnar et al 1979 if the model is modified in one respect the rate of heating of the slab is not constant it is substantially slower during passage of the slab beneath the accretionary prism and overriding plate the structural style in the overriding plate is determined by its stress state though the stress state of overriding plates cannot be quantified one can classify each individual subduction zone into one of seven semiquantitative strain classes that form a continuum from strongly extensional class 1 back arc spreading to strongly compressional class 7 active folding and thrusting this analysis indicates that strain class is probably determined by a linear combination of convergence rate slab age and shallow slab dip interplate coupling controlled by convergence rate and slab age is an important control on strain regime and the primary control on earthquake magnitude arc parallel strike slip faulting is a common feature of convergent margins forming a forearc sliver between the strike slip fault and trench optimum conditions for the development of forearc slivers are oblique convergence a compressional environment and a continental overriding plate the primary factor controlling presence of strike slip faulting is coupling strongly oblique convergence is not required the rate of strike slip faulting is affected by both convergence obliquity and convergence rate maximum trench depth is a response to flexure of the underthrusting plate the amount of flexural deflection at the trench depends on the vertical component of slab pull force which is very sensitive to slab age and shallow slab dip shallow slab dip conforms to the cross sectional shape of the overriding plate which is controlled by width of the accretionary prism and duration of subduction deep slab dip is affected by the mantle trajectory established at shallow depth but may be modified by mantle flow much of the structural complexity of convergent margins is probably attributable to terrane juxtaposition associated with temporal changes in both forearc strike slip faulting and strain regime empirical equations relating subduction parameters can provide both a focus for future theoretical studies and a conceptual and kinematic link between plate tectonics and the geology of subduction zones

statistical tests of additional plate boundaries from plate motion inversions we have investigated the application of the f ratio test a standard statistical technique to the results of relative plate motion inversions the method tests whether the improvement in fit of the model to the data resulting from the addition of another plate to the model is greater than that expected purely by chance this approach appears to be useful in determining whether additional plate boundaries are justified we confirm previous results favoring separate north american and south american plates with a boundary located between 30 n and the equator using chase s global relative motion data we show that in addition to separate west african and somalian plates separate west indian and australian plates with a best fitting boundary between 70 e and 90 e can be resolved these results are generally consistent with the observation that the indian plate s internal deformation extends somewhat westward of the ninetyeast ridge the relative motion pole is similar to minster and jordan s and predicts the nw se compression observed in earthquake mechanisms near the ninetyeast ridge

seismic stratigraphy of western colombian basin caribbean sea multichannel seismic reflection profiles disclose the regional stratigraphy of the western colombian basin the basement complex is the seismic unit below the deepest continuous reflection horizon that can be traced throughout the basin the basement complex reflection signature on the flanks of the mono rise and adjacent areas is smooth continuous and characterized by local occurrences of internal reflectors and is equivalent to the late cretaceous horizon b in the venezuelan basin in the central basin the reflection signature is rough with abundant diffractions typical of normal oceanic crust the sediment overlying the basement complex is subdivided into five mapping units unit cb5 which directly overlies the basement complex is thickest on the mono rise and thins down the flanks of the rise this unit is equivalent to the upper cretaceous to the middle eocene pelagic unit bounded by seismic horizons a and b in the venezuelan basin unit cb4 characterized by pervasive small offset faulting is restricted to the crest of the mono rise units cb3 and cb2 contain subparallel variable amplitude continuous reflectors that fill the regional basement complex relief they are middle tertiary terrigenous distal turbidites and hemipelagic deposits unit cb1 thickens toward southern central america andmore shows complicated reflection patterns typical of a deep sea fan complex a jump correlation to deep sea drilling project site 154 is used to assign a late miocene to quaternary age to unit cb1 development of unit cb1 was concurrent with the uplift of and magmatic activity in southern central america less

