Age Of Oceanic Lithosphere Research Articles

Global variation of seismic energy release with oceanic lithosphere age

Variations in Mid Ocean Ridge seismicity with age provide a new tool to understand the thermal evolution of the oceanic lithosphere. The sum of seismic energy released by earthquakes during a time, and for an area, is proportional to its lithospheric age. Asthenospheric temperatures emerge on ridge centers with new crust resulting in high seismic activity; thus, the energy released sum is highest on the young lithosphere and decreases with age. We propose a general model that relates the systematic variation of seismic energy released with the lithospheric age. Our analysis evaluates the main physical factors involved in the changes of energy released sum with the oceanic lithosphere age in MOR systems of different spreading rates. These observations are substantiated based on three cross-sections of the East Pacific Rise, six sections in the Mid Atlantic Ridge, and three profiles in the Central Indian Ridge. Our global model provides an additional tool for understanding tectonic processes, including the effects of seismicity and mid-plate volcanism, and a better understanding of the thermal evolution for the young oceanic lithosphere.

The effects of the thermal state of overriding continental plate on subduction dynamics: Two-dimensional thermal-mechanical modeling

The dynamic process of ocean-continent subduction depends on not only the properties of the subducting oceanic plate, but also the characteristics and state of the overriding continental plate. Numerical models conducted to date have mostly focused on the oceanic lithosphere in this regard; research on the properties of overriding continental lithosphere remains relatively limited, especially the influence of its thermal state on subduction dynamics. Here we explored the performance of continental lithosphere with different thermal states during the subduction process using two-dimensional thermal-mechanical modeling and systematically investigated the effects of the thermal state of overriding continental plate, the age of subducting oceanic plate, and relative convergence rate on subduction dynamics. Modeling results show that: (1) When the geothermal gradient of continental crust is low (between 10 and 15 °C km−1), the oceanic plate first subducts at a low angle. As subduction continues, the slab dip gradually increases and the slab begins to retreat rapidly driven by its negative buoyancy, opening an ocean basin ranging from 600 to 1100 km in width. This leads to the decoupling between the overriding continental plate and oceanic plate. As the trench retreat continues, the horizontal deviatoric stress inside the overriding continental crust alternates between being positive and negative in a local area. Thinning of the overriding lithosphere mainly occurs at the region adjacent to the subduction zone, where the surface experiences significant subsidence. (2) When the geothermal gradient of continental crust is higher (greater than 15 °C km−1), oceanic plate retreat causes the overriding continental plate to be strongly stretched. In this case, the trench retreat distance decreases and the width of the ocean basin also reduces by between 100 and 1000 km. The horizontal deviatoric stress inside the whole overriding continental crust first manifests as compression and then changes into extension, which causes the surface to first uplift and then slowly subside. (3) Increasing the age of oceanic lithosphere accelerates trench retreat and promotes overriding plate thinning. (4) An advancing overriding continental plate slows down trench retreat. In cases where the geothermal gradient of continental crust is greater than 17.5 °C km−1, the hot continental crust experiences gravitational collapse and is overthrusted onto oceanic lithosphere, resulting in slow trench retreat. We analyzed the subduction process of the western Paleo-Pacific Plate in the Early Cretaceous based on our modeling results and discussed its possible control on the tectonic evolution of the rift basins in east Asia. We suggest that the development of a wide rift basin system on the Amurian Superterrane in the Early Cretaceous was likely related to slow trench retreat and the collapse of the hot crust, and the formation of a series of passive rift basins in the North China Craton was likely caused by the relatively cold thermal state of the lithosphere and the rapid retreat of the Paleo-Pacific Plate.

Plume‐Induced Subduction Initiation: Single‐Slab or Multi‐Slab Subduction?

