past forearc deformation in nicaragua and coupling at the megathrust interface evidence for subduction retreat 1 deformation at the boundary between the cocos and caribbean plates in nicaragua is currently focused in the volcanic arc in the form of arc parallel shearing in the middle miocene however there was widespread subsidence with local uplift in the outer forearc and inner forearc broad uplift to understand this complex deformation we use numerical modeling to investigate the effect of inhomogeneous friction at the megathrust fault on strain localization in the upper plate a good fit is obtained when 1 the interface between the slab and upper plate is not situated at the current wadati benioff zone but rather is moved 50 km landward to where subduction may have been active in the early tertiary and 2 there is high stress accumulation between 15 and 25 km depths and low stress accumulation updip and downdip of the thrust interface the results are consistent with a high velocity block being the remnant of an oceanic plateau that subducted in the eocene and was later incorporated into the upper plate we suggest that this geometry persisted into the middle miocene before the slab broke off and the thrust interface jumped seaward

structure of the makran subduction zone from wide angle and reflection seismic data abstract makran is one of the largest accretionary wedges on the globe formed by the convergence between the eurasian and the arabian plates it is characterised by an extremely high sediment input of 7 km and a shallow subduction angle we present seismic velocity models from four wide angle seismic lines that image the wedge sediments and the subducted oceanic crust three strike lines show complex but rather one dimensional structures where the observed seismic phases have been studied with the reflectivity method a 160 km long dip line was surveyed coincident with a mcs line collected by cambridge university in 1986 prestack depth migration of this data with the new wide angle velocities shows improved images compared to earlier results interpretation of the wide angle and the mcs data indicates that a decollement is developed along a bright reflector within the turbiditic sequence more than 3 km of sediment bypasses the first accretionary ridges underthrusting and is transported to greater depth this might help to explain the sparse earthquake activity associated with subduction here in the strike lines low velocity layers occur landward of the deformation front in zones where active thrusting or underplating takes place the oceanic crust is characterised by a strong velocity contrast between layer ii and layer iii

deformation of erosive and accretive forearcs during subduction of migrating and non migrating aseismic ridges results from 3 d finite element models and application to the central american peruvian and ryukyu margins subduction of aseismic oceanic ridges causes considerable uplift and deformation of the upper plate and may lead for example to the indentation of the forearc the formation of marine terraces or distinct fault patterns in the upper plate depending on the orientation of the ridge relative to the plate convergence direction the ridge may either be stationary or migrate along the margin here we use three dimensional numerical models to investigate the tectonic evolution of forearcs affected by ridge subduction in different experiments we distinguish between migrating non migrating ridges and accretive erosive margins respectively our results reveal that displacement and strain fields above migrating and non migrating ridges are asymmetric with respect to the ridge axis unless both ridge and plate convergence direction are perpendicular to the trench as the asymmetric deformation pattern shifts along the margin through time uplift caused by the underthrusting ridge is followed by subsidence when the ridge crest passed by and regions initially experiencing shortening may subsequently undergo extension and vice versa if the forearc comprises an accretionary prism the ridge induced reentrant is larger than those in models with erosive forearcs and strain localizes in the frontal part of the wedge additional models with a setup adjusted to the cocos and gagua ridges provide constraints on the onset of their subduction at the central american and ryukyu margins at 2 ma and 1 ma respectively displacement and strain fields from a model for the nazca ridge collision zone show good agreement with geological data from marine terraces and quaternary faulting

focused subaerial erosion during ridge subduction impact on the geomorphology in south central peru data obtained from low temperature thermochronometers such as apatite fission track and u th he are combined with morphometric information extracted from digital elevation models this combination shows several geomorphological effects that are caused by the migration of the nazca ridge along the peruvian coastal margin offshore the depth of the deep sea trench decreases by 1500 m where the nazca ridge collides with the continental south american plate onshore the ridge causes an uplift of at least 800 m in the coastal cordillera this uplift results in a westward shift of the coastline thereby focusing and increasing erosion in the uplifted areas at the trailing edge the shelf subsides and the coastline retreats eastwards producing at least part of the indentation observed between paita and pisco the ridge acts therefore like a wave uplifting the andean margin as it traverses inland and southwards leaving a clear fingerprint on the topographic evolution of the peruvian coastal margin

