zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

paleogene evolution of the kodiak islands alaska consequences of ridge trench interaction in a more southerly latitude the kodiak islands of alaska are principally composed of a late cretaceous and paleogene complex of accreted deep sea rocks an early paleocene magmatic event invaded the accretionary complex with mid ocean ridge basalt morb andesite and granodiorite geochemical evidence suggests both the andesite and granodiorite are mixtures of morb and sediment the volcanic rocks are interlayered with coarse clastic sediments and deformed with the latter the granodioritic intrusions with k ar ages as old as 62 ma postdate emplacement of the early paleocene sedimentary and volcanic rocks interaction of the subduction complex with the kula farallon ridge most simply explains this early paleocene magmatic episode paleomagnetic studies of the volcanic rocks indicate this interaction occurred at 40 3 6 north latitude which is 25 3 9 south of the expected position of the kodiak islands relative to north america the magmatic arc presently northwest of the kodiak accretionary complex was active during the late cretaceous and early paleocene diminished after the ridge trench interaction and resurged from late eocene into miocene time the magmatic resurgence is coeval with the obductive offscraping of eocene to oligocene submarine fan deposits in the kodiak islands the petrology of these fan deposits indicates they are the most likely proximal equivalent for the zodiac fan thereby limiting its relative motion with respect to the kodiak islands possible models accounting for the northward motion of the kodiak accretionary complex include 1 coupling the accretionary complex and possibly related arc to the northward moving kula plate and closing an ocean basin north of the arc in paleogene time 2 emplacing the kodiak accretionary complex at the northward limit of the paleomagnetic data with subsequent northward motion accomplished by closure of a small back arc basin and or intracontinental shortening and 3 transcurrent faulting and northward motion of the kodiak accretionary complex without the presently adjacent arc

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

paleomagnetic rotation pattern of the southern chile fore arc sliver 38 s 42 s a new tool to evaluate plate locking along subduction zones the chile fore arc at 37 s 47 s represents the coseismic deformation zone of the 1960 mw 9 5 valdivia earthquake here we report on the paleomagnetism of 43 oligocene pleistocene volcanic sites from the fore arc sliver between 38 s and 42 s sites were gathered west of the 1000 km long liquine ofqui dextral fault zone lofz that represents the eastern fore arc sliver boundary nineteen reliable sites reveal that the fore arc is characterized by counterclockwise ccw rotations of variable magnitude except at 40 s 41 s where ultrafast 50 myr clockwise cw rotations occur within a 30 km wide zone adjacent to the lofz ccw rotation variability even at close sites and rapidity 10 myr suggest that the observed block rotation pattern is related to nw se seismically active sinistral faults crosscutting the whole fore arc according to previously published data cw rotations up to 170 also occur east of the lofz and have been related to ongoing lofz shear we suggest that the occurrence and width of the eastern fore arc sliver undergoing cw rotations is a function of plate coupling along the subduction zone interface zones of high coupling enhance stress normal to the lofz induce high lofz strength and yield a wide deformation zone characterized by cw rotations conversely low coupling imply a weak lofz a lack of cw rotations and a fore arc entirely dominated by ccw rotations related to sinistral fault kinematics our locking inferences are in good agreement with those recently derived by gps analysis and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 ma

archean plate tectonics constraints and inferences the earth has been cooling since archean time the higher temperatures beneath archean ridges resulted in more partial melting which extended down to greater depths than at present the archean oceanic crust was much thicker 20 km than modern crust 5 km this inference is compatible with previous ideas of suspected oceanic crust the thicker oceanic crust in the archean tended to resist subduction similar to modern aseismic ridges but could not prevent it short lived episodes of intra arc spreading followed by cordilleran type compression may have produced archean greenstone belts archean meta tonalites represent a significant part of the continental crust and were probably produced by melting of the subducted slab which was favored by the higher mantle temperatures