AbstractInitiation of subduction following the impingement of a hot buoyant mantle plume is one of the few scenarios that allow breaking the lithosphere and recycling a stagnant lid without requiring any preexisting weak zones. Here, we investigate factors controlling the number and shape of retreating subducting slabs formed by plume‐lithosphere interaction. Using 3‐D thermomechanical models we show that the deformation regime, which defines formation of single‐slab or multi‐slab subduction, depends on several parameters such as age of oceanic lithosphere, thickness of the crust and large‐scale lithospheric extension rate. Our model results indicate that on present‐day Earth multi‐slab plume‐induced subduction is initiated only if the oceanic lithosphere is relatively young (<30–40 Myr, but >10 Myr), and the crust has a typical thickness of 8 km. In turn, development of single‐slab subduction is facilitated by older lithosphere and pre‐imposed extensional stresses. In early Earth, plume‐lithosphere interaction could have led to formation of either episodic short‐lived circular subduction when the oceanic lithosphere was young or to multi‐slab subduction when the lithosphere was old.

ArcCRUST: Arctic Crustal Thickness From 3‐D Gravity Inversion

AbstractThe ArcCRUST model consists of crustal thickness and estimated crustal thinning factors grids for the High Arctic and Circum‐Arctic regions (north of 67°N). This model is derived by using 3‐D forward and inverse gravity modeling. Updated sedimentary thickness grid, an oceanic lithosphere age model together with inferred microcontinent rifting ages, variable crystalline crust and sediment densities, and dynamic topography models constrain this inversion. We use published high‐quality 2‐D seismic crustal‐scale models to create a database of Depths to Seismic Moho (DSM) profiles. To check the quality of the ArcCRUST model, we have performed a statistical analysis of misfits between the ArcCRUST Moho depths and DSM values. Systematic analysis of the misfits within the Arctic sedimentary basins provides information about tectonic processes unaccounted by the assumed model of pure‐shear lithospheric extension. In particular, our model implies a less dense and/or thin mantle lithosphere underneath microcontinents in the deep Arctic Ocean where the ArcCRUST depth to Moho values exceed the DSM. A systematically larger gravity‐derived crustal thickness (~3 km) under the western and northern Greenland Sea points to a hotter upper mantle implied by the seismic tomography models in the North Atlantic.

Global Eocene tectonic unrest: Possible causes and effects around the North American plate

Many of our planet's “crises” were the result of sudden changes in plate tectonic configuration or catastrophic outbursts of volcanism caused by mantle plume impingement at the base of the lithosphere. At the Paleocene-Eocene boundary and in the Early Eocene several mantle plumes, continental collision and mid-ocean ridge subduction triggered a series of changes in seafloor spreading dynamics. We have constructed a detailed global model of oceanic lithosphere age and spreading rates for the 60 to 35 Ma interval. We revise evidence for changes in seafloor spreading direction in the North Atlantic, Arctic and NE Pacific oceans. At least two periods of spreading rate highs, which are separated by sharp value decrease, occurred along the entire eastern North American plate boundary from C25 to C18 time (c. 57 to 40 Ma). The collision and incipient subduction of the Early Eocene Siletzia oceanic LIP may have caused the sharp decrease in spreading rate at C23 time in the Labrador Sea and north of Charlie-Gibbs fracture zone. The post C23 rapid Farallon slab-break-off and subsequent upper mantle flow upwelling may have led to further variations in North Atlantic spreading rates at C22-21 time. Eastward Pacific subduction may have resumed at c. 43 Ma as indicated by a steady NE Pacific seafloor-spreading regime which resumed at or shortly after C21. The North Atlantic realm shows a delayed response to tectonic events west of North America, with an increase in spreading rate south of Charlie-Gibbs fracture zone from C20 to C18 time, followed by a steady decrease until the Oligocene. North American Late Paleocene-Early Eocene kimberlite magma that erupted more than 1000 km from its western plate boundary constitutes additional evidence that tectonic stresses due to changes in the mantle-lithosphere interactions may have affected the entire plate, and therefore also its eastern boundaries.

Eurasia Basin and Gakkel Ridge, Arctic Ocean: Crustal asymmetry, ultra-slow spreading and continental rifting revealed by new seismic data