the migration history of the nazca ridge along the peruvian active margin a re evaluation abstract the collision zone of the 200 km wide and 1 5 km high nazca ridge and the peruvian segment of the convergent south american margin between 14 s and 17 s is characterized by deformation of the upper plate and several hundred meters of uplift of the forearc this is evident by a narrowing of the shelf a westward shift of the coastline and the presence of marine terraces as the nazca ridge is oblique with respect to both trench and convergence direction of the nazca plate it migrates southward along the active plate boundary for reconstructing the migration history of the nazca ridge this study uses updated plate motion data resulting from a revision of the geomagnetic time scale the new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity of 75 mm a before 10 8 ma 61 mm a 10 8 4 9 ma and 43 mm a 4 9 ma to present intra plate deformation associated with mountain building in the peruvian andes since the miocene reduces the relative convergence rate between nazca plate and peruvian forearc taking an intra plate deformation at a rate of 10 mm a estimated from space geodetic and geological data into account does not significantly reduce these lateral migration velocities constraining the length of the original nazca ridge by its conjugate feature on the pacific plate yields a length of 900 km for the subducted portion of the ridge using this constraint ridge subduction began 11 2 ma ago at 11 s therefore the nazca ridge did not affect the northern sites of ocean drilling program odp leg 112 located at 9 s this is supported by benthic foraminiferal assemblages in odp leg 112 cores indicating more than 1000 m of subsidence since at least middle miocene time and by continuous shale deposition on the shelf from 18 to 7 ma recorded in the ballena industrial well at 11 5 s the model predicts the passage of the ridge crest 9 5 ma ago this agrees with the sedimentary facies and benthic foraminiferal stratigraphy of odp leg 112 cores which argue for deposition on the shelf in the middle and late miocene with subsequent subsidence of a minimum of several hundred meters onshore at 12 s the sedimentary record shows at least 500 m uplift prior to the end of the miocene also in agreement with the model

geophysical characteristics of the ninetyeast ridge andaman island arc trench convergent zone abstract the convergence tectonics of the ninetyeast ridge ner upon the andaman island arc trench system is examined through an analysis of etopo2 bathymetry satellite derived free air gravity and seismic data oblique subduction and the buoyancy forces arising from subduction of the ner render the subduction processes near the andaman arc highly complex the bathymetric expression of the ner is visible up to lat 10 n but seismic reflection data indicate that it extends up to about lat 17 n the gravity anomalies are strongly positive over the exposed segment of the ridge but are subdued over the buried portion there is a prominent break in the continuity of the trench gravity low where the ner seems to impinge upon the island arc further a strong curvilinear belt of negative anomalies just behind and running parallel to the island arc associated with the forearc basin is a dominant feature of the gravity map an offset in the continuity of this strong negative anomaly occurs at about the same latitude where the ner seems to be converging upon the island arc seismic reflection data indicate that the ner is very close to the trench flexural modeling of the gravity anomalies for the subducting indian ocean lithosphere loaded by sediments and the ner indicates that the ner is at the starting phase of its collision with the island arc and may not have started affecting the subduction process itself we infer that the en echelon block structure of the ner in the proximity of the convergent zone is a consequence of complex strike slip and subduction related tectonic forces