archaean plate tectonics revisited 1 heat flow spreading rate and the age of subducting oceanic lithosphere and their effects on the origin and evolution of continents a simple model which relates the rate of seafloor creation and the age of the oceanic lithosphere at subduction to the rate of continental accretion can successfully explain the apparent differences between archaean and phanerozoic terrains in terms of plate tectonics the model has been derived using the following parameters 1 the spreading rate at mid ocean ridges 2 the age of the oceanic lithosphere at the time of subduction 3 the area age distribution of the seafloor 4 the continental surface area as a fraction of the total surface area of the earth and 5 the erosion rate of continents as a function of continental surface area and the total number of continental masses observations in phanerozoic terranes suggest that there are profound differences in the nature and volume of subduction zone igneous activity depending upon the age of the oceanic lithosphere being subducted and the nature of the overriding plate that is either continental or oceanic the subduction of young oceanic lithosphere less than 50 m y old which is thermally buoyant appears to result in a reduced volume of igneous activity most of the igneous activity caused by subduction of young oceanic lithosphere is either siliceous plutonism or bimodal tholeiitic rhyolitic volcanism when very young lithosphere is being subducted 50 m y old appears to result in greater volumes of igneous activity including the eruption of andesitic magmas thus andesites could only begin to be abundant in the rock record when older oceanic lithosphere began to be subducted our model predicts that as the earth aged and as heat flow from the interior of the earth diminished the proportion of old oceanic lithosphere being subducted increased fundamentally changing the nature of subduction zone igneous activity and the rate of continental accretion if the subduction of old oceanic lithosphere results in an 8 10 times greater volume of subduction zone magmatism our model predicts or explains all of the following observed features of earth history 1 archaean terranes appear to record two periods of rapid continental accretion between 3 8 and 3 5 b y ago and between 3 1 and 2 6 b y ago 2 there are very few differences and many marked similarities between rocks from archaean terranes and equivalent rocks from phanerozoic terranes 3 the total continental area appears to have remained essentially constant for the past 2 b y 4 archaean andesites are comparatively rare and the relative abundances of mafic and siliceous rocks appear to change during the archaean and the proterozoic with siliceous volcanics becoming proportionately more abundant in the geologic record with time 5 plutonic tonalites and trondhjemites appear to have been relatively much more abundant during the archaean plate tectonics is thus shown to have evolved over time due to a gradually decreasing rate of creation of oceanic lithosphere meaning that archaean tectonics and phanerozoic tectonics are but two points on an evolutionary continuum

growth and differentiation of the continental crust declining radiogenic heat production since the archaean has resulted in a secular evolution from a regime of numerous fast moving small thin torsionally weak plates to the present regime of larger thicker torsionally stronger plates moving at an average rate of less than one sixth of the archaean rate this has been accompanied by episodic changes in geological effects by 2500 ma b p about 85 of the present crustal mass had grown by the addition and amalgamation of mafic and calc alkaline rocks in oceanic arcs at an average rate of 11 17 pg a during the early proterozoic the first large cratons were stabilized and locally thickened and differentiated the proterozoic was an era of little continental growth falling average sea level and intracontinental deformation by 700 ma b p cratons had become much more stable marginal accretionary terrians had begun to develop with an average phanerozoic growth rate of 1 64 pg a and blueschists and ophiolites sensu stricto witness the advent of the extant plate tectonic regime

