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Visit for additional information and data download details and for global terrain visualisation of the vertical gravity gradient (vgg) data and for plate tectonic raster reconstructions of the vgg from 200 Ma to present. Gravity models are powerful tools for mapping tectonic structures, especially in the deep ocean basins where the topography remains unmapped by ships or is buried by thick sediment. We combined new radar altimeter measurements from satellites CryoSat-2 and Jason-1 with existing data to construct a global marine gravity model that is two times more accurate than previous models. We found an extinct spreading ridge in the Gulf of Mexico, a major propagating rift in the South Atlantic Ocean, abyssal hill fabric on slow-spreading ridges, and thousands of previously uncharted seamounts. These discoveries allow us to understand regional tectonic processes and highlight the importance of satellite-derived gravity models as one of the primary tools for the investigation of remote ocean basins.
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New global marine gravity model
from CryoSat-2 and Jason-1 reveals
buried tectonic structure
David T. Sandwell,
*R. Dietmar Müller,
Walter H. F. Smith,
Emmanuel Garcia,
Richard Francis
Gravity models are powerful tools for mapping tectonic structures, especially in the deep
ocean basins where the topography remains unmapped by ships or is buried by thick
sediment. We combined new radar altimeter measurements from satellites CryoSat-2
and Jason-1 with existing data to construct a global marine gravity model that is two times
more accurate than previous models. We found an extinct spreading ridge in the Gulf of
Mexico, a major propagating rift in the South Atlantic Ocean, abyssal hill fabric on
slow-spreading ridges, and thousands of previously uncharted seamounts. These
discoveries allow us to understand regional tectonic processes and highlight the
importance of satellite-derived gravity models as one of the primary tools for the
investigation of remote ocean basins.
Fracture zones (FZs) spanning the ocean ba-
sins reveal the breakup of the continents
and the geometry of sea-floor spreading (1).
The exact intersection points of the FZs
along conjugate continental margins are
used for precise reconstruction of the continents
(24). These FZ intersections are commonly
buried by several kilometers of sediments that
flow off the continents to fill the voids created
by the thermal subsidence of the rifted margins
(5). This sediment cover extends hundreds to
thousands of kilometers out onto the oceanic
lithosphere, resulting in a relatively flat and
featureless sea floor. Reflection seismic profiles
can reveal the underlying basement topogra-
phy of the FZs, but the data coverage is usually
insufficient to map out the intersections. In areas
of thin sediment cover, the topographic ridges
and troughs along the FZs produce large gravity
anomalies that are easily traced across the ocean
basins (Fig. 1). However, when the topography
becomes buried by sediment, the original density
contrast of the sea-floor topography is reduced,
resulting in more-subdued, and sometimes sign-
reversed, gravity signatures (6). Moreover, as the
lithosphere ages and cools, the sea floor subsides,
causing a blurring of the gravity anomalies; smaller
wavelengths of the gravity field become less well-
resolved with increasing water depth. Previous
global marine gravity models derived from satel-
lite altimetry had sufficient accuracy and coverage
to map all FZs in unsedimented sea floor (7), but
the 3 to 5 mGal of gravity noise blurred the small
signatures of sediment-covered topography such
as seamounts and FZs. Here we report on a new
global marine gravity model having ~2-mGal ac-
curacy that is providing a dramatically improved
resolution of the 80% of the sea floor that remains
uncharted or is buried beneath thick sediment.
Gravity-field accuracy derived from satellite al-
timetry depends on three factors: altimeter range
precision, spatial track density, and diverse track
orientation. Two altimeter data sets with high
track density have recently become available
(CryoSat-2 and Jason-1) to augment the older
altimeter data (Geosat and ERS-1), resulting in
improvement by a factor 2 to 4 in the global
marine gravity field. Their newer radar technol-
ogy results in a 1.25-times improvement in range
precision that maps directly into gravity-field
improvement (8). The new altimeters also con-
pared with the 31 months provided by the older
satellites. CryoSat-2 has provided the most dense
track coverage, because although it has a nominal
369-day repeat orbit period, the ground tracks are
allowed to drift within a 5-km band, so after 4 years
in orbit it has provided a nominal track spacing of
about 2.5 km. Jason-1 provided 14 months of dense
track coverage during its geodetic phase, resulting
in a track spacing of 7.5 km.
Most of the improvement in the altimeter-
derived gravity field occurs in the 12- to 40-km
wavelength band, which is of interest for the in-
vestigation of structures as small as 6 km. The
current version of the altimeter-derived gravity
field has an accuracy of about 2 mGal (8). Unlike
terrestrial gravity, where coverage is uneven, these
accuracies are available over all marine areas and
large inland bodies of water, so this gravity pro-
vides an important tool for exploring the deep
ocean basins. At scales smaller than 200 km,
variations in marine gravity primarily reflect
Scripps Institution of Oceanography, La Jolla, CA 92093,
EarthByte Group, School of Geosciences, University of
Sydney, New South Wales, Australia.
Laboratory for Satellite
Altimetry, National Oceanic and Atmospheric Administration
(NOAA), College Park, MD 20740, USA.
European Space
Agency/European Space Research and Technology Centre,
Keplerlaan 1, 2201AZ Noordwijk, Netherlands.
