Barry Parsons

Bio: Barry Parsons is an academic researcher from University of Oxford. The author has contributed to research in topics: Fault (geology) & Slip (materials science). The author has an hindex of 62, co-authored 137 publications receiving 13239 citations. Previous affiliations of Barry Parsons include Massachusetts Institute of Technology.

An analysis of the variation of ocean floor bathymetry and heat flow with age

Abstract: Two models, a simple cooling model and the plate model, have been advanced to account for the variation in depth and heat flow with increasing age of the ocean floor. The simple cooling model predicts a linear relation between depth and t½, and heat flow and 1/t½, where t is the age of the ocean floor. We show that the same t½ dependence is implicit in the solutions for the plate model for sufficiently young ocean floor. For larger ages these relations break down, and depth and heat flow decay exponentially to constant values. The two forms of the solution are developed to provide a simple method of inverting the data to give the model parameters. The empirical depth versus age relation for the North Pacific and North Atlantic has been extended out to 160 m.y. B.P. The depth initially increases as t½, but between 60 and 80 m.y. B.P. the variation of depth with age departs from this simple relation. For older ocean floor the depth decays exponentially with age toward a constant asymptotic value. Such characteristics would be produced by a thermal structure close to that of the plate model. Inverting the data gives a plate thickness of 125±10 km, a bottom boundary temperature of 1350°±275°C, and a thermal expansion coefficient of (3.2±1.1) × 10−5°C−1. Between 0 and 70 m.y. B.P. the depth can be represented by the relation d(t) = 2500 + 350t½ m, with t in m.y. B.P., and for regions older than 20 m.y. B.P. by the relation d(t) = 6400 - 3200 exp (−t/62.8) m. The heat flow data were treated in a similar, but less extensive manner. Although the data are compatible with the same model that accounts for the topography, their scatter prevents their use in the same quantitative fashion. Our analysis shows that the heat flow only responds to the bottom boundary at approximately twice the age at which the depth does. Within the scatter of the data, from 0 to 120 m.y. B.P., the heat flow pan be represented by the relation q(t) = 11.3/t½ μcal cm−2s−1. The previously accepted view that the heat flow observations approach a constant asymptotic value in the old ocean basins needs to be tested more stringently. The above results imply that a mechanism is required to supply heat at the base of the plate.

Toward mapping surface deformation in three dimensions using InSAR

Abstract: [1] One of the limitations of deformation measurements made with interferometric synthetic aperture radar (InSAR) is that an interferogram only measures one component of the surface deformation — in the satellite's line of sight. We investigate strategies for mapping surface deformation in three dimensions by using multiple interferograms, with different imaging geometries. Geometries for both current and future missions are evaluated, and their abilities to resolve the displacement vector are compared. The north component is always the most difficult to determine using data from near-polar orbiting satellites. However, a satellite with an inclination of about 60°/120° would enable all three components to be well resolved. We attempt to resolve the 3D displacements for the 23 October 2002 Nenana Mountain (Alaska) Earthquake. The north component's error is much larger than the signal, but proxies for eastward and vertical motion can be determined if the north component is assumed negligible. Inversions of hypothetical coseismic interferograms demonstrate that earthquake model parameters can be well recovered from two interferograms, acquired on ascending and descending tracks.

Mantle convection and the thermal structure of the plates

Abstract: Though a simple thermal model of plate creation matches the observed bathymetry and heat flow with considerable accuracy, it only does so because a constant temperature is imposed as a lower boundary condition. A more realistic model is developed here, based on the idea that the thermal structure of the plate becomes unstable and leads to the development of small-scale convection. Convective heat transport then supplies the heat flux needed to match the observations rather than an artificial constant temperature boundary condition. The temperature dependence of the rheology is represented in a simple manner. Below a given temperature the material is assumed to move rigidly, defining an upper mechanical boundary layer. Beneath this rigid layer, where the temperatures are greater, the material is assumed to have a constant Newtonian viscosity. The part of the viscous region where there are significant vertical temperature gradients, immediately below the mechanical boundary layer, forms a thermal boundary layer. As the plate cools, both the mechanical and thermal boundary layers increase in thickness. A local critical Rayleigh number criterion is used to test the stability of the thermal boundary layer. On this basis a convective instability is predicted, its occurrence coinciding with the breakdown of the linear dependence of the depth of the ocean floor on the square root of age. Though the small-scale convection which develops from this instability modifies the thermal structure, the basic observational constraints are shown to be satisfied. The stability criterion is further tested in two different laboratory experiments. These experiments also illustrate a possible form for the instability, with cold dense material breaking away from the base of the plate and being replaced by hotter material from below. The effects of the cooling of the oceanic plate as it moves away from a midocean ridge have received a good deal of attention, both theoretical and observational. Forsyth [1977] recently reviewed the different measurements that have been made. Variations in mean heat flow, depth, and velocity of ! - 

