Density Structure of the Earth

In this topic we concentrate on joint analyses of gravity, seismic and other geophysical, geodetic and geological data aiming for the construction of comprehensive models of the lithosphere and underlying mantle and finding a connection of these models with ongoing tectonic processes and geodynamics. In particular, we focus on the determination of the Earth’s density structure, since density heterogeneity is one of the main factors, which control all processes shaping the Earth. Another principal objective is the determination of the rheological and thermal structure of the lithosphere, which provides a link to the processes acting at the surface associated, e.g., with atmospheric, hydrological, or glaciation dynamics. The constructed models have a hierarchical structure: high resolution regional models are embedded into a generalized global one.

A new integrative model of the European lithosphere has been constructed based on the analysis of several data sets (primarily data on crustal structure, a new seismic tomography model showing heterogeneity of the upper mantle and gravity data) which are principally improved compared to previous studies (Tesauro et al., 2008; 2009a; 2009b, 2010a; 2010b; Koulakov et al., 2009; Kaban et al., 2010) This model includes all principal physical (density, temperature, velocity) and rheological parameters. Therein, a new high-resolution (15’x15’) reference crustal model (EuCRUST-07) has been constructed for most of Europe which is based on the integration of nearly all existing seismic determinations in the area (Tesauro et al. 2008). It offers a starting point in any kind of numerical modeling, which has to resolve a trade-off between crustal and mantle effects.

The crustal model has been employed in a new tomography model for P- and S-velocity anomalies beneath Europe, which was a priori corrected for the crustal effect (Koulakov et al., 2009). This enables us to much better resolve the velocity structure in the uppermost mantle by removing a trade-off with the crustal structures. Inversion of the P-wave velocity of the tomography model for temperature allowed to derive a new thermal model of the lithosphere and to estimate lithospheric thickness variations (Tesauro et al., 2009a, 2010a).

For construction of the density model of the crust and upper mantle, we jointly analyze the above results and the gravity field (Kaban et al., 2010). EuCRUST-07 is employed to calculate a gravity effect of the crust and to remove it from the observed field.

The obtained residual mantle gravity anomalies and residual topography are analyzed to characterize principal factors controlling mantle heterogeneity: temperature and compositional variations. The effect of temperature variations is estimated using the new thermal model (Figure 2). Then, the residual anomalies might be used to characterize compositional density anomalies in the upper mantle.

The obtained results have been used to estimate distribution of the strength of the European lithosphere. Identification of intraplate areas that are mechanically weaker or stronger provide a possibility to examine the heterogeneous response of the European lithosphere to large-scale plate tectonic forces and various modes of loading. It has been also calculated the effective elastic thickness of the lithosphere (Tesauro et al., 2009b).

Publications:

  • Tesauro M.; Kaban M.K.; Cloetingh S.A.P.L. (2008): EuCRUST-07: A new reference model for the European crust. Geophysycal Research Letters 35, L05313, doi:10.1029/2007GL032244.
  • Koulakov I.; Kaban M.K.; Tesauro M.; Cloetingh S.A.P.L. (2009): P and S velocity anomalies in the upper mantle beneath Europe from tomographic inversion of ISC data. Geopys. J. Int., 179, 345-366, doi:10.1111/j.1365-246X.2009.04279.x.
  • Tesauro M.; Kaban M.K.; Cloetingh S.A.P.L. (2009a): A new thermal and rheological model of the European lithosphere. Tectonophysics, 476, 478-495, doi:10.1016/j.tecto.2009.07.022.

In cooperation with the US Geological Survey we investigate the density structure of the North American upper mantle based on the integrative analysis of the gravity field and seismic data. The results provide a possibility to understand the thermal state and composition of the lithosphere and to reconstruct its evolution. The basis of our study is the removal of the gravitational effect of the crust to determine mantle gravity anomalies (Kaban and Mooney, 2001, 2010). The effect of the crust is removed in three steps by subtracting the gravitational contributions of (1) topography and bathymetry, (2) low-density sedimentary accumulations, and (3) the 3D density structure of the crystalline crust as determined by seismic observations. In order to separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocity anomalies to temperature anomalies, and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. This thermally corrected mantle-gravity anomaly map reveals density anomalies that are chiefly due to compositional variations.