*Corresponding author. E-mail:
Fig. 1. Ocean gravity maps. (A) New marine gravity anomaly map derived
from satellite altimetry reveals tectonic structures of the ocean basins in
unprecedented detail, especially in areas covered by thicksediments. Land areas
show gravity anomalies from Earth Gravitational Model 2008 (15). (B)VGGmap
derived from satellite altimetry highlights FZs crossing the South Atlantic Ocean
basin (yellow line). Areas outlined in red are small-amplitude anomalies in areas
where thick sediment has diminished the gravity signal of the basement to-
pography.The full-resolution gravity anomaly and VGG models can be viewed in
Google Earth using the following files:
1min/global_grav.kmz and
grav_gradient.kmz.The grids are available in the supplementary material, as well
as at the following FTP site:
sea-floor topography generated by plate tecton-
ics such as ridges, FZs, and abyssal hills. Many
continental margins, and one can also better in-
terpret buried, migrating, unstable FZs, which has
the potential to improve the use of FZs as tie
points for reconstructions of the boundaries be-
tween continental fit reconstructions (Fig. 1). In
addition to FZs, there are other tectonic features
associated with continental margins, such as the
boundaries between continental and oceanic crust
[continent-ocean boundaries (COBs)], that can
now be mapped in greater detail.
The first example (Fig. 2) is in the Gulf of
Mexico, where thick sediments obscure the FZs
and extinct ridges. Reconstruction models pro-
vide the overall framework of counterclockwise
rotation of the Yucatan plate with respect to
North America, as well as a generalized position
for the COB (9). The new vertical gravity gradient
images confirm and refine the positions of these
tectonic boundaries. Extinct spreading ridges pro-
duce a negative gravity signature, because the
relatively high-density sediment cover largely
cancels the positive gravity effect of the topo-
graphic ridge, leaving the negative gravity sig-
nature of the compensating Moho topography
(6). In this region, the Moho is more than 15 km
beneath the sea surface, so the effects of upward
continuation reduce and smooth the anomaly.
The second example is on the African ridge
flank, where the new data reveal a major tectonic
feature that was not visible in previous satellite
gravity data sets because of high-frequency noise.
The newly discovered feature is a set of tectonic
lineaments roughly between 8°S and 12°S, strik-
ing northwest-southeast and obliquely dissected
by individual en-echelon faults, stretching from
the Bodo Verde Fracture Zone in the north into
the middle of the Cretaceous Magnetic Quiet Zone
at its southeastern extension (Fig. 1b). This feature
not follow either the azimuth of nearby sea-floor
isochrons or FZs. A reconstruction of this feature
at magnetic chron 34 [83.5 million years ago
(Ma)] (Fig. 3) reveals that it has a mirror-image
counterpart on the South American plate, but
this conjugate feature is represented only by a
relatively faint gravity lineament (Fig. 3). This
feature is visible in the filtered vertical gravity
gradient image, marking a boundary between
Fig. 2. Gulf of Mexico VGG. (A) Uninterpreted. (B) Our interpretation of tectonic structures, after Pindell and Kennan (9). TheVGG reveals subtle signatures of
the extinct spreading ridges and FZs as well as a significant change in amplitude across the boundary between continental and oceanic crust (COBs).This is a
Mercator projection; grayscale saturates at T20 eotvos units.
Fig. 3. South Atlantic filtered VGG. Reconstructed at chron 34 (83.5 Ma, orthographic projection) with
Africa fixed (16). Major tectonic and volcanic sea-floor features and offshore sedimentary basins are
labeled. The mid-ocean ridge is outlined in red, the extinct Abimael spreading ridge is shown as a dashed
red line, and the reconstructed position of the Cardno hot spot (CS) is outlined by a red star. Most of the
sea floor shown in this reconstruction was formed during the Cretaceous Normal Superchron. Also note
that the extinct Abimael spreading ridge between the Santos Basin and Sao Paulo Plateau offshore of
Brazil is now visible as a negativeVGGanomaly (dashed red line), as compared to previous interpretations
(17,18). This region is of great interest for oil and gas exploration, as it is one of the most extensive
deepwater oil and gas frontiers globally, with several recent discoveries (19).
swaths of differently textured sea-floor fabric to
the east and west of the lineament (Fig. 3). The
geometry of the two features suggests that they
form a pair of an extinct ridge (on the African
side) and a pseudofault (on the South American
side), created by a northward ridge propagation
episode between ~100 and 83 Ma. An absolute
hot spotbased plate reconstruction using the
rotation parameters from ONeill et al.(10)in-
dicates that the Cardno hot spot (Fig. 3) may
have been situated not far north of the northern
tip of the ridge propagator, where it abuts the
propagator came to a halt. These observations
conform with the inference that ridges have a
tendency to propagate toward hot spots/plumes
and that propagation events and resulting spread-
ing asymmetries are frequently contained within
individual spreading corridors bounded by FZs
(11). The existence of major previously unknown
ridge propagation events will also be relevant for
interpreting marine magnetic anomaly sequences
during the Cretaceous Normal Superchron on
conjugate ridge flanks (12).
