Anny Cazenave

Regional variations in subsidence rate of oceanic plates: a global analysis

A global analysis of the dependence of seafloor depth on crustal age has been performed for each large oceanic plate independently. Each plate has been divided into regions bounded by major tectonic features. A total of 32 regions have been considered. Bathymetry corrected for sediment loading has been plotted as a function of crustal age for each region. Except for three regions (western Pacific between 10°S and 40°N, North American plate between 24°N and 38°N and African plate between 10°N and 24°N) where ocean floor older than 80–100 Ma flattens with age, in other regions, depth increases linearly with age1/2. Subsidence rate over each region has been computed by linear regression. Large variations (up to a factor 2) are reported from one region to another. Asymmetrical subsidence is also observed in a number of areas. However, the largest asymmetries are less than the regional variations in subsidence rate occurring on individual plates. Regional variations in subsidence rate appear linearly related to regional variations in ridge crest topography, shallow ridge subsiding quickly and deep ridge subsiding slowly. Hence seafloor depth subsides at a rate modulated by the value of the initial (axial) depth. This observation suggests that regional variations in subsidence rate reflect the thermal regime of the mantle beneath mid-ocean ridges rather than lateral temperature variations inside the asthenosphere involving small-scale convection.

A model for the plate tectonic evolution of the east-central Pacific based on SEASAT investigations

Abstract A complete SEASAT dataset is used to investigate the small-scale bathymetry of a 15° by 15° area of the east-central Pacific, northeast of Pitcairn Island, which was involved in the Miocene ridge jump and reorientation. Two fracture zones (FZ1 and FZ2) are mapped along 450 km, at an azimuth of N95°E, while the signature of the Austral Fracture Zone disappears east of 135°W. Several seamounts are discovered or confirmed along the Oeno-Henderson-Ducie lineament, and the degree of isostatic compensation of a number of islands at the eastern end of the Tuamotu group is studied: islands of the northern branch have practically no signatures in the geoid, while the southern ones must have formed on lithosphere at least 10 Ma old. On the basis of these constraints, we present a model of the evolution of spreading since 40 Ma B.P., in which the deactivation of the Austral Fracture Zone and the transfer of its offset to FZ1 in the Late Oligocene is interpreted as due to rift propagation, triggered by a northern Tuamotu hotspot; we propose that the Oeno-Ducie chain is the surface expression of a southern Tuamotu hotspot, deviated 15° in azimuth by the presence of a young and fresh fracture zone. Thus our model predicts that hotspots and ridge systems can have a moderate influence on each others kinematics and surface expressions, which does not however extend to permanent trapping.

Geosat‐derived geoid anomalies at medium wavelength

Geosat profiles of the Exact Repeat Mission have been averaged over a 1-year period and high-pass-filtered using inverse method techniques. The geoid surface constructed with both ascending and descending profiles shows at medium wavelengths band-shaped anomalies preferentially elongated in the east-west direction. These anomalies have an average amplitude of ∼30 cm and dominant wavelengths to 750km and 1100 km. We have performed numerous tests to show that the lineations are not artefacts created by the filtering process. Moreover, two-dimensional (2-D) filtering with the inverse method applied on a regional basis over the Pacific gives essentially similar results, indicating that the filtered geoid is not affected by directional bias. Seafloor topography in the Pacific filtered by 2-D inverse method also shows east-west trending depth anomalies positively correlated to medium-wavelength geoid lineations. Along the East Pacific Rise, there is a clear correlation between geoid lineations and regional variations in axial depth. Cross-spectral analysis carried out on geoid and topography data over the Pacific area gives coherence maxima at 750-km and 1100-km wavelengths and admittance values of 2–3 m/km. Observed admittance matches the magnitude and shape of admittance predicted by lithospheric cooling models, suggesting that the lineations are related to regional variations in the plate cooling process. Convection models produce much higher admittances than observed unless a low-viscosity layer is assumed so that dynamical support cannot be completely discarded. In most instances, however, the position and direction of the lineations seem to be controlled by major fractures zones which is in favor of a lithospheric origin. In the south central Pacific where the lineations appear parallel to absolute plate motion, there may be a combination of both lithospheric and sublithospheric processes.