In order to separate the contributions of mantle temperature anomalies from mantle compositional anomalies, we apply an additional correction to the mantle anomaly map for the thermal structure of the uppermost mantle. The thermal model is based on the conversion of seismic shear-wave velocities to temperature, and is consistent with mantle temperatures that are independently estimated from heat flow and heat production data. The thermally-corrected mantle density map reveals density anomalies that are chiefly due to compositional variations.

The low-density zone (A, Canadian Shield) appears after removing the thermal effect. It relates to the depleted mantle material. The horizontal shift of the root may be a result of the basal drag resulting from mantle flows. The strongest positive anomaly is co-incident with the Gulf of Mexico, and indicates a positive density anomaly in the upper mantle, possibly an eclogite layer that has caused subsidence in the Gulf. Two linear positive anomalies are also seen south of 40° N: one with a NE-SW trend in the eastern USA roughly coincident with the Grenville-Appalachians, and a second with a NW-SE trend beneath the states of Texas, New Mexico, and Colorado. These anomalies are interpreted as due to: (1) the presence of remnants of an oceanic slab in the upper mantle beneath the Grenville-Appalachian suture; and (2) mantle thickening caused by a period of shallow, flat subduction during the Laramie orogeny, respectively.
Based on these geophysical results, the evolution of the NA upper mantle is depicted in a series of maps and cartoons that display the primary processes that have formed and modified the North American crust and lithospheric upper mantle (Mooney and Kaban, 2010).

Publications:

  • Kaban M.K.; Mooney, W.D. (2001): Density structure of the lithosphere in the Southwestern United States and its tectonic significance. J. Geophys. Res. 106(B1), 721-739, doi:10.1029/2000JB900235
  • Kaban, M.K.; Mooney, W.D. (2010): A new density model of the upper mantle of North America. General Assembly European Geosciences Union (Vienna, Austria 2010), Geophysical Research Abstracts ; Vol 12, EGU2010-5301.
  • Mooney, W.D; Kaban, M.K. (2010): The North American Upper Mantle: Density, Composition, and Evolution, J. Geophys. Res., 115, B12424, doi: 10.1029/2010JB000866.

The tectonic structure of Asia is extremely complex and represents a result of interaction of various tectonic processes. Despite many studies providing geological and geophysical data for Asia, the existing models still do not explain completely deep structure, tectonic processes and dynamics of this extremely complicated region. The study of Central Asia was conducted within the framework of the Project CASE (KA-2669/2-1, DFG SPP 1257). In this study we integrate the new gravity field models from the combined satellite-terrestrial models, seismic data and geological data for modelling the density structure of the crust and upper mantle.

The main features of the obtained 3-D model are illustrated by the vertical cross-section to a depth of 200 km from Kazakh Shield in the North to Tarim Basin in the South.

The study of the Central Tien-Shan is continued within a framework of the project CASE (KA-2669/2-1, SPP 1257). In this study we integrate gravity data from the new combined satellite-terrestrial model, available seismic data, regional GPS observations and tectonics to model density structure of the crust and upper mantle and to understand style of the geodynamic processes responsible for high rate deformations in the region. Here we demonstrate selected preliminary results.

The existing models do not offer strong constraints on the lithospheric processes and geodynamics of Tien Shan. Several possible scenarios are usually discussed. The most popular model (Fig. 3-top) implies "simple" crustal shortening, which results from a strong compression between India and Eurasia. In contrast, the second model (Figure 3-bottom) implies partial underthrust (or continental subduction) of the Tarim lithosphere under Tien Shan. However, isostatic state of the lithosphere for these cases is principally different. Therefore, gravity modelling might be used to choose the right one.

A preliminary map of the isostatic anomalies has been computed for the Central Tien Shan. The result clearly shows that the model with underthrust of the Tarim plate under Tien Shan works much better: we found a strong positive anomaly on the southern flank of Central Tien Shan, which corresponds to the area with large crustal thickness and deep-seated earthquakes.