One of the most important uses of this new
marine gravity field will be to improve the esti-
having no depth soundings. The most accurate
method of mapping sea-floor depth uses a mul-
tibeam echosounder mounted on a large research
vessel. However, even after 40 years of mapping
by hundreds of ships, one finds that more than
50% of the ocean floor is more than 10 km away
from a depth measurement. Between the sound-
ings, the sea-floor depth is estimated from marine
gravity measurements from satellite altimetry
(13). This method works best on sea floor where
sediments are thin, resulting in a high correla-
tion between sea-floor topography and gravity
anomalies in the 12-kmto160-km wavelength
band. The shorter wavelengths are attenuated
because of Newtons inverse square law, whereas
the longer wavelengths are partially cancelled by
the gravity anomalies caused by the isostatic
topography on the Moho (13). The abyssal hill
fabric created during the sea-floor spreading
process has characteristic wavelengths of 2 to
12 km, so it is now becoming visible in the ver-
tical gravity gradient (VGG) models, especially
on the flanks of the slower-spreading ridges
(14). Additionally, seamounts between 1 and 2 km
tall, which were not apparent in the older gravity
models, are becoming visible in the new data.
As CryoSat-2 continues to map the ocean sur-
face topography, the noise in the global marine
gravity field will decrease. Additional analysis
of the existing data, combined with this steady
decrease in noise, will enable dramatic improve-
ments in our understanding of deep ocean tec-
tonic processes.
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2. S. Cande, J. LaBrecque, W. Haxby, J. Geophys. Res. Solid Earth
93, 1347913492 (1988).
3. C. Heine, J. Zoethout, R. D. Müller, Solid Earth 4, 215253
4. L. A. Lawver, L. M. Gahagan, I. W. Dalziel, Mem. Natl. Inst. Polar
Res. 53,214229 (1998).
5. M. S. Steckler, A. B. Watts, Earth Planet. Sci. Lett. 41,113
6. C. S. Liu, D. T. Sandwell, J. R. Curray, J. Geophys. Res. 87,
76737686 (1982).
7. K. Matthews, R. D. Müller, P. Wessel, J. M. Whittaker,
J. Geophys. Res. Solid Earth 116,128 (2011).
8. Materials and methods are available as supplementary
materials on Science Online.
9. J. Pindell, L. Kennen, in The Geology and Evolution of the
Region Between North and South America, K. James,
M. A. Lorente, J. Pindell, Eds. (Special Publication, Geological
Society of London, London, 2009), vol. 328, pp. 155.
10. C. O'Neill, R. D. Müller, B. Steinberger, Geochem. Geophys.
Geosyst. 6, Q04003 (2005).
11. R. D. Müller, W. R. Roest, J. Y. Royer, Nature 396, 455459
12. R. Granot, J. Dyment, Y. Gallet, Nat. Geosci. 5, 220223
13. W. H. F. Smith, D. T. Sandwell, Science 277, 19561962
14. J. A. Goff, W. H. F. Smith, K. A. Marks, Oceanography 17,2437
15. N. K. Pavlis, S. A. Holmes, S. C. Kenyon, J. K. Factor,
J. Geophys. Res. 117, B04406 (2012).
16. M. Seton et al., Earth Sci. Rev. 113, 212270 (2012).
17. W. Mohriak, M. Nóbrega, M. Odegard, B. Gomes, W. Dickson,
Petrol. Geosci. 16, 231245 (2010).
18. I. Scotchman, G. Gilchrist, N. Kusznir, A. Roberts, R. Fletcher, in
The Breakup of the South Atlantic Ocean: Formation of Failed
Spreading Axes and Blocks of Thinned Continental Crust in the
Santos Basin, Brazil and Its Consequences For Petroleum
System Development (Petroleum Geology Conference Series,
Geological Society of London, London, 2010), pp. 855866.
19. W. U. Mohriak, P. Szatmari, S. Anjos, Geol. Soc. London Spec.
Publ. 363, 131158 (2012).
The CryoSat-2 data were provided by the European Space Agency,
and NASA/Centre National d"Etudes Spatiales provided data
from the Jason-1 altimeter. This research was supported by
NSF (grant OCE-1128801), the Office of Naval Research (grant
N00014-12-1-0111), the National Geospatial Intelligence Agency
(grant HM0177-13-1-0008), the Australian Research Council
(grant FL099224), and ConocoPhillips. Version 23 of global grids
of the gravity anomalies and VGG can be downloaded from the
supplementary materials and also at the following FTP site: ftp:// The manuscript contents
are the opinions of the authors, and the participation of W.H.F.S.
should not be construed as indicating that the contents of the
paper are a statement of official policy, decision, or position on
behalf of NOAA or the U.S. government.
Supplementary Text
Figs. S1 and S2
References (2033)
2 July 2014; accepted 2 September 2014
Chiral nanophotonic waveguide
interface based on spin-orbit
interaction of light
Jan Petersen, Jürgen Volz,*Arno Rauschenbeutel*
Controlling the flow of light with nanophotonic waveguides has the potential of
transforming integrated information processing. Because of the strong transverse
confinement of the guided photons, their internal spin and their orbital angular
momentum get coupled. Using this spin-orbit interaction of light, we break the mirror
symmetry of the scattering of light with a gold nanoparticle on the surface of a
nanophotonic waveguide and realize a chiral waveguide coupler in which the handedness
of the incident light determines the propagation direction in the waveguide. We control
the directionality of the scattering process and can direct up to 94% of the incoupled
light into a given direction. Our approach allows for the control and manipulation of
light in optical waveguides and new designs of optical sensors.
The development of integrated electronic cir-
cuits laid the foundations for the informa-
tion age, which fundamentally changed
modern society. During the past decades,
a transition from electronic to photonic in-
formation transfer took place, and nowadays,
nanophotonic circuits and waveguides promise
to partially replace their electronic counterparts
and to enable radically new functionalities (13).