the cenozoic subduction history of the pacific margin of the antarctic peninsula ridge crest trench interactions new magnetic anomaly identifications w of the shackleton fracture zone show 5 spreading sections separated by fracture zones in the 2 most southerly the ridge crest collided with a trench at the margin of the antarctic peninsula only 6 5 and 4 ma ago the latest of a series of collisions starting at the base of the peninsula so ma ago following each collision spreading and subduction both stopped opposite the south shetland trench and actively extending bransfield strait spreading also stopped 4 ma ago but before the spreading centre reached the trench the tendency of the subducting plate to continue sinking probably initiated the opening of the bransfield strait the parallelism between fracture zone orientation on the descending plate and convergence direction previously thought responsible for the tectonic segmentation of the peninsula was effective for only 10 30 ma before collision before that either the fracture zones did not extend to the trench or the subducting ocean floor formed at the farallon phoenix nazca aluk boundary with fracture zones oblique to the subduction direction the aleutian arc kula ridge model in which arc magmatism virtually ceases when ocean floor younger than 25 30 ma occurs beneath the arc fits the distribution of antarctic peninsula radiometric dates explaining a 50 60 ma gap between collisions and youngest ages in the s and possibly the migration of the youngest activity towards the trench thus gaps in the geologic record of arc magmatism need not imply cessation of subduction the progressive steepening of the peninsular margin towards the 4 ma collision site suggests tectonic erosion of the fore arc as the ridge crest approached

rapid growth of some major segments of continental crust some major segments of continental crust display a narrow range of sm nd crustal formation ages the sizes of the canadian shield the svecokarelian province of northern europe the west central united states and the arabian nubian shield suggest rapid crustal growth island arc accretion models rank among the most favored tectonic models for the formation of these areas a quantitative comparison of the growth rates of these crustal segments to mesozoic cenozoic arc addition rates shows however that island arc accretion alone seems insufficient to account for the amount of crust that was produced in each of these terrains other additional mechanisms such as hot spot volcanism and underplating may have been active in addition to arc accretion alternatively large amounts of preexisting basement have gone so far undetected

recent movements of the juan de fuca plate system analysis of the magnetic anomalies of the juan de fuca plate system allows instantaneous poles of rotation relative to the pacific plate to be calculated from 7 ma to the present by combining these with global solutions for pacific america and absolute relative to hot spot motions a plate motion sequence can be constructed this sequence shows that both absolute motions and motions relative to america are characterized by slower velocities where younger and more buoyant material enters the convergence zone pivoting subduction the resistance provided by the youngest portion of the juan de fuca plate apparently resulted in its detachment at 4 ma as the independent explorer plate in relation to the hot spot framework this plate almost immediately began to rotate clockwise around a pole close to itself such that its translational movement into the mantle virtually ceased after 4 ma the remainder of the juan de fuca plate adjusted its motion in response to the fact that the youngest material entering the subduction zone was now to the south differences in seismicity and recent uplift between northern and southern vancouver island may reflect a distinction in tectonic style between the normal subduction of the juan de fuca plate to the south and a complex underplating occurring as the explorer plate is overridden by the continent the history of the explorer plate may exemplify the conditions under which the self driving forces of small subducting plates are overcome by the influence of larger adjacent plates the recent rapid migration of the absolute pole of rotation of the juan de fuca plate toward the plate suggests that it too may be nearing this condition

faster spreading or greater ridge length in the archean dissipation of more heat from a hotter early earth by the plate tectonics mechanism would require subduction of on average younger lithosphere in principle this could be achieved equally well by faster spreading by having greater ridge length or some combination thereof present day evidence however suggests that younger oceanic lithosphere subducts more slowly than older if this applied in the archean then the greater ridge length alternative prevailed a quantitative expression is derived which indicates that oceanic heat loss is proportional to the cube root of ridge length this relationship means that if archean heat flow was 3 times that of the present 27 times as much ridge would have been required which would suggest that the archean earth was covered by many small plates moving slowly

fluvial incision into bedrock insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys western alps france bedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief using digital elevation models topographic maps and field reconnaissance we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the romanche watershed french western alps longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches we reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat from morphometric analyses we find that mean channel gradients and widths as well as knickpoint retreat rates display a drainage area dependence modulated by bedrock lithology however there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints assuming a postglacial origin of these gorges our results imply high postglacial fluvial incision 0 5 15 mm yr 1 and knickpoint retreat 1 200 mm yr 1 rates that are however consistent with previous estimates numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges results from simple end member models suggest transport limited behavior of the bedrock gorges a more sophisticated model including dynamic width adjustment and sediment dependent incision rates predicts present day channel geometry only if a significant supply of sediment from the gorge sidewalls 10 mm yr 1 is triggered by gorge deepening combined with pronounced inhibition of bedrock incision by sediment transport and deposition