From a comparison of the new Moho map (Figure 1) and topography (Figure 2), one may see that correlation between these parameters is relatively low, which might also give preferences to the non-isostatic (subduction) model. To validate this assumption we calculate deflections of the observed Moho from the boundary, which should correspond to the local isostatic compensation of the crust (combined Airy-Pratt). We may conclude that the model with underthrusting of the Tarim plate under Tien Shan works much better (Figure 4).

We have found a strong negative mantle anomaly under Central Tien Shan, which indicates that the mantle under Central Tien Shan is likely hot. This is important conclusion; it supports the idea that this structure is an aborted rift, which was terminated by the collision between Indian and Eurasian lithospheric plates. In this case, the hot and mechanically weak lithosphere might be that factor, which stimulated growing of the Tien Shan ridge far away from the plate collision zone.
These are only preliminary results, the study is still in progress.

New crustal model for the whole Asia


Current crustal models are often unclear about the methodology and data that are used. Furthermore, differences in e.g. depth to Moho between previous models and current available data are often larger than 10km. It is with this in mind that a new methodology to obtain crustal models has been developed and applied to Asia (Stolk et al., 2011). The resulting new Moho estimate for Asia leads to a much better fit to the data, especially in Russia and Eastern China. The reliability of our estimation in these regions is confirmed by relatively low estimation uncertainties. Previous models have been unable to cater for locally varying velocity depth relations in Asia. Our model easily adapts to the data and brings out more crustal heterogeneities than before. 

Publications:

  • Stolk, W.; Kaban, M.K.; Tesauro, M.; Beekman, F.; Cloetingh, S. (2011): New Moho estimate and velocity model for Asia. General Assembly European Geosciences Union (Vienna, Austria 2011).
  • Vinnik, L.; Aleshin, I.; Kaban, M.; Kosarev, G.; Oreshin, S.; Reigber, Ch. (2006): Deep structure of the Tien Shan imaged by receiver function tomography. - Physics of the Solid Earth,  Vol. 42, No. 8, 639-651.
  • Vinnik, L.; Reigber, Ch.; Aleshin, I.; Kosarev, G.; Kaban, M.; Oreshin, S.; Roecker, S., (2004): Receiver Function Tomography of the Central Tien Shan, Earth and Planetary Science Letters, 225/1-2, 131-146.

MANTIS und die Fortsetzung MANTIS II sind Projekte im DFG-Schwerpunktprogramm SPP1788 DynamicEarth.

Der antarktische Kontinent ist fast vollständig von einer dicken Eisschicht bedeckt, die klassische In-situ-Messungen behindert. Es bleibt daher eines der geophysikalisch gesehen am wenigsten bekannten Gebiete der Erde. Über die Struktur und die thermischen und rheologischen Eigenschaften der Lithosphäre ist wenig bekannt. Da der Zustand der Lithosphäre stark mit oberflächennahen Prozessen wie der Eisdynamik oder der glazialisostatischen Anpassung (GIA) sowie dem tieferen, konvektierenden Mantel zusammenhängt, ist die Kenntnis dieser Eigenschaften bei der Modellierung der gekoppelten Systeme von entscheidender Bedeutung. Ziel dieses Projekts ist es, ein umfassendes Modell der antarktischen Lithosphäre in Bezug auf Stärke, Temperatur, Dichte und Zusammensetzung zu erstellen und die Wechselwirkungen der Lithosphäre mit der Eisdecke darüber und dem Mantel darunter zu modellieren.