The strong confinement of light provided by such
waveguides leads to large intensity gradients on
the wavelength scale. In this strongly nonparaxial
regime, spin and orbital angular momentum of
light are no longer independent physical quan-
tities but are coupled (4,5). In particular, the spin
and on the propagation direction of light in the
waveguidean effect referred to as spin-orbit in-
teraction of light (SOI). This effect holds great
promises for the investigation of a large range of
physical phenomena such as the spin-Hall effect
(6,7) and extraordinary momentum states (8)
and has been observed for freely propagating light
fields (9,10) in the case of total internal reflection
(11,12), in plasmonic systems (1315), and for
radio frequency waves in metamaterials (16). Re-
cently, it has been demonstrated in a cavity-
quantum electrodynamics setup in which SOI
fundamentally modifies the coupling between a
single atom and the resonator field (17).
Vienna Center for Quantum Science and Technology, TU
WienAtominstitut, Stadionallee 2, 1020 Vienna, Austria.
*Corresponding author. E-mail: (J.V.); arno. (A.R.)
Supplementary Materials for
New global marine gravity model from CryoSat-2 and Jason-1 reveals
buried tectonic structure
David T. Sandwell,* R. Dietmar Müller, Walter H. F. Smith, Emmanuel Garcia, Richard
*Corresponding author. E-mail:
Published 3 October 2014, Science 346, 65 (2014)
DOI: 10.1126/science.1258213
This PDF file includes:
Supplementary Text
Figs. S1 and S2
References (20–33)
Supplementary Text
Gravity Anomaly Recovery
Gravity anomalies are small differences in the pull of gravity associated with lateral
variations in mass. The best approach to measuring marine gravity is to mount a very
precise accelerometer on a ship. Unfortunately this ship coverage of the oceans is very
sparse (20). A second, now equally precise approach is to use an orbiting radar to
measure the topography of the ocean surface, which is nearly an equipotential surface.
The methods for recovering maps of marine gravity anomaly from radar altimeter data
are discussed in many previous publications [e.g., (21-24)]. Some of the key technology
developments related to this new marine gravity model are provided in two recent
publications (25, 26). For our investigation of crustal structure we use Laplace’s equation
to construct the first and second vertical derivatives of the potential called gravity
anomaly and vertical gravity gradient, respectively. Images of these two fields over the
South Atlantic Basin are shown in Fig. 1. The full resolution maps are best viewed using
a computer display program such as Google Earth. The reader can download two small
KMZ files to bring these full resolution maps into their computer. In addition they can
download the gridded files to construct custom maps from .
Improved Radar Technology
The most important contribution of the new altimeters is related to a 1.25 times
improvement in range precision (26). This improvement is mainly related to an increase
in the pulse repetition frequency (PRF) of the newer altimeters with respect to the older
altimeters. The coherent nature of the radar signal results in speckle in the echoes, which
masks the echo waveform and leads to imprecision in retrieval of its parameters. This can
be alleviated by averaging successive echoes, but only up to the point that they become
correlated, the onset of which has been generally assumed at a PRF of somewhat above
2 kHz at the common transmitter frequency of 13.5 GHz (27). The newer altimeters
CryoSat-2 and Jason-1 have PRFs of 1950 Hz and 2060 Hz, respectively while the older
instruments were technologically limited to lower values of 1020 Hz. Theoretically this
approximate doubling of PRF should result in a
improvement in range
precision; the actual improvement is somewhat smaller (1.25) perhaps reflecting the onset
of echo correlation at the 2 kHz PRF. Nevertheless, this improvement in range precision
maps directly into an improvement in gravity field accuracy.
CryoSat-2 was also operated in a new Synthetic Aperture Radar (SAR) mode over
very limited areas of the oceans. This mode has a much higher PRF of 18.2 kHz and the
highly correlated echoes are summed coherently in bursts of 64 pulses to form a long
synthetic aperture. This enhances along-track resolution in the form of a set of narrow
beams distributed in the along-track direction (27–30). Unlike the conventional pulse-
width limited geometry, the resulting echo waveforms have useful information in both
the leading and trailing edges. This, together with an increase in the effective number of
independent samples resulting from the SAR technique, reduces the height noise by a
factor of ~1.4 compared to conventional LRM (31). Comparison of height noise
performance (26) indeed shows this expected improvement for CryoSat-2’s SAR but
similar gains for pulse-width limited echoes are obtained by a two-pass processing
scheme in which the slowly varying ocean wave-height is first estimated and smoothed
and then excluded from the estimation process in the second pass. These results show that
CryoSat-2’s LRM performs slightly better than Jason-1 (which is already excellent),
despite its reduced PRF. Much of the design of the two radars is common but it is likely
that the improvements introduced for CryoSat-2’s mission, particularly the higher
transmitter power needed for operation over sloping ice surfaces and the extreme phase
stability required for SAR interferometry, are contributing to this performance.