High Arctic new seismic data, collected by Russian Federation from 2011 to 2014, and additional geological and geophysical information, are used to interpret the basement and sedimentary structure of central and eastern Eurasia Basin, the Gakkel Ridge, and their transition into the Laptev Sea. We find that significant changes in basement topography occur in Nansen Basin at C20 (43.43Ma) and in the Amundsen basins at C21 (45.7Ma), and in both basins at C13 (33Ma). A long seismic profile, that documents for the first time the structure of conjugate flanks and their margins in the central-eastern Eurasia Basin, confirms that oceanic accretion was asymmetric, with 10% less crust developed in the Amundsen Basin since continental break-up. In the eastern Amundsen Basin, we observe mid-ocean ridge uplift since C13 (33Ma). We identify four distinct sedimentary packages in the Eurasia Basin: Early to Mid Eocene (c. 56 to 45.7Ma), Mid Eocene to Early Oligocene (45.7 to 33.2Ma), Early Oligocene to Early Miocene (33.2 to 19.7) and Early Miocene to Present (19.7 to 0Ma); they are linked to the oceanic lithosphere age determined from magnetic data.The deepest part of the Gakkel Ridge (5215m), situated close to the easternmost part of this mid ocean ridge, is imaged for the first time by seismic data that reveals volcanic constructions within the older axial ridges and on the flanks. Gakkel Ridge's asymmetric flanks with shallow, regularly-spaced, and rugged structure, typical to ultra-slow spreading ridges, imply periodicity of tectonic phases. The Khatanga-Lomonosov Fault between Lomonosov Ridge and the Laptev Sea region, is identified on few seismic profiles; kinematic models predict that it may have been active only for a maximum of 10myr after continental break-up.

Break-up and seafloor spreading domains in the NE Atlantic

Abstract An updated magnetic anomaly grid of the NE Atlantic and an improved database of magnetic anomaly and fracture zone identifications allow the kinematic history of this region to be revisited. At break-up time, continental rupture occurred parallel to the Mesozoic rift axes in the south, but obliquely to the previous rifting trend in the north, probably due to the proximity of the Iceland plume at 57–54 Ma. The new oceanic lithosphere age grid is based on 30 isochrons (C) from C24n old (53.93 Ma) to C1n old (0.78 Ma), and documents ridge reorganizations in the SE Lofoten Basin, the Jan Mayen Fracture Zone region, in Iceland and offshore Faroe Islands. Updated continent–ocean boundaries, including the Jan Mayen microcontinent, and detailed kinematics of the Eocene–Present Greenland–Eurasia relative motions are included in this model. Variations in the subduction regime in the NE Pacific could have caused the sudden northwards motion of Greenland and subsequent Eurekan deformation. These events caused seafloor spreading changes in the neighbouring Labrador Sea and a decrease in spreading rates in the NE Atlantic. Boundaries between major oceanic crustal domains were formed when the European Plate changed its absolute motion direction, probably caused by successive adjustments along its southern boundary. Supplementary material: Figures showing the long wavelength of the NAG-TEC magnetic anomaly grid, detailed magnetic anomalies and isochrons, and a Table documenting aeromagnetic surveys for NAG-TEC magnetic compilation are available at https://doi.org/10.6084/m9.figshare.c.3661925

Topography caused by mantle density variations: observation-based estimates and models derived from tomography and lithosphere thickness

Large-scale topography may be due to several causes, including (1) variations in crustal thickness and density structure, (2) oceanic lithosphere age differences, (3) subcrustal density variations in the continental lithosphere and (4) convective flow in the mantle beneath the lithosphere. The last contribution in particular may change with time and be responsible for continental inundations; distinguishing between these contributions is therefore important for linking Earth's history to its observed geological record. As a step towards this goal, this paper aims at such distinction for the present-day topography: the approach taken is deriving a ‘model’ topography due to contributions (3) and (4), along with a model geoid, using a geodynamic mantle flow model. Both lithosphere thickness and density anomalies beneath the lithosphere are inferred from seismic tomography. Density anomalies within the continental lithosphere are uncertain, because they are probably due to variations in composition and temperature, making a simple scaling from seismic to density anomalies inappropriate. Therefore, we test a number of different assumptions regarding these. As a reality check, model topography is compared, in terms of both correlation and amplitude ratio, to ‘residual’ topography, which follows from observed topography after subtracting contributions (1) and (2). The model geoid is compared to observations as well. Comparatively good agreement is found if there is either an excess density of ≈0.2 per cent in the lithosphere above ≈150 km depth, with anomalies below as inferred from tomography, or if the excess density is ≈0.4 per cent in the entire lithosphere. Further, a good fit is found for viscosity ≈1020 Pa s in the asthenosphere, increasing to ≈1023 Pa s in the lower mantle above D′. Results are quite dependent on which tomography models they are based on; for some recent ones, topography correlation is ≈0.6, many smaller scale features are matched, topography amplitude is less than ≈30 per cent too large, while geoid variance reduction exceeds 70 per cent—overall a considerable improvement compared to previous models. Correlation becomes less if smaller scale features (corresponding to spherical harmonic degrees 15 and higher), which are probably largely due to anomalies in the lithosphere, are also considered. Comparison of results with different viscosity structures, and a regional comparison of amplitude ratios, indicates that lateral viscosity variations can be quite strong, but only leading to moderate variations in model topography of a factor probably less than two.