helium isotopes in paleofluids and present day fluids of the larderello geothermal field constraints on the heat source 1 the he isotope composition of paleofluids entrapped in fluid inclusions of hydrothermal minerals is compared with the present day fluid composition of the larderello geothermal field almost constant values of 3he 4he m 3he 4he air r ra over time indicate that no important changes have occurred in the deep source of gases at least during the last 3 8 million years on a regional scale a correlation has been found between the r ra spatial distribution heat flow and bouguer gravity anomaly high values of r ra and heat flow and low bouguer anomaly values indicate that the larderello field is an area of preferential escape for mantle derived fluids a positive correlation has also been found between the r ra spatial distribution and a major seismic reflector named the k horizon a deep magma source refilled by periodic gas input from the mantle is the most likely source of 3he enriched fluids and the anomalously high heat flow the nearly constant value of r ra clearly indicates that input of fresh mantle material has occurred up to recent times clear evidence of mixing between mantle and crustal fluids indicates that the high r ra is the lower limit of the actual mantle value which is suggested to be similar to the subcontinental european mantle the decrease of r ra over time in the peripheral part of the larderello field indicates that important changes in the feeding fracture system and or cooling rate have occurred in these areas

three dimensional finite element models on the deformation of forearcs caused by aseismic ridge subduction the role of ridge shape friction coefficient of the plate interface and mechanical properties of the forearc abstract geological and geophysical data show that the forearc of subduction zones experiences strong deformation during the subduction of aseismic oceanic ridges in order to better understand ridge related forearc deformation patterns we performed a series of three dimensional finite element models in which we varied the ridge shape the friction coefficient of the plate interface and the mechanical strength of the forearc experiments were carried out for migrating non migrating ridges and accretive erosive margins respectively our results show that the subducting ridge uplifts the forearc and induces horizontal displacements that alter the strain regime of both erosive and accretive forearcs generally shortening prevails in front of the ridge while domains of shortening and extension exist above the ridge models with stationary ridges show high uplift rates only above the ridge tip whereas the forearc above migrating ridges experiences uplift above the leading ridge flank and subsequent subsidence above the trailing flank the height and width of the ridge as well as the friction coefficient of the plate interface have the largest effect on the forearc deformation patterns whereas the mechanical strength of the forearc plays a lesser role forearc indentation at the trench is largest for high and broad ridges high friction coefficients and or weak forearc material shortening and extension of the forearc above the ridge are more intense for high and narrow ridges our model results provide information about the distribution of ridge induced displacements and strain fields and hence help to identify deformation patterns caused by subducting aseismic ridges in nature

vergence boundaries an extension of the vergence concept abstract the difficulty in distinguishing between vergence changes on adjacent limbs of tight primary folds and the similar effects produced by non coaxial refolding of primary folds by larger scale folds is outlined it is suggested that a distinction should be made between typical and atypical vergence boundaries atypical vergence boundaries falsely suggest the presence of congruent major folds whereas major folds attributed to typical vergence boundaries are real practical rules are formulated that should be applied to vergence boundaries in non coaxially refolded regions in order to recognize atypical vergence boundaries

time series analysis of magnetic susceptibility variations in deep marine sedimentary rocks a test using the upper danian lower selandian proposed gssp spain abstract it has been clearly demonstrated that past climate fluctuations are recorded in marine sedimentary rocks however for many reasons extracting the climate signature is difficult initial low field mass specific magnetic susceptibility ms data can potentially provide a measure of climate variability and thus become a proxy characterizing climate cyclicity in a wide range of marine sediments this is due to the fact that climate change warm wet versus cold dry drives cyclic weathering and erosional variations that are recorded as the detrital components of marine sediment that dominates the ms to test the utility of ms to yield climate proxies in marine sediments showing major changes in lithology we have sampled the well studied danian selandian boundary interval lower paleocene at zumaia zumaya spain this interval represents a dramatic rapid lithologic change from danian carbonate dominated limestone marl couplets to a detrital dominated marl shale sequence in the selandian indicating onset of a major regression erosional event beginning in the lowest selandian sampling included a continuous sequence from the uppermost danian stage 3 71 m into the lowermost selandian stage 5 2 m a suite of 175 samples collected at 5 cm intervals our results indicate that ms measurements reflect changes in detrital sediment at the site first by closely tracking high frequency limestone marl couplets second with a large rapid shift toward higher ms values beginning at the danian selandian boundary resulting from a major regression and third by tracking low frequency climate controlled variations known to have occurred during deposition of these sediments ms zones developed from the cyclicity observed throughout the sequence supported by time series analysis using fourier transform ft methods applied to the ms results exhibit milankovitch cyclicities in the precessional 19 24 kyr obliquity 41 54 kyr and eccentricity bands 100 kyr this is in excellent agreement with previous ft work on the section using measured variations in cyclic bed thicknesses with the new ms data set and ft results we then developed a floating point time scale fpts for the sequence sampled covering 550 kyr through the danian selandian boundary interval yielding a time scale resolution for the uppermost danian to 10 000 years however only the 100 000 year eccentricity band for the selandian is sufficiently well developed for an fpts estimate and yields a time scale resolution of 50 000 years our test of the utility of ms data sets in this varying depositional setting demonstrated that these data can provide a climate proxy that is not disrupted by large lithologic changes