Coherence-Enhancing Filter
Despite the advances in satellite gravity anomaly image quality described in this
paper, some high-frequency noise remains. In order to further improve the interpretation
of linear tectonic features seen in the new vertical gravity gradient images, we have
applied a filtering technique called coherence-enhancing diffusion to a selected region in
the South Atlantic (Fig. 3) (32). This filter combines anisotropic diffusion (a low-pass
filter) with texture analysis, such that a diffusion tensor is computed from the local image
structure so that the diffusion is parallel to linear features in the data. This type of filter
has been successfully applied for enhancing noisy seismic reflection images to facilitate
improved tracking of seismic horizons (33), and is applied to vertical gravity gradient
data here for the first time. While high-frequency noise has been suppressed, linear,
coherent seafloor structures have been enhanced. In particular the internal en-echelon
structure of the extinct mid-ocean ridge on the African Plate has been enhanced, while
the juxtaposed differences in seafloor structure west and east of the conjugate pseudofault
have been enhanced as well. Deeply buried linear structures of the Santos Basin and Sao
Paulo Plateau offshore Brazil have been equally enhanced (Fig. 3), illustrating that
improved satellite data combined with well-targeted filtering have a great potential to
reveal previously hidden structures on abyssal plains and along passive margins.
Gravity Anomaly Uncertainty
We estimated the uncertainty in the gravity by calculating the rms difference in slope
between individual altimeter profiles and the mean north and east slopes used to compute
gravity (Fig S1.) The uncertainties were calibrated by comparisons with shipboard data
from two completely different proprietary sources. First we computed the rms difference
between the altimeter-derived gravity and more accurate shipboard gravity in a small
region in the Gulf of Mexico. The ship data, provided by EDCON Inc., were collected on
a very fine grid and have an rms crossover error of 0.5 mGal (25). For this first
comparison we found an rms difference of 1.60 mGal. In the second case, the altimeter-
derived gravity data were compared with 30 million of the best shipboard data by the
National Geospatial Intelligence Agency (NGA personal communication) resulting in an
rms difference of 2.6 mGal. The rms difference is somewhat higher (3.6 mGal) in
shallow areas (< 1 km) and somewhat lower (2.3 mGal) in deeper areas (3 - 6 km). On
average, the NGA ship data have an rms error of 1 - 2 mGal. Assuming the mean rms
error is 1.6 mGal then the mean rms error in the altimeter-derived gravity is ~2 mGal in
agreement with the calibration derived from the Gulf of Mexico comparison. As shown
in our previous study (25) most of the error reduction between this new gravity model
and the older models occur in the 12 to 40 km wavelength band.
The noise reduction over the short wavelength band provides a dramatic improvement
in the clarity of the vertical VGG signals. We used the Gulf of Mexico region to
illustrate this noise reduction (Fig. S2). The upper plot shows the VGG derived from
only Geosat and ERS-1 altimetry data (24) while the middle plot shows also includes the
new measurements from CryoSat-2 and Jason-1. The reduction in noise between the old
and new models reveals the extinct spreading ridges and transforms as well as the
continent ocean boundary. One can also see some of these features in the old model but
they are largely obscured by noise. The difference between the new and old model (Fig.
S2 c) reveals the noise in the old model. The rms difference between the two models is
6.9 eotvos units and in terms of gravity anomaly (not shown) the rms difference is 2.2
mGal. The rms differences are zero over land, where the VGG and gravity anomaly are
set by the EGM08 model (9). Differences are greatest near the shorelines where the raw
altimeter waveforms are sometimes contaminated by stray echoes off the land. To
understand the contribution of each of the satellite data sets to the accuracy of the gravity
grid, we have constructed a suite of gravity models after removing one of the non-repeat
data sets. We find that because the four non-repeat altimeter data sets have differing
orbital inclinations and noise levels they are all important for achieving the best overall
Fig. S1. Gravity error. (a) Estimated error in marine gravity anomaly to 81 degrees
latitude. Color scale ranges from 0-10 mGal. Relatively larger noise occurs in areas of
high mesoscale variability such as the Gulf Stream. Sharp changes in gravity noise occur
at the maximum inclination of Jason-1, Geosat, ERS/Envisat ground tracks. (b)
Longitude-averaged gravity error versus latitude. Noise is higher in polar regions due to
lower track density and altimeter noise caused by sea ice. (Note CryoSat-2 collects data
to 88 degrees latitude but this plot only extends to 81.)
Fig. S2. Gulf of Mexico
VGG. (a) Old VGG model
based on Geosat and ERS-1.
(b) New VGG model also
includes data from CryoSat-
2 and Jason-1. (c)
Difference between the two
models plotted using the
same greyscale shows noise
in the old VGG model.
Mercator projection, grey
scale saturates at +/- 20
eotvos units.
References and Notes
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... The high-resolution PAD-13 and PAD-14 seismic profiles were acquired by R/V BGP Explorer for ENI Ireland BV during 2013-14. The vessel was equipped with Sercel G-Gun-II as a source, placed at a depth (Sandwell et al., 2014) of the study area. The seismic line crosses the highest gravity anomaly in Rockall Basin. ...
... The gravity data ( Fig. 3a) from Sandwell et al., (2014), is used in this study to investigate the large-scale crustal structure. It has a resolution of 2 mGal. ...
... The new gravity data was acquired during the acquisition of the DCENR-ENI seismic data in the Porcupine and Rockall Basin (Geotrace Technologies Ltd, 2015). Tomar and O'Reilly (2020) compiled this data with the satellite gravity data (Sandwell et al., 2014) and observed a good agreement (within ∼2 mGals). Therefore, the satellite gravity data (Sandwell et al., 2014) was used for gravity modelling. ...
... · 7 Figure F3. A. Bouguer anomaly from satellite-derived free air anomaly (Sandwell et al., 2014). Bouguer anomalies are obtained by subtracting from free air anomalies the attraction of seafloor topography and of unconsolidated sediments using a density contrast of 1630 Kg/m 3 and of 770Kg/m 3 , respectively. ...