geological manifestations of ridge collision evidence from the golfo de penas taitao basin southern chile recent geological and geophysical studies in the southern andes adjacent to the intersection of the chile rise with the peru chile trench ant naz sam triple junction have revealed a number of features and a neogene geologic history that are unique along the pacific margin of south america this history includes 1 development of a tertiary quaternary marine basin with up to 3 km of sediment infill golfo de penas taitao basin gtb 2 disruption of the region by a series of faults with both normal and strike slip movements and 3 localization of silicic near trench volcanism and epizonal plutonism and related hydrothermal activity the northern portion of the gtb began to subside in the late miocene possibly earlier and has subsequently been deformed uplifted and exposed gravity and seismic reflection data suggest that the basin continues offshore where it is still actively subsiding today golfo de penas subsidence and uplift have thus occurred diachronously in the region although it is unclear when subsidence began in the golfo de penas tectonic disruption of the region is likely related to the liquine ofqui fault lof a major ns trending crustal shear zone that curves westward and terminates in the golfo de penas the lof has both down to the west and right lateral offset and separates the main andean cordillera on the east from a large crustal block the chiloe block on the west we hypothesize that the gtb has developed as a pull apart basin in response to northward movement of the chiloe block along the lof we propose a dynamic model whereby a stress gradient that decreases longitudinally away from the chile rise peru chile trench intersection is set up because the youngest most buoyant oceanic lithosphere is being subducted at the triple junction the chile rise is viewed as a type of indenter which is acting to drive the chiloe block northward in front of the northward migrating triple junction this model explains the unique set of geologic features found in the region and suggests that ridge trench interactions may be an important factor in orogenesis at active continental margins

complexities in the tectonic evolution of the west philippine basin abstract the oceanic west philippine basin can be subdivided into a main basin a northwest sub basin and a southern sub basin these basins have undergone different tectonic histories magnetic anomalies 21 17 are present in the main basin and indicate that it formed by two limbed seafloor spreading the now extinct spreading center is located within a zone of rough topography associated with the northwest trending central basin fault cbf the morphology of the cbf is primarily the result of post spreading tectonism the magnetic anomaly pattern of the main basin does not extend into the southern sub basin or the northwest sub basin geometrical constraints indicate that the southern sub basin did not form as part of the southern limb of the main basin the southern sub basin may have formed immediately before a ridge jump to the main basin spreading axis alternatively possible shallower crustal depth in the southern sub basin may indicate that the sub basin is younger than the main basin and formed separately from it in the northwest sub basin which originated as part of the main basin the dominant topographic trends are perpendicular to those in the main basin this difference as well as higher heat flow and shallower depth in the northwest sub basin indicate that it has undergone a deformational event which did not affect the main basin this deformation may be related to subduction along the east luzon margin in mid tertiary time the gagua ridge an uplifted sliver of oceanic crust along the western edge of the northwest sub basin may have originated during this event

kurosegawa zone and its bearing on the development of the japanese islands abstract the kurosegawa zone in southwest japan is a 600 km long serpentinite melange in the chichibu terrains it runs generally e w but is slightly oblique to the subparallel arrangement of the ryoke sanbagawa and chichibu belts of southwest japan a variety of geological units occurs in the kurosegawa zone 1 1 granodiorite gneiss and amphibolite of ca 400 ma 2 2 siluro devonian formations 3 3 upper carboniferous to jurassic formations 4 4 upper jurassic to lower cretaceous formations 5 5 serpentinite and 6 6 low to medium grade metamorphic rocks of various baric types ages 220 320 360 and 420 ma by k ar the most widespread is a high pressure intermediate group of metamorphic rocks serpentinite is emplaced along the faults between and within the constituent units rocks of the kurosegawa zone represent a mature orogenic belt along a continental margin or an island arc its original site as constrained by paleomagnetism was near the equatorial area here 400 ma old paired metamorphism and related magmatism took place the island arc or microcontinent migrated northward to collide with the eurasia plate during late jurassic thus consuming the intervening ocean