backstop geometry and accretionary mechanics of the sunda margin the convergent sunda margin off indonesia displays all geological features characteristic of an accretion dominated subduction zone a combined interpretation of prestack depth migrated seismic reflection data and velocity information gained from refraction studies is supplemented by high resolution bathymetric data and for the first time allows the exact mapping of backstop regimes initially the outer high evolved as material was pushed against a static rigid arc framework backstop underlying a forearc basin increasing material strength of the outer high due to lithification formed a dynamic backstop which controls accretion today an out of sequence thrust marks the transition from the recent active frontal accretionary prism to the outer high and may be traced in the seismic and bathymetric data over the whole extent of the study area the existence of a static as well as a dynamic backstop controls the forearc geometry and is associated with the segmentation of the forearc which is observed in regimes of frontal as well as of oblique subduction mass balance calculations which account for porosity changes and metamorphism indicate a subduction history dominated by accretionary processes since the late eocene accretion is associated with the low values of basal friction inferred for the sunda margin structural investigations of conjugate fault planes indicate a very weak basal detachment effective stress analyses reveal that intrinsically weak material causes the high strength ratio of the detachment to the overlying sediments whereas overpressuring within the frontal accretionary prism is negligible

what drives orogeny in the andes the andes the world9s second highest orogenic belt were generated by the cenozoic tectonic shortening of the south american plate margin overriding the subducting nazca plate we use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening from the modeling we infer that the most important factor was accelerated westward drift of the south american plate changes in the subduction rate were less important other important factors are crustal structure of the overriding plate and shear coupling at the plates9 interface the model with a thick 40 45 km at 30 ma south american crust and relatively high friction coefficient 0 05 at the nazca south american interface generates 300 km of tectonic shortening during 30 35 m y and replicates the crustal structure and evolution of the high central andes the model with an initially thinner

response of the tectonically erosive south peruvian forearc to subduction of the nazca ridge analysis of three dimensional analogue experiments 1 the collision zone of the obliquely subducting nazca ridge and the erosive peruvian forearc is characterized by enhanced tectonic erosion normal faulting forearc uplift and a minor indentation we apply two three dimensional sandbox experiments monitored by an optical particle imaging velocimetry system to decipher the spatial and temporal evolution of forearc deformation during oblique convergence the rigid model ridge causes a wave of uplift followed by subsidence to shift along the forearc by varying the strength of the analogue material we demonstrate that the forearc response to ridge subduction in particular the amount and rate of uplift and wedge indentation is controlled by the mechanical strength of the forearc the model using the high strength material yields results more compatible with the geological record of the peruvian forearc first the modeled amount of uplift scales to 1 2 km in nature which agrees with 1 0 km of uplift recorded by marine terraces second the calibrated model uplift rate of 0 9 km myr is similar to the natural rate of 0 7 km myr third the development of normal faults above the model ridge is in accordance with the style of faulting above the nazca ridge finally oversteepening of the uplifted wedge and local slope failure as observed in the experiment has also been identified off southern peru a comparison between the erosive peruvian and accretive margins emphasizes that the style of ridge induced deformation depends on the forearc strength and crustal structure which in turn are controlled by the long term evolution of the margin s mass transfer regime