... A method that is often used to estimate the level of water reservoirs is satellite altimetry (Duan and Bastiaanssen 2013). Satellite altimeter sensors can measure sea surface height (SSH) based on the center of mass of the earth over the oceans (Fu and Haines 2013;Sandwell et al. 2014). Although the main purpose of altimeter satellites is to study the altimetric level of the high seas, they are also able to monitor the water level changes of small lakes (Nguy-Robertson et al. 2018;Yang et al. 2017). ...
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In this study, the impact of vegetation, land surface temperature, and precipitation on changes in water level and area of seven inland lakes (Urmia lake in Iran, Tharthar, Mosul lakes and Hammar 4 wetland in Iraq, and Beyşehir and Erçek lakes in Turkey) is analyzed to evaluate the variability of these lakes due to climate change. The altimetric data from four remote sensing databases (TOPEX/POSEIDON and Jason 1, 2, and 3), the area of the lakes from the images of Landsat OLI and ETM + sensors, the vegetation from MOD13Q1-NDVI 250 m database, land surface temperature from LST-MOD11A1, and precipitation from GPM_3IMERGM product were used in this study to assess the changes occurring in the period of the last 20 years (2000–2019). The results showed that in the analyzed area the values of the land surface temperature and vegetation indices increased, whereas annual precipitation sums decreased. Although temperature and vegetation changes in all three countries were almost consistent with each other, changes in the water level and area of the studied lakes were different. The highest decrease in the water level was observed for Urmia lake. Although decreases in the water level were also observed in other lakes, their water level returned after a time to its initial level (1992). This was not the case for Urmia lake, where the water level after 1999 never returned to the initial value, finally lowering by 7 m. The fluctuations of the water level and area of Iran, Turkey, and Iraq lakes are however caused by factors other than only those related to climate, which needs more investigations to determine more precisely the changes in the water level of these lakes.
... The green line is the interpreted domain boundary and the dark blue line is the 1948s U-shaped line. Yao et al. (2006) also gave similar results for tectonic zoning roughly along the U-shaped line based on gravity anomalies shown in Fig.11, where the gravity data from satellite altimetry (Sandwell et al., 2014). Although gravity and magnetic anomalies are often not generated by the same source, the results of the SCS interpreted domain are highly consistent, of which boundary is approximately along the U-shaped line. ...
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Magnetic data has been widely applied in the tectonic division. High-resolution magnetic data were used to analyze the geotectonic zoning of the South China Sea. Based on the newly compilated magnetic data, the processing results and the distribution of known faults, we consider that the U-shaped line approximately along the South China Sea national boundary of China shown in the magnetic map is a significant geological and geophysical boundary. We first described the linear characteristics of the magnetic data and then applied pseudo-gravity, Euler deconvolution, tilt derivatives, and the texture segmentation method to process the data. Results show that the dividing line between the South China Sea and the surrounding blocks is approximately along this U-shaped line. The dividing line between the South China domain and the South China Sea domain is along with the Dongsha Islands to Xisha Trough, which is different from the previous geophysical zoning results. Our results are almost consistent with those of the gravity data indicating roughly the tectonic zonation along the U-shaped line.
... The free-air gravity and bathymetric data used in the Moho depth inversion were obtained from the global satellite gravity anomaly database (gravity: V24.1; bathymetry: V18.1) (Sandwell et al., 2014), which is maintained by the Scripps Marine Association of the University of California and the National Oceanic and Atmospheric Administration Laboratory for satellite altimetry. The resolution of the data was 1 min. ...
The Jurassic oceanic crust is the oldest existing oceanic crust on earth, and although distributed sparsely, carries essential information about the earth’s evolution. The area around the Pigafetta Basin in the west Pacific Ocean (also known as the Jurassic Quiet Zone, JQZ) is one of a few areas where the Jurassic oceanic crust is present. This study takes full advantage of high-resolution multichannel seismic reflection profiles in combination with bathymetry, magnetic, and gravity data from the JQZ to examine the structure, deformation, and morphology of the Jurassic oceanic crust. Our results show the following insights: 1) The Moho lies at 2–3s in two-way travel time beneath the seafloor with the segmented feature. The gaps between the Moho segments well correspond to the seamounts on the seafloor, suggesting the upward migration of magma from the mantle has interrupted the pre-existing Moho. 2) The oceanic crust is predominantly deformed by crustal-scale thrust faults, normal faults cutting through the top of basement, and vertical seismic disturbance zones in association with migration of thermal fluids. The thrust faults are locally found and interpreted as the results of tectonic inversion. 3) Seafloor morphology in the JQZ is characterized by fault scarps, fold scarps, seamounts, and small hills, indicating the occurrence of active faults. 4) The oceanic crust in the JQZ and East Pacific Rise has many structural and geometrical variations, such as the thickness of sediments, seafloor topography, basement morphology, fault size and type.
Various earthquake source models predict that aseismic slip modulates the seismic rupture process. However, observations of aseismic slip associated with earthquakes are scarce, which has left the earthquake source model controversial. Here, we characterise seismic and aseismic processes for 3 days during the 2014 Iquique earthquake sequence in northern Chile by analysing seismicity and crustal deformation time series measured by high-rate Global Positioning System (GPS). We demonstrate that the early afterslip started immediately after the M 8.1 mainshock and led to the largest M 7.6 aftershock 27 hours later, located ~120 km to the south. At the mainshock latitude, the interevent early afterslip is located downdip of the mainshock rupture, and is associated with aftershocks. These afterslip and aftershocks exhibit a rapid temporal decay. In contrast, south of the mainshock slip patch, a peak of afterslip separates the mainshock rupture from the largest aftershock, suggesting that this area acted as a barrier to the southward propagation of the mainshock rupture. Seismicity count and moment accelerate in this southern area during the interevent stage. We conclude that the largest aftershock nucleation was driven by the interevent afterslip. The mechanical connection between sequential great earthquakes can therefore be mediated by aseismic slip.