Satellites provide the big picture

INSIGHTS Mountain range formation p. 687 Rethinking vascular therapy for cancer p. 694 ▶ PERSPECTIVES WATER Downloaded from www.sciencemag.org on September 14, 2015 Watching water: From sky or stream? Monitoring and management of freshwater resources has long depended upon on-the-ground measurements. Satellite remote sensing has brought new complementing capabilities. In this final of three debates, Science invited arguments about the appropriate roles for, and balance between, each approach. By J. S. Famiglietti, 1, 2, 3 * A. Cazenave, 4, 5 A. Eicker , 6 J. T. Reager, 1 M. Rodell , 7 I. Velicogna 1 ,2 Satellite observations have revolutionized our understanding of hydrology, water availability, and global change, while catalyzing modern advances in weather, flood, drought, and fire prediction in ways that would not have occurred with relatively sparse POLICY ground-based measurements alone. Earth-observing satellites provide the necessary “big-picture” spatial coverage, as well as the regional-to-global understanding essential for improving predictive models and informing policy-makers, re- source managers, and the general public. Sustained investments in a robust satellite hydrology program have enabled a plethora of discoveries, along with modernization of water management, that have increased the human, economic, and water security of many nations. We now recognize distinct human- and climate-driven fingerprints on the water landscape that are dramatically changing the distribution of freshwater on Earth ( 1). Improved understanding and heightened societal aware- ness of the global extent of sea-level rise ( 2), ice sheet and glacial melt ( 3), changing rainfall patterns ( 4), declining snow cover ( 5), groundwater depletion ( 6), and the changing extremes of flooding ( 7) and drought ( 8) simply would not have occurred without satel- lite observations. As we look ahead, ongoing and near-future missions will soon provide routine global monitoring of the stocks of soil moisture ( 9), surface water ( 10), and total water storage ( 11)—which will im- prove estimates of groundwater storage changes ( 12)—and of the fluxes of precipitation ( 4) and evapotranspiration ( 13). Taken to- NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA. Department of Earth System Science, University of California, Irvine, CA, USA. 3 Department of Civil and Environmental Engineering, University of California, Irvine, CA, USA. 4 Centre National d’Etudes Spatiales–Laboratoire d’Etudes Geophysique et Oceanographique Spatiales (CNES/ LEGOS), Toulouse, France. 5 International Space Science Institute, Bern, Switzerland. 6 Institute of Geodesy and Geoinformation, University of Bonn, Germany. 7 NASA Goddard Space Flight Center, Greenbelt, MD, USA. *Corresponding author. E-mail: james.famiglietti@jpl.nasa.gov. sciencemag.org SCIENCE 14 AUGUST 2015 • VOL 349 ISSUE 6249 Published by AAAS ILLUSTRATION: DAVIDE BONAZZI Satellites provide the big picture

Thermal cooling of the oceanic lithosphere: new constraints from geoid height data

Abstract Up to now, tests of thermal models of the oceanic lithosphere as it cools and moves away from the ridge crest have been based mainly on topography and heat flow data. However, large areas of the ocean floor deviate from the normal subsidence due to thermal contraction and heat flow data are not very sensitive to the form of the model. Cooling of the lithosphere causes a short-wavelength step in the geoid across fracture zones that can also be used to constrain thermal models. We have analyzed geoid data at fracture zones from the SEASAT altimeter measurements in the entire Pacific Ocean and redetermined parameters of the cooling models. We find that the data reveal two distinct regimes of cooling; one for seafloor ages in the range 0–30 Ma, the other beyond 30 Ma; this does not appear to be correlated with particular fracture zones but rather it is representative of the whole area studied, i.e., the entire south Pacific and northeast Pacific Ocean. These two trends may be interpreted in terms of two different (asymptotic) thermal thicknesses of the plate model. The smaller thermal thickness (∼ 65 km) found for ages From the results obtained in this study, we conclude that the half-space cooling model is unable to explain the data, that beyond 30 Ma, a simple plate model gives a satisfactory fit to the data but in the younger plate portion (ages

Long wavelength topography, seafloor subsidence and flattening

Seafloor subsidence effects due to cooling of the oceanic lithosphere have been removed from bathymetry data. The corrected ocean floor topography presents long wavelength highs, in particular over western Pacific. We show in this study that this long wavelength residual topography can be interpreted as either a term of seafloor flattening at old ages or a dynamic response to large-scale convection. Whatever its origin, this long wavelength residual topography is dominated by a degree 2 pattern highly correlated with geoid, lower mantle heterogeneities and plate age.

Seasat gravity undulations in the central Indian Ocean

Abstract In the southeast Indian ocean, geoid and gravity maps derived from the Geos3 and Seasat satellites data show clear evidence of a lineated pattern located between the southeast Indian ridge and the southern branch of the Ninetyeast Ridge. We have carried out two-dimensional spectral analyses of the gravity data over four regions, each of 10° sx 10° dimension, three being located over the Indo-Australian plate, one over the Antarctica plate. We have also analysed 13 Seasat geoid height profiles roughly parallel to the southeast Indian ridge, covering both sides of the ridge. Power spectra of the gravity and geoid data over the Indo-Australian plate confirm the existence of undulations oriented N40°E of ∼ 200 km wavelength, and of ∼ 10 mGal and 40 Ma. The wavelength remains constant with increasing plate age. Such undulations are not seen west of the southeast Indian ridge, i.e. over the Antarctica plate. Where observed, the geoid undulations have a sinusoidal signature quite different from the typical step-like signature of fracture zones. The lineations observed in the southeast Indian ocean have characteristics comparable to those reported by Haxby and Weissel in the central Pacific and interpreted as evidence of small-scale convection occurring in a low viscosity layer just beneath the lithosphere.