collision rotation and the initiation of subduction in the evolution of sulawesi indonesia the island of sulawesi indonesia has been shaped and deformed as a result of collision with the sula platform a sliver of continental material from the northern margin of australia new guinea the collision has resulted in rotation of the north volcanic arm of sulawesi and the development of the accretionary wedge of the north sulawesi trench the north sulawesi trench changes laterally from a zone of no active deformation in the eastern part to a wide accretionary wedge in the west early stages of thrusting produce a steep frontal slope 8 16 indicative of relatively high basal shear stress whereas the more advanced western zone of thrusting produces a gentle 2 slope consistent with low basal shear stress reported paleomagnetic data suggest post late eocene counter clockwise rotation of the north arm and the offshore geophysics are explained by a pivot of the north arm with respect to the celebes basin about the eastern end of the arc convergence between the north banda basin and southeast sulawesi is documented by the presence of the tolo thrust its outcrop is strongly arcuate and its accretionary wedge varies in width from a minimum of a few kilometers at each end to a maximum of 30 40 km in the central part the northern end transforms to the leftlateral matano fault with a reported offset of 20 km the southern end of the thrust projects toward the deformed rocks of buton but the structural relations there are not clear the matano fault zone appears to connect westward with the palu fault which forms the western transform of the north sulawesi trench the palu matano fault system acts as a trench trench transform between the north sulawesi trench and the tolo thrust and this system is described by the same rotation pole as that for the sulawesi north arm

northern east pacific rise magnetic anomaly and bathymetric framework the oceanic crust in the eastern pacific between 7on and 30on and east of 127ow contains a fairly complete history of the spreading centers associated with the east pacific rise since 25 m y b p late oligocene in this paper we have summarized the seafloor spreading magnetic anomaly data and the bathymetric data that reflect the record of this tectonic history the well defined magnetic lineations north of the clarion fracture zone in the mouth of the gulf of california and on the east flank of the east pacific rise epr are carefully examined and used to provide a guide for interpreting the spreading pattern between the clarion and clipperton fracture zones southward of the rivera fracture zone over the mathematician ridge and over the entire epr east of the mathematician ridge between the rivera and siqueiros fracture zones the bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas the fracture zone bathymetry and the seafloor spreading magnetic lineations on the epr south of the rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations all these data provide a good record of the plate reorganizations in the middle miocene at magnetic anomaly 5a time 12 5 to 11 m y b p in the late miocene at magnetic anomaly 3 4 time 6 5 m y b p and in the pliocene at magnetic anomaly 2 3 time 3 5 m y b p several abandoned spreading centers including the mathematician ridge were left behind as a result of these reorganizations the mathematician ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5a

uranium lead isotopic ages of the samail ophiolite oman with applications to tethyan ocean ridge tectonics plagiogranites are a minor but widespread component of the samail ophiolite plutonic member they crystallized from the most fractionated melts generated by magmatic crystallization and differentiation of a steady state magma chamber beneath the tethyan spreading ocean ridge and their ages are thought to mark the time of ocean crust formation isotopic u pb ages of zircons from 13 plagiogranites collected along a 270 km segment of the samail ophiolite subparallel to the regional trend of the sheeted dike complex the former spreading ridge axis direction define a narrow time interval of 93 5 97 9 m y with a pronounced clustering about 95 m y the zircon ages of the plagiogranites agree remarkably well with the early cenomanian to early turonian biostratigraphic ages of sediments that are intercalated within the ophiolite pillow lavas and that lie just above them tippit et al 1981 the agreement of radiometric and biostratigraphic ages provides strong support for the conclusion that the plagiogranite u pb ages closely date the time span of ocean crust formation no step changes in age patterns are observed along the ridge axis sheeted dike direction suggesting that there are no major internal offsets of the ophiolite by transform or other faults along most of the traverse one possible exception occurs at the southeastern end of the sampled interval ibra area where a 3 m y discontinuity might be caused by an unmapped fault assuming that the regional trend of the sheeted dikes n10 25 w marks the direction of the former spreading ridge axis the present array of sample localities spans a distance of 130 to 195 km normal to that axis i e in the spreading direction the data as a whole do not define a clear cut age trend normal to the spreading axis but by eliminating samples that may be aberrant due to faulting the data array suggests a pattern of increasing ages from east to west this would indicate that the samail ophiolite is derived from the western southwestern side of the tethyan spreading system a conclusion reached independently from geological evidence also the gentle age gradient in the spreading direction i e between 130 and 195 km of ocean crust were formed within an age span of no more than 2 m y suggests a very fast spreading rate this conclusion also is supported by independent geologic evidence