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Plain Language Summary The Blanco transform fault system (BTFS) northwest off the coast of Oregon is seismically very active. We used 1 year of ocean bottom seismometer data collected between September 2012 and October 2013 to locate 138 local earthquakes. The events align perfectly with the morphologic features of the BTFS, dividing the BTFS into five transform segments and two short intra‐transform spreading centers. Furthermore, we observe different seismotectonic behaviors of the western and eastern BTFS based on the along‐strike variation in morphology, magnetization, focal depth distribution, and strain partitioning. Although many segmented oceanic transform systems were formed from a single transform fault in response to rotations in plate motion, the BTFS turns out to be originated from non‐transform offsets between ridge segments, as we observed no prominent fracture zone traces neither in morphology nor gravity field data. A clockwise shift in the Juan de Fuca/Pacific pole of rotation at ∼5 Ma followed by a series of ridge propagation events initiated the formation of the BTFS, integrated each segment of the BTFS by shortening the ridge segments in between . Our observations suggest that the Blanco Ridge and the Gorda transform segment in the eastern BTFS were formed at ∼1.6 and 0.6 Ma, respectively, and ever since, the eastern BTFS became a mature transform boundary. In contrast, seismic slip vectors comparing to plate motion directions reveal that stresses in the western BTFS are systematically skewed, suggesting the immature transform plate boundary is still adjusting to the new stress regime.
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More than 60% of the Earth's land and shallow marine areas are covered by > 2 km of sediments and sedimentary rocks, with the thickest accumulations on rifted continental margins (Figure 1). Free-air marine gravity anomalies derived from Geosat and ERS-1 satellite altimetry (Fairhead et al., 2001; Sandwell and Smith, 2009; Andersen et al., 2009) outline most of these major basins with remarkable precision. Moreover, gravity and bathymetry data derived from altimetry are used to identify current and paleo-submarine canyons, faults, and local recent uplifts. These geomorphic features provide clues to where to look for large deposits of sediments. While current altimeter data delineate large offshore basins and major structures, they do not resolve some of the smaller geomorphic features and basins (Yale et al., 1998; Fairhead et al., 2001). Improved accuracy and resolution is desirable: to facilitate comparisons between continental margins; as an exploration tool and to permit extrapolation of known structures from well-surveyed areas; to follow fracture zones out of the deep-ocean basin into antecedent continental structures, to define and compare segmentation of margins along strike and identify the position of the continent-ocean boundary; and to study mass anomalies (e.g., sediment type and distribution) and isostatic compensation at continental margins. In this article, we assess the accuracy of a new global marine gravity model based on a wealth of new radar altimetry data and demonstrate that these gravity data are superior in quality to the majority of publicly available academic and government ship gravity data.
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Improving the accuracy of the marine gravity field requires both improved altimeter range precision and dense track coverage. After a hiatus of more than 15 yr, a wealth of suitable data is now available from the CryoSat-2, Envisat and Jason-1 satellites. The range precision of these data is significantly improved with respect to the conventional techniques used in operational oceanography by retracking the altimeter waveforms using an algorithm that is optimized for the recovery of the short-wavelength geodetic signal. We caution that this new approach, which provides optimal range precision, may introduce large-scale errors that would be unacceptable for other applications. In addition, CryoSat-2 has a new synthetic aperture radar (SAR) mode that should result in higher range precision. For this new mode we derived a simple, but approximate, analytic model for the shape of the SAR waveform that could be used in an iterative least-squares algorithm for estimating range. For the conventional waveforms, we demonstrate that a two-step retracking algorithm that was originally designed for data from prior missions (ERS-1 and Geosat) also improves precision on all three of the new satellites by about a factor of 1.5. The improved range precision and dense coverage from CryoSat-2, Envisat and Jason-1 should lead to a significant increase in the accuracy of the marine gravity field.
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OPEN ACCESS - Full text and supplementary data available at The South Atlantic rift basin evolved as a branch of a large Jurassic–Cretaceous intraplate rift zone between the African and South American plates during the final break-up of western Gondwana. While the relative motions between South America and Africa for post-break-up times are well resolved, many issues pertaining to the fit reconstruction and particularly the relation between kinematics and lithosphere dynamics during pre-break-up remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-break-up evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight-fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African Rift Zones, we have been able to indirectly construct the kinematic history of the pre-break-up evolution of the conjugate west African–Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic pre-salt sag basin and the São Paulo High. We model an initial E–W-directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities from 140 Ma until late Hauterivian times (≈126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ≈14 Myr-long stretching episode the pre-salt basin width on the conjugate Brazilian and west African margins is generated. An intermediate stage between ≈126 Ma and base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE–SW. From base Aptian onwards diachronous lithospheric break-up occurred along the central South Atlantic rift, first in the Sergipe–Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final break-up between South America and Africa occurred in the conjugate Santos–Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piauí-Ceará margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of these conjugate passive-margin systems and can explain the first-order tectonic structures along the South Atlantic and possibly other passive margins.