On the interaction of two scales of convection in the mantle

Abstract: A system in which convection takes place in the upper mantle on two distinct horizontal length scales is proposed. This is consistent with the existence of the plates themselves, the relatively constant heat flux background in older ocean basins, and the knowledge of convection in fluid layers gained from laboratory and numerical experiments. The large-scale circulation consists of the plates themselves and the return flow necessary to conserve mass. The small-scale flow, analogous to Rayleigh-Benard convection or variants of this, which have been the main target of numerical study, provides the necessary vertical heat transport in the upper mantle that supplies the required heat flux at the base of the lithosphere. The depth of convection is taken to be down to the 650-km seismic discontinuity, and this depth characterizes the horizontal length scale of the small-scale convection. This system is studied by means of a set of laboratory experiments that explore the interaction of the small-scale convection with the large-scale flow. The experiments show the plausibility of convection on two scales. Furthermore, they suggest that beneath fast-moving plates (absolute velocities around 10 cm y−1) the small-scale convection will align itself as rolls in the direction of the large-scale flow in geologically short times. However, beneath very slow moving plates the times required for the alignment of convective rolls are long in comparison with times over which no changes in plate motions are to be expected. Here the convective planform is more likely to take the form of upwelling and downwelling spouts. Thus this simple system of a convecting layer beneath a moving boundary contains the possibility of explaining a wide variety of surface features. Observational tests of the consequences of the two-scale idea are suggested, and the assumptions on which this idea is based are critically discussed.

Oceans and continents: Similarities and differences in the mechanisms of heat loss

Abstract: The principal objective of this paper is to present a simple and self-consistent review of the basic physical processes controlling heat loss from the earth To accomplish this objective, we give a short summary of the oceanic and continental data and compare and contrast the respective mechanisms of heat loss In the oceans we concentrate on the effect of hydrothermal circulation, and on the continents we consider in some detail a model relating surface heat flow to varying depth scales for the distribution of potassium, thorium, and uranium From this comparison we conclude that the range in possible geotherms at depths below 100 to 150 km under continents and oceans overlaps and that the thermal structure beneath an old stable continent is indistinguishable from that beneath an ocean were it at equilibrium Oceans and continents are part of the same thermal system Both have an upper rigid mechanical layer where heat loss is by conduction and a lower thermal boundary layer where convection is dominant The simple conductive definition of the plate thickness is an oversimplification The observed distribution of area versus age in the ocean allows us to investigate the dominant mechanism of heat loss which is plate creation This distribution and an understanding of the heat flow through oceans and continents can be used to calculate the heat loss of the earth This heat loss is 1013 cal/s (42 × 1013W) of which more than 60% results from the creation of oceanic plate The relation between area and age of the oceans is coupled to the ridge and subducting slab forces that contribute to the driving mechanism for plate motions These forces are self-regulating and maintain the rate of plate generation required to achieve a balance between heat loss and heat generation

Global Sea Floor Topography from Satellite Altimetry and Ship Depth Soundings

Abstract: A digital bathymetric map of the oceans with a horizontal resolution of 1 to 12 kilometers was derived by combining available depth soundings with high-resolution marine gravity information from the Geosat and ERS-1 spacecraft. Previous global bathymetric maps lacked features such as the 1600-kilometer-long Foundation Seamounts chain in the South Pacific. This map shows relations among the distributions of depth, sea floor area, and sea floor age that do not fit the predictions of deterministic models of subsidence due to lithosphere cooling but may be explained by a stochastic model in which randomly distributed reheating events warm the lithosphere and raise the ocean floor.