pliocene near trench magmatism in southern chile a possible manifestation of ridge collision eight shallow level intrusions and an ophiolite complex have recently been discovered in a remote region along the pacific coast of southern chile known as the taitao peninsula the magmatic rocks are found only 10 to 15 km from the buried extension of the peru chile trench the ophiolite body includes serpentinized ultramaflc rocks gabbro dikes and an interbedded sequence of volcanic and marine sedimentary rocks more regionally distributed granodiorite plutons and porphyritic stocks and sills intrude pre late jurassic basement as well as tertiary marine sequences of the south american forearc k ar ages of 3 5 to 4 0 ma of the ophiolite and other intrusions suggest that these rocks represent a short lived pulse of pliocene magmatism directly seaward of the taitao peninsula a segment of the chile rise bounded by the tres monies and taitao fracture zones collided with the chilean forearc and was subducted about 2 5 to 4 0 ma because of this good spatial and temporal correlation the magmatic activity of the taitao peninsula is likely to be a direct result of ridge subduction

an early miocene ridge crest trench collision on the south scotia ridge near 36 w abstract preliminary interpretation of marine geophysical data and geochemical analyses of dredged rocks has led to the identification of a ridge crest trench collision zone southeast of the south orkney islands the existence of a series of such collision zones along the south scotia ridge had earlier been hypothesised to explain changes in scotia sea evolution the collision zone takes the form of a double ridge with dissecting trough aligned ne sw and about 200 km long oceanic magnetic lineations in the northern weddell sea young northwestward towards the zone with the youngest identified as anomaly 6 20 ma reaching the central trough volcanics dredged from jane bank the northwestern ridge are chemically very similar to the more siliceous of the low k tholeiite series which characterises the presently active south sandwich arc jane bank is interpreted as an in situ remnant of the arc and upper fore arc of an intra oceanic arc produced by subduction of south american oceanic lithosphere like the present south sandwich islands part of the upper fore arc and all of any accretionary wedge appear to have been subducted unlike ocean floor at collision zones off chile and the antarctic peninsula the seabed in the trough and on the ridge to the southeast does not conform to the normal oceanic age depth relationship the estimated collision time 20 ma is close to the time of onset of n s extension in the central scotia sea the limited data set described here however does not establish the synchroneity of the collision over the entire 200 km of jane bank so an assessment of the extent of any causal relationship between the two events must await the analysis of a more extensive survey

ridge collision at convergent margins implications for archean and post archean crustal growth abstract growth of continental crust today occurs dominantly at convergent plate margins through magmatic addition and tectonic accretion lateral and vertical related to 1 1 steady state subduction 2 2 non steady state episodic processes ridge forearc collision is one episodic process that can cause significant highly diachronous thermal and tectonic effects when a buoyant bathymetrically high and thermally active oceanic spreading ridge approaches collides with and potentially subducts below a continental margin near field effects within 100 km of collision site in the forearc may include anomalous heat anomalous magmatism ophiolite obduction forearc deformation and tectonic erosion of the base of the forearc far field effects generally postdate collision and may include changes in the thermal structure of the continent changes in the composition of arc magmas a reduction or hiatus in arc magmatism uplift and or subsidence of the continental margin marginal basin formation underplating of subducted slabs and intraplate deformation effects of ridge collision can vary depending on the triple junction geometry the chile margin triple junction cmtj where the actively spreading chile ridge is colliding with the south american forearc is the best modern example of ridge collision near field effects of pliocene recent ridge collisions near the cmtj include 1 1 emplacement of the pliocene taitao ophiolite less than 15 km from the trench 2 2 intrusion of pliocene granitic plutons and 3 3 forearc deformation formation of a pull apart basin on the continental shelf and late tertiary recent movement on a major fault along the arc forearc boundary far field effects include reduction in subduction related seismicity and volcanism chemical changes in arc magmatism and possibly uplift and or subsidence ridge collison features in southern chile are similar to features of archean greenstone belts suggesting that some greenstone sequences may have formed in a ridge collision setting and that ridge collision may have been an important process in archean crustal growth these features include granitic rocks closely associated with ophiolitic rocks formed near a continental margin mafic ultramafic section overlain by basaltic to andesitic volcanic rocks of mixed calc alkaline and morb character intercalated with volcaniclastic sedimentary rocks differences between archean and phanerozoic tectonic processes mainly related to higher heat production in the past probably have affected the nature and importance of ridge collision in archean crustal growth during the archean greater ridge length and smaller plates probably resulted in more ridge collisions and related processes contributing to a higher continental growth rate in particular underplating of subducted slabs and direct slab melting may have been important processes in archean continental growth but are of relatively minor importance today