thermal modeling of the geometry of subduction with implications for the tectonics of the overriding plate the path which subducting lithosphere follows can be broken into three zones zone i a subhorizontal region of nearly constant dip near the trench zone ii a region of rapidly increasing dip and zone iii a region generally deeper than 125 km again of nearly constant dip we have modeled the thermal evolution of slabs as they pass through these three zones using the method of lines a numerical technique well suited to the solution of time dependent partial differential equations by use of appropriate initial conditions and time dependent boundary conditions we can simulate subduction with variations in age of subducting lithosphere slab velocities and paths followed by the slab thermal modeling results indicate that the geometry of the slab in region ii can be empirically related directly to the thermal structure of the slab in that region and thus indirectly to age and velocity it follows that low angle subduction can extend several hundred kilometers inland only in cases of high convergent velocities 15 cm yr 1 and old 120 m y lithosphere we further suggest that rapid variations in the convergence velocities can provide an enabling mechanism for back arc spreading while slow changes in convergence velocities and or age variations in the subducting lithosphere lead to migration of volcanic arcs

a summary of cenozoic tectonic history along the ipod japan trench transect along the international program of ocean drilling ipod japan trench transect a mid cenozoic period of tectonism without arc volcanism separated an earlier cretaceous to paleogene and a later neogene period of plate convergence and subduction the cretaceous to early paleogene period of convergence is evidenced by andesitic volcanism associated with a large syncline probably a forearc basin the seaward flank of the basin is a tectonically thickened sediment sequence inferred to be an accretionary complex the arc the syncline and the thickened sediment sequence resemble an arc trench system of the size and structure of the great valley forearc basin and franciscan accretionary complex of california and they were probably continuations of the yezo geosyncline and associated sequences exposed to the north on hokkaido the arc trench system is contemporaneous with the adjacent shimanto arc trench system southwest of tokyo bay in the early paleogene volcanism ceased and part of the thickened sediment sequence of the cretaceous paleogene margin was emergent as a 160 km wide landmass which is recognized in seismic records as a later paleogene sediment source at the end of the paleogene this landmass began to subside and volcanism began again but it was located first briefly on the east and then on the west of the cretaceous arc along the present arc of northern honshu the land mass erosion surface is clearly visible as an angular unconformity in multichannel seismic reflection records the sequence of neogene sediment explosive volcanism and benthic foraminiferal assemblages recording subsidence from subareal conditions to the present bathyal depths have been studied in deep sea drilling project dsdp cores cretaceous subduction appears to have resulted in a very extensive accretionary complex whereas the neogene subduction period appears to have been marked by little net accretion in the forearc area but massive subsidence and some erosion of the front of the convergent margin

hotspots in the southern oceans an absolute frame of reference for motion of the gondwana continents abstract the geometry and geochronology of aseismic ridges and oceanic islands in the southern oceans provide a good test of the proposition that hotspots remain fixed over long periods of time that is motion of an order of magnitude less than the relative motion between plate pairs in most cases it is concluded that inter hotspot movement cannot be discerned for the period 100 m y to present and that widely distributed hotspots in the atlantic and indian oceans provide a frame of reference for plate motions following the disintegration of gondwanaland which is independent of paleomagnetism this frame of reference is absolute in that it gives the motion of the lithosphere with respect to the mantle hotspots the absolute motion model indicates that africa and antarctica are now moving only very slowly that there has been significant relative movement between east and west antarctica since the cretaceous and prescribes the relative motion between the somali and african plates