Global model of the upper mantle lateral density structure based on combining seismic and isostatic models

We compile the global model of the upper mantle lateral density structure with a 2×2 arc-deg spatial resolution using the values of the crust-mantle density contrast estimated relative to the adopted crust density model. The combined least-squares approach based on solving Moritz’s generalization of the Vening-Meinesz inverse problem of isostasy is facilitated to estimate the crust-mantle density contrast. The global geopotential model (EGM08), the global topographic/bathymetric model (DTM2006.0) including ice-thickness data, and the global crustal model (CRUST2.0) are used to compute the isostatic gravity anomalies. The estimated upper mantle densities globally vary between 2751 and 3635 kg/m3. The minima correspond with locations of the divergent oceanic tectonic plate boundaries (along the mid-oceanic ridges). The maxima are found along the convergent tectonic plate boundaries in the Andes and Himalayas (extending under the Tibetan Plateau). A comparison of the estimated upper mantle densities with the CRUST2.0 data shows a relatively good agreement between these two models within the continental lithosphere with the differences typically within ±100 kg/m3. Much larger discrepancies found within the oceanic lithosphere are explained by the overestimated values of the CRUST2.0 upper mantle densities. Our result shows a prevailing pattern of increasing densities with the age of oceanic lithosphere which is associated with the global mantle convection process.

Subduction of young oceanic plates: A numerical study with application to aborted thermal-chemical plumes

We investigated numerical models of initiation and subsequent evolution of subduction of young (10–30 Myr) oceanic lithosphere. Systematic numerical experiments were carried out by varying the age of the subducting plate (10, 12.5, 15, 17.5, 20, 25 and 30 Myr), the rate of induced convergence (2, 4 and 5 cm/yr) and the degree of hydration (0 and 2 wt% H2O) of the pre-existing weak oceanic fracture zone along which subduction is initiated. Despite the prescribed plate forcing, spontaneously retreating oceanic subduction with a pronounced magmatic arc and a backarc basin was obtained in a majority of the experiments. It was also found that the younger age of oceanic lithosphere results in more intense dehydration and partial melting of the slab during and after the induced subduction initiation due to the shallow dispositions of the isotherms. Partial melting of the subducted young crust may create thermal-chemical instabilities (cold plumes) that ascend along the slab-mantle interface until they either freeze at depth or detach from the slab and penetrate the upper plate lithosphere contributing to the nucleation and growth of a volcanic arc. Freezing of the plumes in the slab-mantle interface is favored by subduction of very young lithosphere (i.e., 10 Myr) at moderate rate (4 cm/yr) of convergence. Such aborted plumes may correspond to Cretaceous partially melted MORB-derived slab material and associated adakitic tonalitic-trondhjemitic rocks crystallized at ca. 50 km depth in the slab-mantle interface and exhumed in a subduction channel (serpentinite melanges) in eastern Cuba.

