T21C-01
Dynamic topography of the East European Craton: Shedding light upon lithospheric structure, composition and mantle dynamics
While most of the East European Craton (EEC) lacks surface relief, the amplitude of topography at the top of the basement exceeds 20 km, variations in the thickness of the crystalline crust reach 50 km, and variations in the lithosphere thickness exceed 200 km. This study (Global Planet. Change, 2007, 58, 411-434) examines the relative contributions of the crust, the subcrustal lithosphere, and the dynamic support of the sublithospheric mantle to maintain surface topography, using regional seismic data on the structure of the crust and the sedimentary cover, and thermal and large-scale P- and S-wave seismic tomography data on the structure of the lithospheric mantle. The isostatic contribution of the crust to the surface topography of the EEC is found to be almost independent of age (ca. 4.5 km) due to an interplay of age- dependent crustal and sedimentary thicknesses and lithospheric temperatures. On the contrary, the contribution of the subcrustal lithosphere to the surface topography strongly depends on the age, being slightly positive (+0.3+0.7 km) for the regions older than 1.6 Ga and negative (-0.5-1 km) for younger structures. This leads to age- dependent variations in the residual topography, i.e. the topography which cannot be explained by the assumed thermal and density structure of the lithosphere, and which can (at least partly) originate from the dynamic component caused by the mantle flow. Positive dynamic topography at the cratonic margins, which exceeds +2 km in the Norwegian Caledonides and in the Urals, clearly links their on-going uplift with deep mantle processes. Negative residual topography beneath the Archean- Paleoproterozoic cratons (-1-2 km) indicates either a smaller density deficit (ca 0.9 per cent) in their subcrustal lithosphere than predicted by mantle-derived xenoliths or the presence of a strong convective downwelling in the mantle. Such mantle downflows can effectively divert heat from the lithospheric base, leading to a long-term survival of the Archean Paleoproterozoic lithosphere. http://www.lithosphere.info
T21C-02
Toward a Next Generation of Isostatic Analysis
Much of the effort to improve upon coherence analysis of isostatic response since Forsyth's&p [1985] introduction has focused on optimizing resolution/bias using more advanced power spectral estimators or wavelets. Less attention has been paid to Forsyth's innovative recognitions that (1) internal loads significantly impact Te estimates, (2) any arbitrary choice of Te exactly models observed gravity and topography given the right combination of surface and internal loading, and (3) an incorrect choice of Te or other parameter to deconvolve surface from internal loads artificially correlates load estimates, making the parameterization that minimizes correlation the likely optimal choice. However, a drawback of parameterizing isostasy by minimizing load correlation is that accuracy of the result hinges on the assumption that surface and internal load processes truly are uncorrelated. Simulations demonstrate that error in recovery of Te arises because, even when synthetic load fields are truly uncorrelated, sub-sampling the data in windows, tapers or wavelets incurs random, spurious correlation at certain locations and wavelengths. As the inexorable march of EarthScope's transportable array of seismic stations proceeds across the country, we are approaching a time when assumptions about load statistics may be downweighted or even unnecessary in isostatic analyses (in the conterminous US at least). Seismic velocities (e.g. from Pn, body and surface wave tomography) and layer thicknesses (from P-S conversions) may be used via regression and/or correlation analyses with gravity/topography data to arrive at independent estimates of internal loading that can then be used to better constrain Te. Seismic data will also aid in overcoming another significant limitation of Forsyth's deconvolution approach. Load deconvolution assumes the internal load mass occurs at a single depth (dictated by the fact that we can solve for at most two unknown amplitudes from the two observations of gravity and topography), whereas true internal loading occurs at multiple layer interfaces and as density variations within layers. Incorporation of seismic data into the isostatic analyses holds the promise of overcoming these limitations altogether and substantially reducing errors in isostatic analyses introduced by erroneous load estimates. I will present a new approach to incorporating seismic data in Te estimation procedures. Preliminary tests on data profiles from the Himalayas and European Alps suggest that adding even a skeletal seismic constraint may provide a more robust estimate of Te than minimization of the load coherence alone.