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This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east west direction, and locally it is marked by rift structures aligned along a NW SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east-west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporite sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.
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We have examined 4918 track line geophysics cruises archived at the U.S. National Geophysical Data Center (NGDC) using comprehensive error checking methods. Each cruise was checked for observation outliers, excessive gradients, metadata consistency, and general agreement with satellite altimetry-derived gravity and predicted bathymetry grids. Thresholds for error checking were determined empirically through inspection of histograms for all geophysical values, gradients, and differences with gridded data sampled along ship tracks. Robust regression was used to detect systematic scale and offset errors found by comparing ship bathymetry and free-air anomalies to the corresponding values from global grids. We found many recurring error types in the NGDC archive, including poor navigation, inappropriately scaled or offset data, excessive gradients, and extended offsets in depth and gravity when compared to global grids. While ~5-10% of bathymetry and free-air gravity records fail our conservative tests, residual magnetic errors may exceed twice this proportion. These errors hinder the effective use of the data and may lead to mistakes in interpretation. To enable the removal of gross errors without over-writing original cruise data, we developed an errata system that concisely reports all errors encountered in a cruise. With such errata files, scientists may share cruise corrections, thereby preventing redundant processing. We have implemented these quality control methods in the modified MGD77 supplement to the Generic Mapping Tools software suite.
The occurrence of failed breakup basins and deepwater blocks of thinned continental crust is commonplace in the rifting and breakup of continents, as part of passive margin development. This paper examines the rifting of Pangaea-Gondwanaland and subsequent breakup to form the South Atlantic Ocean, with development of a failed breakup basin and seafloor spreading axis (the deepwater Santos Basin) and an adjacent deepwater block of thinned continental crust (the Sao Paulo Plateau) using a combination of 2D flexural backstripping and gravity inversion modelling. The effects of the varying amounts of continental crustal thinning on the contrasting depositional and petroleum systems in the Santos Basin and on the São Paulo Plateau are discussed, the former having a predominant post-breakup petroleum system compared with a pre-breakup system in the latter. An analogy is also made to a potentially similar failed breakup basin/thinned continental crustal block pairing in the Faroes region in the NE Atlantic Ocean. © Petroleum Geology Conferences Ltd. Published by the Geological Society, London.
This work addresses the geological and geophysical interpretation of salt structures in selected Brazilian sedimentary basins, from intracratonic Palaeozoic evaporites in the Amazon and Solimões basins to divergent margin evaporite basins formed during the Mesozoic break-up of Gondwana. There is an intriguing correlation between evaporite basins and hydrocarbon accumulations in all the selected basins discussed. The Solimões and Amazonas basins developed evaporite depositing environments as the Middle Carboniferous sea was closing during a plate convergence phase. The salt basin along the eastern Brazilian and western African margins developed along the Mesozoic rifts of the South Atlantic. Regional seismic interpretation and potential field (gravity and magnetic) data over the eastern Brazilian and west African margins suggest a very thick autochthonous salt layer deposited over rifted continental crust and particularly above the thick sag basin sediments over the hyperextended crust that marks the transition from continental to oceanic crust. Most of the hydrocarbon discoveries in the eastern Brazilian and western African margins are in post-salt turbidite and carbonate reservoirs, but recent discoveries in the deepwater salt basins along the southeastern Brazilian margin indicate that pre-salt plays will represent an important contribution to hydrocarbon production in the near future.
We present a new approach to structural interpretation of 3D seismic data with the objectives of simplifying the task and reducing the interpretation time. The essential element is the stepwise removal of noise, and eventually of small‐scale stratigraphic and structural features, to derive more and more simple representations of structural shape. Without noise and small‐scale structure, both man and machine (autotrackers) can arrive at a structural interpretation faster. If the interpreters so wish, they can refine such an initial crude structural interpretation in selected target areas. We discuss a class of filters that removes noise and, if desired, simplifies structural information in 3D seismic data. The gist of these filters is a smoothing operation parallel to the seismic reflections that does not operate beyond reflection terminations (faults). These filters therefore have three ingredients: (1) orientation analysis, (2) edge detection, and (3) edge‐preserving oriented smoothing. We discuss one particular implementation of this principle in some detail: a simulated anisotropic diffusion process (low‐pass filter) that diffuses the seismic amplitude while the diffusion tensor is computed from the local image structure (so that the diffusion is parallel to the reflections). Examples show the remarkable effects of this operation.
Satellite altimetry from the Geosat and the ERS-1 Geodetic Missions provide altimeter data with very dense spatial coverage. Therefore the gravity field may be recovered in great detail. As neighboring ground tracks are very closely distributed, cross-track variations in the sea surface heights are extremely sensitive to sea surface variability. To avoid errors in the gravity field caused by such effects, sea surface variability needs to be carefully eliminated from the observations. Initially, a careful removal of gross errors and outliers was performed, and the tracks were fitted individually to a geoid model and crossover adjusted using bias and tilt. Subsequently, sea surface heights were gridded using local collocation in which residual ocean variability was considered. The conversion of the heights into gravity anomalies was carried out using the fast Fourier transform (FFT). In this process, filtering was done in the spectral domain to avoid the so-called ``orange skin'' characteristics. Comparison with marine gravity was finally carried out in three different regions of the Earth to evaluate the accuracy of the global marine gravity field from ERS-1 and Geosat.