the opening of the woodlark basin subduction of the woodlark spreading system and the evolution of northern melanesia since mid pliocene time abstract magnetic anomaly and seismological data define segments of active seafloor spreading and associated magnetic lineations trending ene in the woodlark basin the total opening rate has been approximately 6 cm yr for the last 1 m y spreading rates diminish by over 10 from east to west along the woodlark spreading system implying a pole of current opening 15 20 to the west commencement of seafloor spreading in the basin has apparently been time transgressive beginning prior to 3 5 m y in the east and at successively later times to the west earthquake focal mechanisms and geological evidence suggest that the land areas bounding the western end of the woodlark basin are undergoing tensional deformation we believe that eventually the woodlark basin plate boundary will propagate westward through the d entrecasteaux islands into the papuan peninsula hitherto unreported shallow seismicity associated with the northern margin of the ne trending section of the woodlark rise probably reflects partial decoupling of the woodlark and solomon basins possibly due to mechanical difficulties in subducting the young woodlark lithosphere analysis of the relative motions between the solomon indo australian and pacific plates shows that the woodlark spreading system has been subducted at high rates 10 cm yr beneath the solomon islands during the opening of the woodlark basin several tectonic and geological features limited to the region of interaction of the woodlark basin with the solomon trench and arc may be symptomatic of ridge subduction these features include high heat flow in the solomon trench which shoals to 4 km low levels of seismicity and only shallow hypocenters and voluminous eruptions of high k 2 o tio 2 olivine basalts and basaltic andesites extremely close to the trench axis this close association in space and time of an unusual volcanic suite with ridge subduction implies a strong dependence of the petrogenesis on the tectonic regime a combination of this study of the woodlark basin and the previous study of the bismarck basin taylor 1979 provides a reconstruction of the positions of the continents ocean basins and island chains in northern melanesia for mid pliocene time in accepting the existence of a solomon plate we can explain the trench like structure off the trobriand margin of new guinea the occurrence of late cenozoic calc alkaline volcanism along the papuan peninsula and the presence of intermediate depth seismicity beneath the north papuan peninsula the rapid changes in relative motions along or across the new ireland solomons chain over the past 3 5 m y may explain the spatial and temporal changes in igneous activity observed on these islands

seismotectonics of subduction and back arc rifting under the andaman sea abstract active underthrusting of the indian plate below the burma plate produces seismic activity under the andaman and burmese arcs it is suggested that significant differences exist in the configuration of the benioff zone underlying the two arcs several parallel and linearly extensive fault zones are outlined by epicentral distribution in the andaman arc the andaman back arc spreading ridge is defined by a relatively narrow shallow seismic zone that orients nne to sse in north and south parts of the andaman sea respectively the spreading ridge is offset by active transform faults on the far north the spreading ridge appears to connect to a major transform fault in burma across the burmese coast focal mechanism study provides evidence for tensional stress near the inner wall of the andaman trench dominant compressive deviatoric stress within the overriding burma plate and tensional stress under the back arc spreading ridge

neocene and quaternary extent and geometry of the subducted pacific plate beneath north island new zealand implications for kaikoura tectonics abstract the extent and geometry of the obliquely subduced oceanic pacific plate beneath north island new zealand for five million year intervals through the mid miocene to quaternary are presented in a series of maps and cross sections these show that the subducted plate progressively increased its extent from ne to sw beneath the north island and in the more northern regions where it was first emplaced concomitantly increased its dip from 10 to 50 the changing extent and geometry of the subducted slab has been established from the age pattern of orogenic andesites and from the geochemical k 2 o h parameter of depth of magma generation the radiometric dates show a migration of the volcanic front back towards the trench at an average rate of 20 km my the trenchward migration is explained by a model of increasing slab dip which is corroborated by the k 2 o data calibrated against the presently active arc taupo volcanic zone with the exception of northern coromandel peninsula the andesitic magmas were generated at 85 100 km depth the interpretation of the dates adopted here indicates that the subducted slab originated at the ne sw trending kermadec hikurangi trench and implies a different and much simpler evolution of the australia pacific plate boundary in the vicinity of north island than other recent models subduction geometry has been found elsewhere to be a principal influence upon the state of stress and deformational style in an over riding plate the possibility is explored that the timing nature and pattern of the neogene to quaternary kaikoura orogeny in north island is due to this influence apart from the effect of oblique subduction in eastern north island there is an accord between the onset of deformation and the emplacement sequence of the shallow slab beneath north island and between the change in subduction geometry and a progressive north to south change in northern north island from compression to extension