T21C-03 INVITED
Lithospheric Edges and Sutures
Various styles of seismic imaging provide evidence for variability within major lithospheric blocks such as the cratons of Australia. The crustal components show clear differentiation that appears to link to the properties of the mantle lithosphere. Although some cratonic sutures have a clear impact on structure in the uppermost mantle, surface wave tomography does not provide support for the concept of near vertical link zones penetrating the full lithosphere. The transition from a craton to Phanerozoic belts may not occur simply at depth. In eastern Australia is evidence for a set of steps in lithospheric thickness whose configuration differs markedly from the surface "Tasman Line" marking the easternmost extent of Precambrian outcrop. The substructure of the lithosphere provides much information on the evolution of the present assemblage. The subtle features appear most clearly when information from multiple styles of seismic probes can be combined, so that both large and small spatial scales can be resolved.
T21C-04
Lithospheric Thickness Modeled From Long Period Surface Wave Dispersion
The behavior of surface waves at long periods is indicative of subcrustal velocity structure. Using recently published dispersion models, we invert surface wave group velocities for lithospheric structure, including lid velocity and lithospheric thickness, over much of the Eastern Hemisphere, encompassing Eurasia, Africa, and the Indian Ocean. Thicker lithosphere keels and faster upper mantle velocities under Precambrian shields and platforms are clearly observed, not only under the large cratons (West African Craton, Congo Craton, Baltic Shield, Russian Platform, Siberian Platform, Indian Shield, Kalahari Craton), but also under smaller blocks like the Tarim Basin and Yangtze Craton. There are also interesting variations within cratons like the Congo Craton. As expected, the thinnest lithospheric thickness is found under oceanic and continental rifts, and also along convergence zones. We compare our results to thermal lithospheric models of the continents, lithospheric cooling models of oceanic lithosphere, lithosphere-asthenosphere boundary (LAB) estimates from S-wave receiver functions, and velocity variations of global tomography models.
T21C-05
Seismic Structure of Precambrian Lithosphere: New Constraints From Broadband Surface-Wave Dispersion
Depth distributions of seismic velocities and their directional dependence (anisotropy) in the crust and mantle beneath cratons yield essential constraints on processes of formation and evolution of continental lithosphere. Despite recent progress in mapping the lateral extent of cratonic roots, profiles of seismic velocities within them remain uncertain. We employ a novel combination of waveform analysis techniques and measure interstation Rayleigh- and Love-wave phase velocities in broad period ranges. The data yield resolution from the upper crust to deep upper mantle. Sampling a selection of 10 Archean and Proterozoic locations, we derive new constraints on the isotropic and radially anisotropic seismic structure of Precambrian lithosphere. S velocity Vs is consistently higher (and, thus, temperature is lower) in the lithosphere of cratons than in the lithosphere of Proterozoic foldbelts, even though temperature in the asthenosphere beneath the units does not appear to correlate with their age. This confirms that the stable, buoyant lithosphere beneath cratons is substantially thicker than beneath younger continental blocks. An increase in Vs between the Moho and a 100-150 km depth is consistently preferred by the data and is likely to be due to the transition from spinel peridotite to garnet peridotite, as proposed earlier. Seismic and mineralogical data available at present are consistent with both a sharp and a gradual increase in Vs: a Hales discontinuity or a `Hales gradient'. Radial anisotropy in the upper crust is observed repeatedly and indicates vertically oriented anisotropic fabric (Vsh < Vsv); this may provide a clue on how cratons grew, lending support to the view that distributed crustal shortening with sub-vertical flow patterns occurred over large scales in ancient orogens. In the lower crust and upper lithospheric mantle, radial anisotropy consistently reveals horizontal fabric (Vsh > Vsv); the fabric can be interpreted as a record of (sub-) horizontal ductile flow at the time of the formation and stabilization of the cratons. We also find indications for radial anisotropy in the 200-400 km depth range, corroborating recent evidence for the occurrence of seismic anisotropy due to current and recent flow in the asthenosphere beneath cratons.