Extrapolating Oceanic Age Distributions: Lessons from the Pacific Region

Extrapolation of the age distribution of oceanic lithosphere has played a significant role in assessments of variations in global mean spreading rate, global mean ocean basin depth, and implications for global mean sea level. Subduction has already removed 50% of oceanic lithosphere younger than 55.7 Ma, making some level of extrapolation a necessary part of global plate reconstructions. An area equal in size to the Pacific Basin oceanic lithosphere must be extrapolated for ages older than 29.1 Ma. Three modes of extrapolation are identified. Mode 1 extrapolation uses the preserved history as recorded on one plate to infer the history of the previously adjacent plate. This mode of extrapolation is exemplified by the inferred history of the Farallon, Vancouver, Nazca, and Cocos plates relative to the Pacific Plate, on which this record is preserved. Mode 2 involves extrapolation beyond the preserved age extent of a given ridge system. No observable data exist that directly constrain the motions beyond the youngest magnetic reversal–dated oceanic lithosphere along such a boundary. This mode has, for example, been employed to extrapolate the age distribution resulting from spreading along the Izanagi-Pacific ridge system for as much as 60 m.yr. beyond the last directly determined record preserved on the Pacific Plate. Mode 3 is extrapolation of age distributions of entirely subducted ocean basins where no information explicitly constrains the relative-motion history of such basins. The age distributions in various neo-Tethyan basins require mode 3 extrapolation. This article examines extrapolations specifically using modes 2 and 3, employing the known spreading histories of the Pacific-Farallon/Vancouver and Pacific-Phoenix plate systems and the Tasman Sea as case studies. These tests demonstrate that extrapolated distributions of ages do not match preserved ages. Important events recorded in the preserved oceanic lithosphere, including both initiation and extinction of spreading ridges, cannot be inferred from the extrapolations and yet constitute important events that control aspects of the preserved oceanic lithosphere age distribution. Hence, reconstructed age distributions that require significant mode 2 and 3 extrapolations cannot provide a rigorous basis for testing hypotheses related to global histories of ridge production, mean age, mean depth, or other potentially correlated phenomena. This may appear to be an obvious result, and hence not worth publishing, but the persistent use of extrapolated age distributions in the published literature suggests that problems with extrapolation have not been appreciated by all.

Mean age of oceanic lithosphere drives eustatic sea-level change since Pangea breakup

The Atlantic and Indian Oceans and the oceanic part of the Antarctic plate have formed at the expense of Panthalassa as a result of Pangea breakup over the last 180 Myr. This major plate reorganization has changed the age vs. surface distribution of oceanic lithosphere and has been a likely driver of sea-level change. Assuming that the age/surface structure of Panthalassa has remained similar to the present-day global distribution from 180 Ma to Present, and using the isochron patterns preserved in the newly formed oceans, we model resulting relative sea-level change. We find a first (slower) phase of sea-level rise (by 90 to 110 m), culminating between 120 and 50 Ma, followed by a (faster) phase of sea-level drop. We show that this result is not strongly sensitive to our hypothesis of constant mean age of Panthalassa, for which much of the information is now erased due to subduction. When the effects of oceanic plateau formation and ice cap development are added, the predicted sea-level curve fits remarkably well the first-order variations of observed sea-level change. We conclude that the changes in mean age of the oceanic lithosphere (varying between 56 and 62 ± 0.2 Myr), which are simply the expression of the Wilson cycle following Pangea breakup, are the main control, accounting for ∼ 70%, of first-order changes in sea-level.

Incipient shortening of a passive margin: the mechanical roles of continental and oceanic lithospheres

SUMMARY By definition, passive margins deform little or no further after continental break-up. Nevertheless, post-break-up deformation and associated uplift have been documented along many margins (for example Norway, Greenland, West Africa, South America, India and Australia). We have conducted a series of analogue experiments in which we subjected scaled multilayered models of the lithosphere (made of sand and viscous layers) to horizontal shortening. A multilayered lithosphere rested on an asthenosphere of low viscosity that allowed isostatic adjustments. Models with various strength profiles represented oceanic lithospheres of different ages. The continental lithosphere reacted to an applied boundary velocity according to the strength and density of oceanic lithosphere. Where oceanic lithosphere was stronger and heavier than continental lithosphere it subducted; where oceanic crust was stronger and lighter than continental crust it obducted. On this basis we suggest that the style of incipient shortening of a passive margin depends on the age of oceanic lithosphere. Where the ocean is young we expect obduction; where it is old we expect incipient subduction. In nature, ophiolites and shortened passive margins appear to conform to this simple rule.