emplacement of the resurrection peninsula ophiolite in the southern alaska forearc during a ridge trench encounter the 57 1 ma resurrection peninsula ophiolite of southern alaska s chugach terrane formed in a near trench environment as the kula farallon ridge was being subducted beneath north america the magmatic sequence includes pillow lavas sheeted dikes gabbros trondhjemites and a poorly exposed ultramafic section the lavas show mid ocean ridge basalt and arc like geochemical signatures interpreted to reflect compositionally diverse melts derived from near fractional melting of a variably depleted mantle source mixed with variable amounts of assimilated continentally derived flysch a sedimentary sequence overlying the ophiolite here named the humpy cove formation of the orca group preserves a continuous 3 6 plusmn 1 4 ma record of turbidite sediments that were deposited on the ophiolite as it was transported to north america and emplaced in the chugach accretionary prism a transition from thinly bedded turbidites at the base of the section to thickly bedded turbidites is interpreted as a distal to proximal facies change recording the migration of the ophiolite toward the continent chemical trends vertically through the sedimentary sequence show an increasing biogenic component diluting terrigenous matter 950 m above ophiolitic basalt signaling a change to more favorable biologic conditions or increased preservation of biogenic matter with proximity to the continent hydrothermal metals are not resolvable in sediments interbedded with pillow lavas or in sediments above the ophiolite suggesting that they were diluted by turbidite sedimentation the top of the section is truncated by the fox island shear zone a 1 km thick greenschist facies west over east thrust related to the emplacement of the ophiolite into the accretionary wedge the fox island shear zone is intruded by a 53 4 0 9 ma granite showing that the ophiolite formed was transported to the north american continent overthrust by a major accretionary prism related thrust and intruded by granite all within 3 6 1 4 ma using these estimates for the timing of formation and emplacement of the ophiolite with palinspastic reconstructions and plate motion vector triangles we suggest that the resurrection peninsula ophiolite was transported up to 500 km 461 79 km relative to north america from the place of its formation at the kula farallon ridge prior to its emplacement into the north american margin since the best estimates of the amount of translation of the chugach terrane come from paleomagnetism of the resurrection peninsula ophiolite current estimates of margin parallel strike slip need to be reduced considerably

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

lithological structural and geochemical characteristics of the mesoarchean târtoq greenstone belt southern west greenland and the chugach prince william accretionary complex southern alaska evidence for uniformitarian plate tectonic processes the mesoarchean t rtoq greenstone belt southern west greenland consists of tectonically imbricated slices of metamorphosed basalt gabbro peridotite and sedimentary rocks and is intruded by fel