In diesem Projekt haben wir iterativ Schwerkraft-, Topographie- und seismische Tomographiedaten, die durch mineralphysikalische Gleichungen eingeschränkt wurden, in einem gemeinsamen Inversionsschema kombiniert, um ein 3D-Dichte-, Temperatur- und Zusammensetzungsmodell der antarktischen Lithosphäre zu entwickeln. Seismische Daten zu Krustenstrukturen wurden außerdem verwendet, um ein neues Moho- und Krustendichtemodell zu erstellen. Als Maß für die Stärke wurde zusätzlich die effektive elastische Dicke Te durch Kreuzspektralanalyse des Schwerefeldes mit der eingestellten Topographie abgeleitet. Die Fan-Wavelet-Technik wurde verwendet, um erstmals Variationen von Te über den gesamten Kontinent mittels Admittanz- und Kohärenzanalyse zu berechnen. Um die Strukturen und Eigenschaften der oberen Kruste zu untersuchen, wurden Korrekturen des Schwerkrafteffekts der isostatischen Kompensation geologischer Lasten auf die isostatischen Schwerefeldanomalien angewendet. Die resultierenden sogenannten dekompensativen Schwerkraftanomalien können verwendet werden, um Informationen über Sedimentverteilungen abzuleiten, die zuvor unter der Eisdecke verborgen waren, wobei negative Anomalien wahrscheinlich durch Sedimente geringer Dichte verursacht werden.

Eine allgemeine Aufteilung der antarktischen Lithosphäre wird durch alle untersuchten Parameter bestätigt. Während die Ostantarktis (EANT) durch eine alte, dicke, starke und kalte Lithosphäre gekennzeichnet ist, hat die Westantarktis (WANT) erst im Känozoikum ihre heutige Form erreicht und weist daher eine viel dünnere, schwächere und heißere Lithosphäre auf. Ein Übergang ist entlang der transantarktischen Berge sichtbar. Ob die Bergkette zu WANT oder EANT gehört, ist fraglich, aber insbesondere die Abschätzungen von Te deuten auf eine engere Verbindung zu WANT hin. Abgesehen von dieser allgemeinen Unterteilung wurde innerhalb von EANT eine lithosphärische Fragmentierung entdeckt. Im Dronning Maud Land, im Wilkes Land und in der Nähe des Südpols wurden kratonische Fragmente präkambrischen Ursprungs mit hoher Verarmung, niedrigen Temperaturen und hoher Te nachgewiesen. Die beiden letzteren sind wahrscheinlich Teil des Mawson-Kratons. Eine lithosphärische Schwächung in Kombination mit einem fast primitiven oberen Mantel besteht im Lambert-Graben und war wahrscheinlich das Ergebnis von Riftaktivität im ostantarktischen Rift-System. Die erhaltenen dekompensativen Schwerkraftanomalien entsprechen gut bekannten Sedimentbecken wie dem Lambert Graben und dem Filchner-Ronne-Eisschelfs. Sie deuten auch auf große Sedimentablagerungen hin, die zuvor nicht nur spärlich kartiert wurden.

Die erste Phase dieses Projekts liefert ein umfassendes Modell der Lithosphäre der Antarktis. Das System Erde besteht jedoch nicht aus statischen unabhängigen Schichten, sondern stellt ein komplexes System interagierender dynamischer Subsysteme dar. Ein solides Verständnis jeder dieser Schichten ist erforderlich, um die dynamischen Prozesse zu verstehen, die sie miteinander verbinden. Das bereitgestellte lithosphärische Modell ist daher eine Grundlage für die weitere Analyse seiner Kopplung mit den Prozessen des tiefen Mantels und der Oberfläche. Als solches erleichtert das Temperaturmodell die Modellierung des Oberflächenwärmeflusses, die Bestimmung der thermischen Lithosphären-Astenosphärengrenze und ermöglicht - kombiniert mit Dichteschwankungen - die Abschätzung der Viskosität des oberen Mantels. Diese Parameter sind entscheidend für die Modellierung von oberflächen(nahen) Prozessen wie Eisdynamik und GIA. Zusätzlich wird ein dynamisches Modell des Lithosphäre-Oberer Mantel-Systems erstellt, da die Mantelkonvektion eine Schlüsselrolle in der Plattentektonik spielt und mit der Basis der Lithosphäre interagiert, wobei der Basiswiderstand und die dynamische Topographie durch den Mantelfluss die Spannungsfeldverteilung verändern. Entgegen konventionellem Glauben wurde gezeigt, dass sogar kratonische Strukturen von der Mantelkonvektion beeinflusst werden.

Contact

Mikhail Kaban
Group Leader
Dr. habil. Mikhail Kaban
Earth System Modelling
Telegrafenberg
Building A 20, Room 320
14473 Potsdam
+49 331 288-1172
Profile