Reevaluation of the Chandler wobble seismic excitation from recent data

Abstract Since 1977, seismic moment tensors of major earthquakes have been systematically determined while high accuracy polar motion data have been made available from space techniques. The seismic excitation of the Chandler wobble during the period 1977–1983 has been obtained using the centroid moment tensor solutions of 1287 strong and moderate earthquakes occurring during that period. Changes in the Earth inertia tensor induced by earthquakes, which are derived from the elastic theory of dislocation, are assumed to be step functions of time. From a comparison between the derived synthetic Chandler wobble and the observed one, it turns out that seismic excitation is far too small (by about two orders of magnitude) to explain the Chandler wobble amplitude variations during 1977–1983, in particular the increase in the wobble amplitude between 1980 and 1983. During the last two decades, seismic moments of major earthquakes remained on the level of 10 27 to 10 28 dyne cm, whereas in the past, events of high magnitudes corresponding to seismic moments on the order of 10 30 to 10 31 dyne cm have been frequently reported. As the later values are those needed to explain the amplitude variations of the Chandler wobble, the problem of the validity of magnitude estimations in the past and frequency of occurrence of great earthquakes is raised, as well as the question of validity of previous positive conclusions on the influence of earthquakes on the Chandler wobble excitation.

Possible dynamical evolution of the rotation of Venus since formation

The past evolution of the rotation of Venus has been studied by a numerical integration method using the hypothesis that only solar tidal torques and core-mantle coupling have been active since formation. It is found quite conceivable that Venus had originally a rotation similar to the other planets and has evolved in 4.5×109 years from a rapid and direct rotation (12-hour spin period and nearly zero obliquity) to the present slow retrograde one.While the solid tidal torque may be quite efficient in despinning the planet, a thermally driven atmospheric tidal torque has the capability to drive the obliquity from ∼0° towards 180° and to stabilize the spin axis in the latter position. The effect of a liquid core is discussed and it is shown that core-mantle friction hastens the latter part of the evolution and makes even stronger the state of equilibrium at 180°. The model assumes a nearly stable balance between solid and atmospheric tides at the current rotation rate interpreting the present 243 day spin period as being very close to the limiting value.A large family of solutions allowing for the evolution, in a few billions years, of a rapid prograde rotation to the present state have been found. Noticeably different histories of evolution are observed when the initial conditions and the values of the physical parameters are slightly modified, but generally the principal trend is maintained.The proposed evolutionary explanation of the current rotation of Venus has led us to place constraints on the solid bodyQ and on the magnitude of the atmospheric tidal torque. While the constraints seem rather severe in the absence of core-mantle friction (aQ≃15 at the annual frequency is required, and a dominant diurnal thermal response in the atmosphere is needed), for a large range of values of the cores viscosity, the liquid core effect allows us to relax somewhat these constraints: a solid bodyQ of the order ∼40 can then be allowed. ThisQ value implies that a semi-diurnal ground pressure oscillation of ≃2 mb is needed in the atmosphere in order for a stable balance to occur between the solid and atmospheric tides at the current rotation rate. No model of atmospheric tides on Venus has been attempted in this study, however the value of 2 mb agrees well with that predicted by the model given in Dobrovolskis (1978).

The Earth's Variable Rate of Rotation: A Discussion of Some Meteorological and Oceanic Causes and Consequences

Aspects of the Earth’s variable speed of rotation, or variations in the length of day, and their geophysical consequences and causes are reviewed. Emphasis is placed on those areas which may benefit most from improved observations of the rotation rate. Seasonal changes in the length of day are primarily of meteorological origin. Zonal winds, in particular, play an important role, and year-to-year variations in the magnitude of the seasonal rotational characteristics provide information on the variability of the year-to-year atmospheric circulation. Changes observed since 1955 in the annual and semi-annual change in the length of day indicate a decreasing strength of the zonal circulation at these frequencies. Changes observed in the astronomical biennial term indicate that the biennial zonal winds propagate downwards to variable depths and that it is of variable period. Higher-frequency variations in length of day are also primarily of meteorological origin and will mask or interfere with other geophysical factors affecting the Earth’s rotation, such as tides or earthquake caused changes in the inertia tensor. Thus improved observations of the variable rotation will have to be accompanied by improved global compilations of zonal winds so that the meteorological contribution can be evaluated with equal accuracy. Present compilations of wind data are inadequate for this. An area where satellite observations can make an important contribution to studies of the Earth’s rotation concerns the separation of the secular tidal and non-tidal changes in length of day by studying the tidal perturbations in satellite orbits.