variations in the velocities of the major plates since the late cretaceous abstract we present models for the relative and absolute velocities of the major plates from 80 m y ago to the present as tests of several alternative plate reconstructions and as a guide in the evaluation of the hypothesis that pronounced changes in plate speeds or directions arise directly from changes in the configuration of plate boundary forces digitized plate boundaries were rotated backwards in time using published rotations to determine positions at 5 m y intervals back to the late cretaceous the oceanic plate boundaries were then corrected so as to properly match magnetic anomalies relative plate velocities were obtained by differencing finite rotations absolute velocities were determined by assuming that no net torque is exerted on the lithosphere rms absolute speeds of each plate were calculated as functions of time as a conceptually simple measure of plate velocities and accelerations we have tested several possible reconstructions for the antarctic and pacific plates proposed to explain the failure of simple rigid antarctic and or pacific plate models to match paleomagnetic data hotspot tracks and equatorial crossings defined by pelagic sediment deposition we find that 1 1 plate reconstruction models with either two antarctic or two pacific plates in the early tertiary are distinguishable from models with single antarctic and pacific plates principally on the basis of faster speeds for the kula plate and the northern pacific greater areas for the kula and farallon plates and faster subduction at the aleutians and along the west coast of south america for that time 2 2 oceanic plates as a group have moved faster than those with continental lithosphere only since 50 m y ago active subduction appears to be a sufficient condition for rapid plate motion regardless of the fractional plate area comprised of continental lithosphere 3 3 the absolute motion of india in the late cretaceous was anomalously fast 15 20 cm yr compared to the speeds of the present plates 4 4 the rms velocities of a number of plates and of the lithosphere as a whole are much higher in the late cretaceous than through most of the cenozoic 5 5 the predicted apparent polar wander for north america is in fairly good agreement with paleomagnetic data with the slight discrepancy attributable to a net contribution to the torque balance from plate boundary forces

earthquakes plate subduction and stress reversals in the eastern aleutian arc plate subduction beneath the 1500 km long segment of the eastern aleutian arc between kodiak and atka islands 154 w and 176 w longitude is studied with observations from teleseismic data the primary data base consists of hypocenters of earthquakes for the period 1965 1975 carefully selected from the bulletins of the international seismological centre and of 44 new focal mechanism solutions the principal results of this study are that hypocenters of intermediate depth earthquakes in the eastern aleutians appear to define a weakly developed double seismic zone at depths between 70 and 170 km additional evidence for a double seismic zone comes from focal mechanisms which generally show downdip directed p axes for earthquakes in the upper zone and downdip directed t axes in the lower zone major features of the double zone can be explained by thermoelastic stresses in the downgoing plate the observed predominant downdip stress polarity at intermediate depths in the descending plate reverses along strike of the arc this stress reversal coincides in map view with a change from a continental to an oceanic arc the coincidence may result from spatial differences either in the coupling between the plates at shallow depths or in the rheology of the surrounding oceanic versus continental mantle alternatively the stress reversal may be related to the time since the last great earthquake if the observed stress polarity relates to the elapsed time since the last great earthquake it may be a significant observation for long to intermediate term prediction of great thrust earthquakes in subduction zones portions of the eastern aleutian arc where downdip tension predominates contain one or more seismic gaps that appear to have a high probability for great earthquakes in the next few decades a moderate sized bathymetric feature in the pacific plate the amlia fracture zone modifies subduction and produces a bend or offset in the downgoing plate as well as an offset in the line of active volcanoes in the upper plate subduction of the fracture zone appears also to cause formation of summit basins on the arc platform and some aseismic behavior of a 100 km long segment of the main thrust zone

flow models for back arc spreading abstract a secondary flow model for back arc spreading is developed in this paper that shows some of the characteristics of observed back arc spreading back arc spreading has formed marginal seas around the west and southwest rim of the pacific the episodic spreading and different directions of opening are not completely understood however there does appear to be a limited lifetime

a back arc basin formed within continental lithosphere the central volcanic region of new zealand abstract the central volcanic region is an area of quaternary volcanism and tensional tectonics within the north island of new zealand that appears to be a direct continuation of a young oceanic back arc basin the havre trough into the continental structure of new zealand volcanism is predominantly rhyolitic yet the region possesses many of the geophysical characteristics often associated with oceanic back arc basins upper mantle seismic velocities are low pn and sn velocities of 7 4 and 3 95 km s respectively the crust is anomalously thin for its continental setting and the heat flow almost all of which is expressed in the discharge of hot water through geothermal systems is about 700 mw m2 i e some twelve times greater than normal continental heat flow in order to account for such a high heat flow a large scale mass transfer of heat is required it is proposed therefore that the central volcanic region represents a site of active back arc spreading within a continental lithosphere