Constructing a Geodynamic A Priori Seismic (GAPS) velocity model of upper mantle heterogeneity

Seismic studies of the deep mantle suffer from the fact that the probing seismic waves must traverse the highly heterogeneous and poorly resolved shallow structure. One potential way forward is to develop high‐resolution models of the crust and upper mantle using other information. Here we describe the construction of a geodynamic a priori model of some aspects of upper mantle seismic velocity heterogeneity. It is based on an equal area tomographic grid and it has been produced at two scales, a 1° × 1° resolution at the equator, (i.e., each cell has an approximate dimension of 100 km by 100 km), and a 5° × 5° resolution at the equator. Both have a constant layer thickness of 100 km. Currently, the model accommodates the subducting lithosphere and global variation in continental crustal thickness and age of oceanic lithosphere. The shape of subducting oceanic lithosphere was derived from profiles through seismicity. The shape was combined with estimates of plate velocities and age of subducting lithosphere using an analytic solution of the thermal field to define the slab thermal anomaly. The temperature perturbation was converted to a slowness (1/velocity) perturbation. For oceanic lithosphere, a plate‐cooling model was used to convert lithosphere age to slowness perturbation via a temperature perturbation. The variation in the thickness of continental crust, around a global average, formed the third element of the slowness perturbation model. This model has already been applied in a high‐resolution mantle tomographic study of lower mantle heterogeneity.

Fluid-mobile trace element constraints on the role of slab melting and implications for Archaean crustal growth models

We use published and new trace element data to identify element ratios which discriminate between arc magmas from the supra-subduction zone mantle wedge and those formed by direct melting of subducted crust (i.e. adakites). The clearest distinction is obtained with those element ratios which are strongly fractionated during refertilisation of the depleted mantle wedge, ultimately reflecting slab dehydration. Hence, adakites have significantly lower Pb/Nd and B/Be but higher Nb/Ta than typical arc magmas and continental crust as a whole. Although Li and Be are also overenriched in continental crust, behaviour of Li/Yb and Be/Nd is more complex and these ratios do not provide unique signatures of slab melting. Archaean tonalite-trondhjemite-granodiorites (TTGs) strongly resemble ordinary mantle wedge-derived arc magmas in terms of fluid-mobile trace element content, implying that they-did not form by slab melting but that they originated from mantle which was hydrated and enriched in elements lost from slabs during prograde dehydration. We suggest that Archaean TTGs formed by extensive fractional crystallisation from a mafic precursor. It is widely claimed that the time between the creation and subduction of oceanic lithosphere was significantly shorter in the Archaean (i.e. 20 Ma) than it is today. This difference was seen as an attractive explanation for the presumed preponderance of adakitic magmas during the first half of Earth's history. However, when we consider the effects of a higher potential mantle temperature on the thickness of oceanic crust, it follows that the mean age of oceanic lithosphere has remained virtually constant. Formation of adakites has therefore always depended on local plate geometry and not on potential mantle temperature.

Evolution of oceanic upper mantle structure

Love and Rayleigh wave phase velocities increase systematically with increasing age of oceanic lithosphere up to 150 Ma. The rates of increase differ between oceans and vary for different age intervals. Modeling of lithospheric age–phase velocity relations indicates that the high velocity seismic lid thickens and the velocities in the sub-lithospheric low velocity zone (LVZ) increase with age at different rates between oceans. The initial thickness of the lithosphere near mid-ocean ridges, as averaged by long-period surface waves, varies between 10 and 45 km, and 100 Ma lithosphere has thickness from 83 to 110 km, for models with isotropic shear velocities. Hotspots in oceanic areas appear to modify oceanic lithosphere evolution, producing deviations from average patterns with increasing age.