T21C-06
The African Lithosphere
There have been a number of prior, large scale surface wave studies of Africa, the majority of which rely on fundamental mode observations. In this study we use a large data set of multi-mode surface waves recorded over epicentral distances most of which are shorter than 6000 km, to investigate the Sv wave speed heterogeneity of the upper mantle beneath Africa. The inclusion of the higher mode data allow us to build an upper mantle model for the African plate with a horizontal resolution of a few hundred kilometers and a vertical resolution of a few tens of kilometers extending to about 400 km depth. Our tomographic images of the upper mantle beneath Africa displays significant shear velocity features, much of which correlate with surface geology. High velocity mantle persists beneath the West African and Congo cratons to 225-250 km depth, but the high velocity root beneath Kalahari Craton extends to only about 175 km depth. Low velocity upper mantle underlies the Pan- African terranes of Africa with the exception of the Damara mobile belt separating the Congo and Kalahari Cratons. The Damara mobile belt is underlain by a thick high velocity upper mantle lid which is indistinguishable from that beneath the Congo Craton to the north and the Kalahari Craton to the south. Low velocity upper mantle underlie the Hoggar, Tebesti and Darfur volcanic areas of northern Africa, and very low velocities underlie the Afar region to at least 400 km depth. We use the relationship between shear velocity and temperature of Priestley & McKenzie (2006) to derive a model for the African thermal lithosphere. Two types of lithosphere underlie Africa. Thick lithosphere underlies most of western Africa and all of southern Africa; in the latter the extent of the thick lithosphere is significantly different from the distribution of Archean crust mapped at the surface. Thick lithosphere forms one continuous structure beneath the Congo and Kalahari Cratons. Other than the Pan-African Damara mobile belt, the only Pan-African terrane of Africa free of recent (<30 Ma) volcanism, all of the Pan- African is underlain by lithosphere whose thickness is too thin to be resolved by our current surface wave analysis.
T21C-07
Stacked uppermost mantle layers within the Slave craton of NW Canada as defined by anisotropic seismic discontinuities
A 20-station seismic array in NW Canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the Mohorovicic and Lehman discontinuities. Following wave field decomposition, groups of seismograms were source-normalized through simultaneous deconvolution to estimate the near-receiver impulse response and thus detect major discontinuities beneath each seismic station. Stations within the Lac de Gras kimberlite field display an unusually strong negative impulse on the radial component within the NW quadrant and two moderate impulses on the transverse component. Forward modeling of these impulses suggests a mantle layer dipping at 22° to the southeast with a mildly anisotropic upper discontinuity at 120 to 135 km depth and another mildly anisotropic discontinuity at about 170 km depth. Superimposed on these layers is another, stronger anisotropic layer between 110 and 180 km depths that dips to the west. Stations outside of the Lac de Gras field, but within the southeastern Slave craton, display more numerous, but weaker impulses. The most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed further north. The prominent negative impulse on the radial component is interpreted to arise from structural- preferred orientation in the form of a stockwork of wehrlite dykes beneath the Lac de Gras field. Interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000-2900 Ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 Ma. The layer beneath the SE Slave has a similar, but distinct tectonic history of underthrusting associated with the 2635 to 2615 Ma Defeat Suite of plutonism. Taken together, these interpretations indicate that the Slave craton was assembled from at least four lithospheric blocks prior to its cratonization about 2580 Ma: each block is 90 to 120 km thick and abut across a near-vertical boundary beneath MacKay Lake in the central Slave craton. Significant amounts of carbon could have become incorporated into the central Slave mantle within the diamond stability field during the proposed underthrusting of lithosphere.
T21C-08 INVITED
High Resolution Modelling of Convective Flow in the Sublithospheric Mantle Below the African Plate
The African continent is unique, both in terms of its surface topography and in terms of the high-temperature, plume-like structures in the subcontinental mantle which have been inferred by seismic tomographic imaging. No other continent on Earth displays the pervasive influence of very large scale tensional stresses and widely distributed topographic domes and late-Cenozoic volcanic eruptions which have impacted the surface geology of Africa. Advances in high resolution seismic tomographic imaging of global Earth structure have revealed that the African continent is sitting atop an immense thermochemical plume which stretches upward from the core- mantle boundary under southern African and extends into the upper mantle (Simmons et al. 2007). The most recent high-resolution tomography model is derived from a joint inversion of both global seismic and surface geodynamic data sets and it incorporates mineral physical constraints on the thermal properties of the mantle. This tomographic inversion yields a 3-D distribution of mantle density anomalies that includes both thermal and compositional heterogeneity and it therefore enables us to incorporate the stabilising effect of compositional buoyancy in the continental tectosphere and in the deep lower mantle. This new inference of the 3-D structure below the African plate are used in a new series of numerical simulations of the present day mantle convective flow below the continent. We obtain a remarkably detailed pattern of shallow, sublithospheric flow below Africa and we trace its dynamical relationship to the deep-mantle flow driven by the African superplume. The asthenospheric flow patterns show clearly focussed upwellings below all the major late-Cenozoic volcanic domes on the African plate. We employ these predictions of shallow mantle flow to explore the implications for present-day dynamic topography and stress on the African plate.