Derivation of some modern arc magmas by melting of young subducted lithosphere

Abstract: MOST volcanic rocks in modern island and continental arcs are probably derived from melting of the mantle wedge, induced by hydrous fluids released during dehydration reactions in the subducted lithosphere1. Arc tholeiitic and calc-alkaline basaltic magmas are produced by partial melting of the mantle, and then evolve by crystal fractionation (with or without assimilation and magma mixing) to more silicic magmas2—basalt, andesite, dacite and rhyolite suites. Although most arc magmas are generated by these petrogenetic processes, rocks with the geochemical characteristics of melts derived directly from the subducted lithosphere are present in some modern arcs where relatively young and hot lithosphere is being subducted. These andesites, dacites and sodic rhyolites (dacites seem to be the most common products) or their intrusive equivalents (tonalites and trondhjemites) are usually not associated with parental basaltic magmas3. Here we show that the trace-element geochemistry of these magmas (termed 'adakites') is consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.

Current plate motions

Abstract: A global plate motion model, named NUVEL-1, which describes current plate motions between 12 rigid plates is described, with special attention given to the method, data, and assumptions used Tectonic implications of the patterns that emerged from the results are discussed It is shown that wide plate boundary zones can form not only within the continental lithosphere but also within the oceanic lithosphere; eg, between the Indian and Australian plates and between the North American and South American plates Results of the model also suggest small but significant diffuse deformation of the oceanic lithosphere, which may be confined to small awkwardly shaped salients of major plates

Current plate motions

Abstract: SUMMARY We determine best-fitting Euler vectors, closure-fitting Euler vectors, and a new global model (NUVEL-1) describing the geologically current motion between 12 assumed-rigid plates by inverting plate motion data we have compiled, critically analysed, and tested for self-consistency. We treat Arabia, India and Australia, and North America and South America as distinct plates, but combine Nubia and Somalia into a single African plate because motion between them could not be reliably resolved. The 1122 data from 22 plate boundaries inverted to obtain NUVEL-1 consist of 277 spreading rates, 121 transform fault azimuths, and 724 earthquake slip vectors. We determined all rates over a uniform time interval of 3.0m.y., corresponding to the centre of the anomaly 2A sequence, by comparing synthetic magnetic anomalies with observed profiles. The model fits the data well. Unlike prior global plate motion models, which systematically misfit some spreading rates in the Indian Ocean by 8–12mm yr−1, the systematic misfits by NUVEL-1 nowhere exceed ∼3 mm yr−1. The model differs significantly from prior global plate motion models. For the 30 pairs of plates sharing a common boundary, 29 of 30 P071, and 25 of 30 RM2 Euler vectors lie outside the 99 per cent confidence limits of NUVEL-1. Differences are large in the Indian Ocean where NUVEL-1 plate motion data and plate geometry differ from those used in prior studies and in the Pacific Ocean where NUVEL-1 rates are systematically 5–20 mm yr−1 slower than those of prior models. The strikes of transform faults mapped with GLORIA and Seabeam along the Mid-Atlantic Ridge greatly improve the accuracy of estimates of the direction of plate motion. These data give Euler vectors differing significantly from those of prior studies, show that motion about the Azores triple junction is consistent with plate circuit closure, and better resolve motion between North America and South America. Motion of the Caribbean plate relative to North or South America is about 7 mm yr−1 slower than in prior global models. Trench slip vectors tend to be systematically misfit wherever convergence is oblique, and best-fitting poles determined only from trench slip vectors differ significantly from their corresponding closure-fitting Euler vectors. The direction of slip in trench earthquakes tends to be between the direction of plate motion and the normal to the trench strike. Part of this bias may be due to the neglect of lateral heterogeneities of seismic velocities caused by cold subducting slabs, but the larger part is likely caused by independent motion of fore-arc crust and lithosphere relative to the overriding plate.