Controls on trench topography from dynamic models of subducted slabs

A finite element method with constrained elements and Lagrange multipliers is used to study tectonic faults in a viscous medium. A fault, representing the interface between overriding and subducting plates, has been incorporated into a viscous flow model of a subduction zone in which both thermal buoyancy and the buoyancy associated with the phase change from olivine to spinel are included. The fault causes stress to concentrate in its vicinity, giving rise to a weak plate margin and a mobile plate if a power law rheology is used. Surface dynamic topography with either a Newtonian or a power law rheology and with typical subduction zone parameters is characterized by a narrow and deep trench and a broadly depressed back arc basin. This suggests that oceanic trenches and back arc basins over subduction zones are dynamically compensated by viscous flow. Our models show that trench depth increases with fault dip angle, slab dip angle, slab length, and age of oceanic lithosphere just prior to subduction. The influence of fault dip angle and age of lithosphere on trench depth is greater than the influence of slab dip angle and slab length. These relationships of trench depth versus subduction zone parameters explain well the statistics of observed trench depths. For those subduction zones with oceanic lithosphere on both sides of the trench, observed trench depths have been corrected for fault and slab dip angles, based on the relationships from the dynamic models. After correction to a common set of parameters, trench depth correlates linearly with age of lithosphere prior to subduction with a slope which is close to what models having high viscosities within the transition zone and lower mantle predict. Comparison between the trench depths, corrected for fault and slab dip angles, and model trench depths suggests that the resisting tangential stress on faults in subduction zones may range from 15 MPa to 30 MPa, depending on model details.

Hypocentral trend surface analysis: Probing the geometry of Benioff Zones

A hypocentral trend surface is a continuous function of latitude and longtitude fitted by least squares to a set of hypocenters so that it predicts depth to the “middle” of a Benioff Zone. In this paper we take a relatively simple approach to hypocentral trend surface analysis. The hypocentral trend surface is constructed from a spherical surface harmonic expansion whose coefficients are selected so as to minimize the standard vertical deviation between hypocenters and trend surface. The vertical deviation of a hypocenter from the hypocentral trend surface is called its residual. Consideration of a hypocentral trend surface cannot be divorced from consideration of the associated residuals and their spatial distribution. Any analysis of Benioff Zone geometry trades‐off presumed thickness of the zone with its presumed shape. This trade‐off can be investigated by examining suites of candidate hypocentral trend surfaces (together with their associated residual distributions) generated by varying the number of degrees of freedom available to the trend surface. Hypocentral trend surfaces are generated for three high quality sets of hypocenters obtained by local seismic networks in Honshu (Japan), Cook Inlet (Alaska) and South Peru. Hypocenters beneath Honshu generate a bimodal distribution of residuals about their trend surface. The vertical separation of the upper and lower sheets of this Double Benioff Zone averages about 33 km. No Double Benioff Zone configuration is discovered below Cook Inlet or South Peru. Regional trend surfaces are established for intermediate‐depth teleseismic data from South and Middle America. In South America (0°–40°S) teleseismic data suggest that along‐strike transitions between “flat” and moderately steeply dipping sections of the Benioff Zone are achieved by flexure of a coherent slab rather than by fragmentation of the slab into tear‐bound flaps or separate tongues of lithosphere. Hypocentral trend surface analysis of teleseismic data from South Peru suggests a geometry similar to that indicated by hypocentral trend surface analysis of the local data of Hasegawa and Sacks. An eastward shallowing Benioff Zone beneath southwest Mexico is closely associated with the geometry of the trans‐Mexican volcanic arc. The Benioff Zone beneath Central America is more steeply dipping. The flat slab‐steep slab transition near 96°W may relate to the difference in age of oceanic lithosphere on either side of the Tehuantepec Ridge. While the strong flexure of the subducted Nazca plate beneath Ecuador is closely associated with the Carnegie Ridge, the Nazca ridge extropolates slightly north of the flexure in subducted lithosphere beneath South Peru. The flexure appears instead more closely associated with the pronounced inflection in the map view shape of the leading edge of the overriding South American plate.

On the origin of the Cocos-Nazca spreading center

To investigate the mechanism underlying the break-up of the Farallon plate into the Cocos plate and Nazca plate, we analyze the state of stress in the Farallon plate at about 30 m.y. B.P., just prior to the fragmentation. To this purpose we use finite element methods and a reconstruction of regional plate boundaries appropriate for 30 m.y. B.P. A key role in the model is played by the dependence of two important plate-tectonics forces (slab pull and ridge push) on the age of oceanic lithosphere. The results show a highly tensional stress field, with maximum principal stresses of 5 to 6 kbar. North-south tension in the vicinity of present-day Panama is proposed to have been the cause for the fragmentation of the Farallon plate and the inception of spreading along the new Cocos-Nazca plate boundary. Because the kind of plate interaction that gave rise to the high level of tensional stresses in the Farallon plate is not restricted to the area of this study, the proposed mechanism seems to shed light on the problem of fragmentation of oceanic plates in general.