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

quaternary uplift astride the aseismic cocos ridge pacific coast costa rica the pacific coast of costa rica lies within the central american forearc and magmatic arc region that was created by northeastward subduction of the cocos plate beneath the caribbean plate at the middle america trench from the peninsula de nicoya south eastward toward the peninsula de osa and the peninsula de burica on the panamanian border the middle america trench loses its physiographic expression where it intersects the aseismic cocos ridge interaction between subduction of the buoyant aseismic cocos ridge and the overriding caribbean plate is invoked to explain the variation in rates of vertical crustal uplift along a coastal transect from nicoya to burica the pliocene and pleistocene stratigraphic record and holocene marine terraces and beach ridge complexes indicate that maximum rates of crustal uplift have occurred on the peninsula de osa immediately landward of the aseismic cocos ridge crustal uplift rates decrease northwest toward the peninsula de nicoya and to a lesser extent southwest toward the peninsula de burica the late quaternary stratigraphy on the peninsula de osa is subdivided into two major chronostratigraphic sequences from groupings of radiocarbon dates crustal uplift rates calculated from these sequences systematically decrease from 6 5 to 2 1 m ka north east across the peninsula deformation of the peninsula is modeled as uplifted and down to the northeast tilted fault blocks with an angular rotation rate of 0 03 to 0 06 per thousand years although less well constrained crustal uplift rates on the peninsula de nicoya 200 km to the northwest of the peninsula de osa vary from 1 m ka for pliocene and pleistocene sediments to 2 5 m ka for holocene marine terraces in the quepos region 100 km to the northwest of the peninsula de osa calculated uplift rates derived from incision of late quaternary fluvial terraces range from 0 5 to 3 0 m ka on the peninsula de burica only 60 km to the southwest of the peninsula de osa calculated uplift rates range from 4 7 m ka for a late holocene marine terrace to 1 2 m ka for post late pliocene deep sea sediments the variations in calculated uplift rates on the peninsula de osa constrain a dynamic model for subduction of the cocos ridge and the resulting uplift of the overriding caribbean plate deflection of the caribbean plate is modeled using various effective elastic thicknesses as the response of an elastic plate to the buoyant force of the subducted cocos ridge because the shape of the subducted end of the cocos ridge is unknown two scenarios are evaluated 1 a radially symmetric ridge with a slope similar to the slope of the flanks of the ridge and 2 a ridge where the subducted end was truncated by the panama fracture zone the best fit model utilizes a truncated ridge that has been subducted during the past 0 5 m y 50 km beneath the overriding caribbean plate which has an effective elastic thickness of 5 km the model predicts that the highest uplift rate should be 3 7 m ka and occur on the southwest coast of the peninsula de osa the rate of uplift slows considerably to the northeast and indicates that the peninsula de osa is tilting to the northeast which agrees with observations in that region the predicted uplift rate attributed to aseismic ridge subduction also decreases along the coast both north and south of the peninsula de osa resulting in little uplift that can be attributed to cocos ridge subduction in the northwestern portions of the peninsula de nicoya

crustal density structure in northwestern south america derived from analysis and 3 d modeling of gravity and seismicity data abstract this paper presents a three dimensional 3 d interpretation of new gravity and seismicity datasets for northern south america a 3 d forward density model was constructed on the basis of deep wide angle seismic refraction sections moho depth from receiver functions and surface geology density values were estimated from published borehole data for sediments by using empirical velocity density functions and considering mineralogical chemical composition variations under typical pressure temperature conditions for upper and lower crustal rocks the modeled 3 d density structure was kept as simple as possible the continental and oceanic plates were formed by two sedimentary bodies one crustal body and one mantle lithosphere body overlying a sub lithospheric mantle the caribbean plate was modeled with an atypical crustal thickness of 18 km including sediments the geometry of the caribbean plate was modeled using a combination of gravity modeling and analyses of the seismicity and focal mechanism solutions intermediate seismicity and the orientation of the t axes appeared aligned along the predicted position of the slab as a result the estimated slab dip angle under maracaibo and the merida andes was 15 and increases up to 20 after 100 km depth the model shows two orientations in the slab strike n150 e 5 in western colombia and southward underneath the maracaibo block the modeling results suggest that the northern south american upper and lower crusts are relatively light and the density of the caribbean crust is typical for an oceanic crust