late permian early triassic paleogeography of northern japan did pacific microplates accrete to japan extensive and detailed field studies were conducted in the kitakami mountains of northern japan in order to sample permian rocks and to make paleogeographic reconstructions for permian time these field studies demonstrate that large landmasses were present on both sides of the honshu geosyncline during permian time i e on both the pacific ocean and the sea of japan sides these landmasses appear on the basis of sediment studies to have been present during permian mesozoic and paleogene time the paleogeographic reconstructions presented here and papers by other authors show that neither the accretion of microplates from the pacific nor major horizontal movements of the japanese part of tethys occurred in mesozoic time

on the evolution of marginal basins this paper presents a critique of the models proposed to explain the formation of back arc basins those marginal basins located behind active or inactive trench systems and whose origin is inferred to be subduction related several marginal basins are not generically back arc but rather are related to the initiation of subduction continental rifting or plate boundary readjustments models of back arc basin formation must be consistent with the observed characteristics of back arc basins which include the following 1 the crustal structure and magnetic lineation patterns of back arc and major ocean basins are similar implying that the processes of crustal accretion in both tectonic environments are also similar 2 arc trench systems without currently spreading back arc basins are in the majority 3 although petrographically similar to and within the compositional range of midocean ridge basalts morb back arc basin basalts babb show consistent geochemical differences from n type morb and are in many respects transitional toward island arc tholeiites iat 4 distinct babb and iat mantle sources may be in extremely close proximity during the initial rifting of an island arc 5 rifting of the volcanic arc may occur on either side of the line of active volcanoes 50 km and may vary from one side to the other along strike 6 a remnant arc is not always developed 7 back arc spreading may be initiated above subducted lithosphere but with time the center of spreading migrates away from the arc and no longer overlies a benioff zone and 8 there is no simple correlation between the timing of global plate reorganizations and the formation of back arc basins the three main classes of models proposed to explain back arc basin formation are mantle diapirism induced aesthenospheric convection and global plate kinematics because the driving mechanism for the first two classes of models is provided by the local subducting slab these models fail to explain the temporal and spatial distribution of back arc basins the third class of models proposes that back arc basins should form whenever global plate interactions require divergence between the overriding plate and the trench line the latter being observed to move only in a seaward direction with respect to a hot spot reference frame however the tectonic setting of several back arc basins suggests that they represent more than just a passive response to kinematic boundary conditions for example some dynamic back arc force seems required to move the new hebrides tonga and south sandwich forearc plates seaward neither the local nor the global models adequately specify the necessary and or sufficient conditions for back arc basin formation a better understanding of the processes associated with the initiation of rifting and of subduction is required arc rifting and back arc spreading are both secondary phenomena to subduction but they are also primary extensional phenomena fundamentally analogous to continental rifting and the initial spreading of major ocean basins

deformation in moffat shale detachment zones in the western part of the scottish southern uplands a study of the d ecollement zones in the moffat shale group in the ordovician northern belt of the southern uplands of scotland reveals a progressive sequence of deformation and increased channelization of fluid flow the study concentrates on exposures of imbricated moffat shale on the western coast of the rhins of galloway initial deformation occurred in partially lithified sediments and involved stratal disruption and shearing of the shales deformation then became more localized in narrower fault zones characterized by polyphase hydrothermal fluid flow veining events deformation continued after vein formation resulting in the development of low temperature crystal plastic microstructures and further veining late stage deformation is recorded as a pressure solution event possibly reflecting the cessation of slip on these faults as the slice became accreted most deformation can be ascribed to se directed thrusting and incorporation of the individual sheets into the southern uplands thrust stack later sinistral shear deformation not observed in overlying turbidites is also localized in these fault zones the study reveals the likely structures formed at levels of an accretionary prism deforming under diagenetic to low grade metamorphic conditions