laramide crustal thickening event in the rocky mountain foreland and great plains the rocky mountain foreland and great plains of the western united states were formerly part of a continental platform adjusted by erosion and deposition in cambrian through jurassic time to near mean sea level and therefore to a near uniform crustal thickness of approximately 33 km today the region stands at regional elevations up to 2 km isostatically supported by a crust exceeding 50 km in thickness reasonable estimates of tertiary sedimentation and laramide strain do not account for more than 15 of the implied thickening however from approximately 70 40 m y b p this region was underlain by a horizontally subducting slab of farallon plate lithosphere moving northeast this slab could have been the cause of the thickening finite difference thermal models with specified kinematics show only a temporary cooling of the base of the north american lithosphere by this slab the excess weight of the slab would have depressed the region plate bending calculations show a quantitative agreement of predicted depression with upper cretaceous isopachs since depression by this slab lasted until the eocene at least the latest cretaceous regression was probably caused by a laramide crustal thickening event the farallon slab might have caused crustal thickening in two ways its excess weight would have drawn in ductile lower crust from surrounding regions however calculations show that this effect is too slow too local and too reversible to explain most of the crustal thickening therefore it seems likely that ductile lower crust was transported from sw to ne by shear stresses which the farallon plate exerted on the base of the north american lithosphere a preliminary finite element calculation based on this hypothesis shows the correct general pattern of crustal thickening an unexpected but encouraging result is that predicted principal compression directions are orthogonal to many laramide basement uplifts

arrival angle anomalies across the usarray transportable array abstract we construct composite maps of surface wave arrival angle anomalies using clustered earthquakes and an array method for measuring wave front geometry this results in observations of arrival angles covering the entire footprint of the usarray transportable array during 2006 2010 bands of arrival angle deviations in the propagation direction indicate the presence of heterogeneous velocity structure both inside and outside of the array we compare the observed patterns to arrival angles predicted using two global tomographic models the mantle model s362ani and the surface wave dispersion model gdm52 we use both ray theory based prediction methods and measurements on synthetic data calculated using a spectral element method both models and all prediction methods produce similar mean arrival angles and long wavelength patterns of anomalies which are similar to the observations predicted short wavelength features generally do not agree with the observations the spectral element method produces some complexity that is not obtained using the ray theory based methods this predicted complexity is similar in character to the observed patterns but does not match them

interaction between the chile ridge and chile trench geophysical and geothermal evidence geophysical and geothermal data are examined from the three southernmost sections of the chile ridge starting at 44 s and continuing south to the triple junction of the nazca antarctic and south america plates at 47 s these sections represent three progressively younger stages in a ridge trench collision event corresponding to 3 m y before the collision 1 m y before the collision and culminating in an ongoing collision at the triple junction magnetic and bathymetric data across the chile rise indicate that there is little change in the configuration of the spreading center as the ridge approaches and then collides with the trench in the collision zone a normal looking rift valley can be traced for 40 km before it disappears beneath the toe of the landward trench slope there is no evidence for the complex pattern of ridge jumps and spreading center rotations that occurred when the pacific farallon spreading center collided with north america in contrast the overriding plate is greatly affected by the ridge collision the landward trench slope steepens and narrows as the collision zone is approached at the southern end of the collision zone a ridge associated with the taitao fracture zone is colliding with the trench and may be in the process of being obducted onto the landward trench slope geothermal measurements were made along three transects of the margin corresponding to the time 3 m y before the collision during the collision and 6 m y after the collision the heat flow measurements in the collision zone document a large pulse of heat associated with the subduction of the ridge values as high as 345 mw m2 were recorded on the lower trench slope a prominent bottom simulating reflector bsr observed over a wide area of the landward trench slope north of the triple junction and more locally south of the triple junction is used to expand the grid of heat flow observations excellent agreement is found between measured heat flow and estimates of heat flow based on the depth to the bsr the heat flow measurements compare favorably with a theoretical model assuming conductive heat flow we estimate that the accretionary prism has been substantially removed in the collision zone and conclude that the landward trench slope is undergoing an episode of rapid tectonic erosion periodic ridge collisions in the past may account for the apparent truncation of the andean forearc region