constraints on inner forearc deformation from balanced cross sections fila costena thrust belt costa rica 1 the fila costena thrust belt in the forearc of costa rica is accommodating a significant portion of the convergence of the cocos plate and panama microplate geologic mapping of the thrust belt depicts a duplex with three horses that incorporate eocene limestones and oligocene to early miocene clastics inboard of the subducting cocos ridge axis by constructing a cross section at this location along a ne sw trending transect perpendicular to the thrust belt we constrain a shortening rate of approximately 40 mm a and propose that as much as 50 of the total plate convergence rate is taken up in the inner forearc the eocene limestones at the base of the thrust sheets pinch out in both directions away from the onland projection of the cocos ridge axis owing to decrease in slip on faults and a lateral ramp in the basal decollement the thrust belt terminates near the panama border at the onland projection of the subducting panama fracture zone these observations suggest that shortening is propagating to the east with the migration of the panama triple junction and the onset of shallow subduction of the thickened edge of the cocos plate the absence of similar features in the nicaraguan forearc where the subducting crust is older subducts more steeply and lacks incoming ridges and seamounts indicates that deformation of the forearc basin in costa rica reflects greater coupling between the converging plates inboard of the cocos ridge

physical characteristics of subduction interface type seismogenic zones revisited 1 based on global earthquake catalogs the hypocenters nodal planes and seismic moments of worldwide subduction plate interface earthquakes were extracted for the period between 1900 and 2007 assuming that the seismogenic zone coincides with the distribution of 5 5 m 7 earthquakes the subduction interface seismogenic zones were mapped for 80 of the trench systems and characterized with geometrical and mechanical parameters using this database correlations were isolated between significant parameters to identify cause effect relationships empirical laws obtained in previous studies were revisited in light of this more complete accurate and uniform description of the subduction interface seismogenic zone the seismogenic zone was usually found to end in a fore arc mantle rather than at a moho depth the subduction velocity was the first order controlling parameter for variations in the physical characteristics of plate interfaces determining both the geometry and mechanical behavior as such the fast subduction zones and cold slabs were associated with large and steep plate interfaces which in turn had large seismic rates the subduction velocity could not account for the potential earthquake magnitude diversity that was observed along the trenches events with mw 8 5 preferentially occurred in the vicinity of slab edges where the upper plate was continental and the back arc strain was neutral this observation was interpreted in terms of compressive normal stresses along the plate interface large lateral ruptures should be promoted in neutral subduction zones due to moderate compressive stresses along the plate interface that allow the rupture to propagate laterally

thermal structure coupling and metamorphism in the mexican subduction zone beneath guerrero summary temperature is one of the most important factors that controls the extent and location of the seismogenic coupled and transition partially coupled segments of the subduction interplate fault the width of the coupled fault inferred from the continuous gps observations for the steady interseismic period and the transient width of the last slow aseismic slip event m w 7 5 that occurred in the guerrero subduction zone in 2001 2002 extends up to 180 220 km from the trench previous thermal models do not consider this extremely wide coupled interface in guerrero subduction zone that is characterized by shallow subhorizontal plate contact in this study a finite element model is applied to examine the temperature constraints on the width of the coupled area the numerical scheme solves a system of 2 d stokes equation and 2 d steady state heat transfer equations the updip limit of the coupling zone is taken between 100 and 150 c while the downdip limit is accepted at 450 c as the transition from partial coupling to stable sliding from the entire coupled zone the seismogenic zone extends only up to 82 km from the trench inferred from the rupture width of large subduction thrust earthquakes corresponding to the 250 c isotherm only a small amount of frictional heating is needed to fit the intersection of the 450 c isotherm and the subducting plate surface at 180 205 km from the trench the calculated geotherms in the subducting slab and the phase diagram for morb are used to estimate the metamorphic sequences within the oceanic subducting crust a certain correlation exists between the metamorphic sequences and the variation of the coupling along the interplate fault