Conference Agenda

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Session Overview
Session
4PM1: Upper mantle and lithosphere
Time:
Thursday, 23/Mar/2017:
2:00pm - 3:35pm

Session Chair: Christopher Finlay
Session Chair: Alexey Kuvshinov

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Presentations
2:00pm - 2:20pm

Keynote: With GOCE and Swarm Towards the 3D Lithosphere

Jörg Ebbing

Kiel University, Germany

Satellite measurements of the Earth gravity and magnetic field as from the GOCE and Swarm satellite missions have an increasing resolution. Their global coverage and specific characteristics (e.g. gradient measurements for GOCE) make them an ideal data sets for studying the lithosphere in combination with seismological models. Such a combined lithosphere is one of the goals of the ESA STSE “3D Earth”.

The prime objective of our project is to integrate seismological models and satellite observation towards a consistent image of the crust and upper mantle in 3D. Satellite gravity and (electro-) magnetic data help to transfer velocity images towards composition and temperature that reflect the tectonic state and evolution of the Earth and offer a novel understanding of the processes that shape our planet.

We will analyse the limitations and sensitivities of the different geophysical methods in the context of their imaging capability and in combination for forward and inverse modelling of the Earth’s internal structure. Such analysis will for example help to assess the role of isostatic (lithospheric) and dynamic (deep Earth) effects in shaping the surface of the Earth.


2:20pm - 2:35pm

Large-Scale Constraints on Tectonics and Macroscopic Magnetic Properties of the Earth's Lithosphere from the Swarm Constellation and CHAMP

Michael E Purucker1, Suzanne McEnroe2

1NASA, Goddard Space Flight Center; 2Norwegian University of Science and Technology

We constrain large-scale macroscopic magnetic properties (susceptibility, remanence and their geometry) of the earth's lithosphere in three regimes (continent, ocean, and subducted slab) using the latest high degree CHAOS and CM models, and compare those constraints to measured compilations of magnetic properties. At the highest degrees, these models still reflect a significant contribution from CHAMP, but the difference with the latest Swarm models is narrowing. In the oceanic realm, the magnetic symmetry across spreading centers, and presence of transform faults, are clearly evident only in the fastest spreading ridges (e.g. East Pacific Rise). Both the CHAOS and CM models can now extract along-track features in the lithosphere. We discuss the implications for large-scale oceanic and continental tectonics in these latest models, and anticipate some of the results when the Swarm satellites are at lower altitudes.


2:35pm - 2:50pm

A New Lithospheric Field Model based on CHAMP and Swarm Magnetic Satellite Data

Nils Olsen1, Dhananjay Ravat2, Christopher C Finlay1

1Technical University of Denmark, Denmark; 2University of Kentucky

We used magnetic field observations from the last two years of the CHAMP satellite mission (at altitudes between 280 and 350 km), augmented with Swarm satellite gradient data, to determine a model of the lithospheric field. We first subtracted predictions of the core and large-scale magnetospheric field as given by the CHAOS-6 model from the observations. No further data treatment was applied (e.g., no orbit-by-orbit filtering or “line leveling” of the individual satellite tracks was done). The lithospheric field is described by 35,000 point sources at 100 km depth below Earth’s surface. We estimate the amplitudes of these point sources from the observations using an Iteratively Reweighted Least Squares approach with robust weighting, to account for non-Gaussian data errors. The model is regularized by minimization of the L1 norm of the vertical magnetic field at the surface (WGS84 ellipsoid). In a final step we expand our point-source model in series of spherical harmonics up to degree and order N=185, accounting for div B = 0.

We will present our modeling approach and discuss the obtained lithospheric model, in particular how it compares with other lithospheric field models and with independent data sets.


2:50pm - 3:05pm

An Updated Global lithospheric Model by Implementing 3D Joint Inversion of Gravity, Geoid, Topography and the Gravity Gradients in Spherical Coordinates

Farshad Salajegheh, Juan Carlos Afonso

CCFS, Department of Earth and Planetary Sciences,Macquarie University, Sydney, New South Wales, Australia

The ever-increasing interest in the generation of high-resolution models of the whole of the lithosphere by industry and academia has stimulated the development of probabilistic inversions of multiple geophysical datasets. These types of inversions are computationally expensive and rely on efficient algorithms to solve the forward problems. Potential field data (e.g. gravity, geoid and GOCE gravity gradients) is of particular interest given their complementary sensitivities to both crustal and deep lithospheric structure, particularly when jointly inverted with seismic data. When the inversion is performed in 3D Cartesian geometry, very efficient solvers exist, and this why current implementations of multi-observable probabilistic inversions rely on such geometry. This, however, precludes the application of the inversion method to large-scale domains (e.g. continental or global studies).

For the forward calculation, we have implemented the prism approximation for calculating the tesseroid’s gravity effects. Thus, the lack of modelling singularities at the prism’s surface allows us to compute the gravity effects on the surface. If a prism instead of a tesseroid is used in the spherical system, the effect of the potential or its first and second derivatives is obtained in the local edge system of the prism. Therefore, these effects have to be transformed to the local reference frame of the observation point.

We have applied a smoothness-constrained least-squares algorithm based on the joint interpretation of free-air gravity, geoid, topography and gravity gradients data. The used inversion method is a linearized iterative inversion procedure in order to obtain variations in MOHO depth, average crustal density, average asthenosphere density and LAB depth. This method is able to provide in relatively efficient time a simplified global lithospheric model, which may utilize as the lateral extension of the model for the large scale study in a more complicated modelling using probabilistic inversion algorithm (e.g. LitMod3D_inv) to directly image the thermal and compositional structure of the lithosphere and sub-lithospheric upper mantle in the spherical coordinates. The prior information for this study has been extracted from the LITHO1.0 model.

In the meantime, the ScaLAPCK package has been used for parallelizing the inversion calculations. ScaLAPACK is a collection of routines written in Fortran77 for solving dense and banded linear systems, least squares problems, eigenvalue problems, and singular value problems in the distributed memory concurrent supercomputers.


3:05pm - 3:20pm

Imaging Small-scale Seafloor and Sub-seafloor Tectonic Fabric Using Satellite Altimetry

David T. Sandwell1, R. Dietmar Müller2, Kara J. Matthews3

1UCSD, United States of America; 2Univ. of Sydney, Australia; 3Univ. of Oxford, United Kingdom

Marine gravity anomalies derived from satellite radar altimetry now provide an unprecedented resolution for mapping small-scale seafloor and sub-seafloor tectonic fabric. Most of the new information comes from the CryoSat-2 satellite, which has routinely collected altimetry data over ice, land, and ocean since July 2010. The satellite has a long 369-day repeat cycle resulting in an average ground track spacing of 3.5 km at the equator. To date it has completed more than 6 geodetic mappings of the ocean surface. These data are augmented by a complete 14-month geodetic mapping of the ocean surface by Jason-1 from its lower inclination orbit of 66 ̊ that compliments the higher inclination orbit CryoSat-2 (88 ̊). The most recent global marine gravity anomaly map based on all of these geodetic mission data with 2-pass retracking for optimal range precision has an accuracy that is 2 times better than the maps derived from Geosat and ERS-1. The new data reveal the detailed fabric of fracture zones, previously unmapped, now extinct oceanic microplates in the central Pacific, and fault networks buried beneath thick sediments along continental margins. By combining satellite altimetry with marine magnetic anomalies and seafloor age dates from rock samples we are able to pinpoint the geometry and age of major plate reorganisations, particularly the enigmatic 100 Ma event, which occurred during the Cretaceous Magnetic Superchron.



 
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