Ulrich Achauer EOST-IPG, UMR 7516 Université de Strasbourg Jaroslava Plomerova Geophysical Institute Czech Academy of Sciences Stefan Jung Universität Hamburg Department Geowissenschaften Mineralogisch-Petrographisches Institut and the LABPAX working group Project-Idea for a new ILP task force starting in 2010 What is the lithosphere-asthenosphere boundary? The lithosphere-asthenosphere boundary depth paradox - LABPAX Rationale: The lithosphere-asthenosphere boundary (LAB) is the most extensive and active plate boundary on the Earth. However, the LAB beneath the continents the LAB is relatively cryptic compared to other first-order structural subdivisions of Earth. Though we face different physical definitions of the LAB in dependence on methods used to map the boundary, a general understanding “WHAT is the LAB is still missing”. One of the major challenges for seismologists presented by the IRIS-NSF-co-funded workshop on “Seismological Grand Challenges in Understanding Earth’s Dynamic Systems” (published by IRIS in January 2009) was as challenge No. 7 (out of the ten grand challenges): “what is the lithosphere-asthenosphere boundary?”. This demonstrates the importance seismologists devote to the above subject. From the recent DefLAB workshop (2009 Dublin, http://www.dias.ie/deflab) it may be concluded that while a lithosphere-asthenosphere boundary is seen by most disciplines in Earth Sciences, the definition, the depth and what it means from the structural, the rheological and physico-chemical point of view is highly debated. There seem to be several “boundaries”, namely the LAB-S (seismological), the LAB-M (mechanical), the LAB-T (thermal), the LAB-C (chemical) and the LAB-E (electromagnetic), all called by the colleagues from the particular fields in Earth Science “LAB” which differ in depth and thickness (i.e. whether they are discrete or more like a transitional layer) and most likely will not “define” at all the same thing! It is evident that only a multi-disciplinary approach, bringing together all disciplines from Earth Sciences will help us to shed light on the above question and to better understand and communicate between the different fields in Earth Sciences, what the lithosphere-asthenosphere boundary is all about, what it’s origin is and what role it has played and still plays in the evolution of our planet.

Where to tackle the problem? Obviously can interesting target areas being found all around the globe, as can be seen from the following figures, which present examples of the complexity of the lithosphere-asthenosphere boundary and the lithosphere structure in three regions - across WECEP (stands for WEstern-Central European Platform), in Europe and Western North America.

Figure 1: Left: Central European Cenozoic Igneous Provinces. Right: Examples of S-P-receiver functions along the profile indicated in the map given on the left. The Moho and the lithosphere-asthenosphere boundary (LAB) are clearly visible in the data. Thinning of the lithosphere beneath the Upper Rhine Graben and its thickening towards the TTZ is evident (Time scale multiplied by a factor 9 gives roughly the depth scale). Combined application of surface wave, S-P receiver function and magnetotelluric techniques, as well as studies of seismic anisotropy (Babuska and Plomerova 2006), will lead to unprecedented resolution of the lithospheric-asthenospheric structure in the West and Central European Platform (from the outline of LABTOP project, submitted to ESF for funding by the Topo-Europe Eurocores; Achauer et al., 2007).

Fig. 2. LAB depths and schematic representation of variable fabrics of the mantle lithosphere along the SW- NE profile across the WECEP and northern Europe. Domain fabrics are

modelled by peridotite aggregates with generally oriented axes of hexagonal symmetry (from Plomerova et al., 2010).

(from the IRIS report, December 2008). Possible targets and themes are: In the following we will highlight some possibilities, either because there are already some projects running (or data have been acquired in recent years) ore because groups are very much interested in a particular area. These project ideas show the wealth of possibilities. Of course is this list of projects neither exhaustive nor exclusive and more ideas are welcome. P1: The LAB beneath the West-Central European Platform (WECEP) For us it would seem that among others the WECEP region (see Topo-Europe rationale) is a perfect natural laboratory to tackle this grand challenge. Not alone because this region is very rich in its scientific expertise in all the necessary fields of Earth Science, but also because there exists already a great wealth of data which can be used as a starting point. Topography of the WECEP resulted from Cenozoic intraplate compression during the Alpine orogeny, development of rift systems and mantle plume impingement, accompanied by widespread volcanism. A better understanding of the interaction between these still on-going processes requires imaging of the entire lithosphere-asthenosphere system by a variety of geophysical methods using innovative approaches. While seismic and electromagnetic observations reveal the current state of the lithosphere-asthenosphere system, geological data document its past deformation with the geochemistry of the Cenozoic magmas reflecting the evolution of mantle plumes. The WECEP is a natural laboratory for studying and modelling entire lithosphere, its structure and bottom, formed during rifting, intraplate compression and mantle plume activity. Merging different approaches will aim at understanding what the LAB is.

Plomerová and J., Babuška V., 2010. Long memory of mantle lithosphere fabric - European LAB constrained from seismic anisotropy. Lithos, 10.1016/j.lithos.2010.01.008 Babuška V and Plomerová J., 2006. European mantle lithosphere assembled from rigid microplates with inherited seismic anisotropy. Phys. Earth. Planet. Inter., 158: 264-280; doi:10.1016/j.pepi.2006.01.010 Jaroslava Plomerová, Ulrich Achauer, Vladislav Babuška, Luděk Vecsey and BOHEMA working group, 2007. Upper mantle beneath the Eger Rift (Central Europe): plume or asthenosphere upwelling?, GJI, 169, 675-682. S. Gregersen, P. Voss, L.V. Nielsen, U. Achauer, H. Busche, W. Rabbbel, Z. H. Shomali, 2009. UNIQUENESS OF MODELING RESULTS FROM TELESEISMIC P-WAVE TOMOGRAPHY IN PROJECT TOR, Tectonophysics, doi:10.1016/j.tecto.2009.01.020. R. Tondi, U. Achauer, M. Landes, R. Davı, L. Besutiu, 2009. Unveiling seismic and density structure beneath the Vrancea seismogenic zone (Romania), JGR, in print. P2: Seismic structure of the Trans-European Suture Zone - from crust to deep mantle by Prof. Marek Grad and Dr. Monika Wilde-Piórko The main target of this part of the project will be recognition of the crustal and lithospheric structure of the western part of the Variscan Belt (VB), particularly in the area of Poland and neighbouring countries. The contact zone between Precambrian East European Craton (EEC) and the Phanerozoic VB is the most distinct border of the European lithosphere. The Trans-European Suture Zone (TESZ) represents the most prominent tectonic boundary in Europe north of Alpine-Carpathian orogenic front. The large seismic refraction and wide-angle reflection experiments (POLONAISE'97, CELEBRATION 2000, ALP 2002, SUDETES 2003) carried out with the most modern techniques gave a high resolution of crustal and uppermost mantle structure of the Central Europe. In NW and central Poland the results of seismic investigations show the presence of relatively low velocity rocks (Vp<6.1 km/s) down to a depth of 20 km beneath the Polish Basin (PB), and a high velocity lower crust (Vp=6.8-7.3 km/s). In general the crustal thickness in the TESZ is intermediate between that of the EEC to the northeast (40-45 km) and that of the VB to the southwest (~30 km, see figure). Beneath Bohemian Massif crustal thickness is about 30-35 km and 40-45 km beneath Eastern Alps. The velocities in the uppermost mantle of the TESZ are relatively high (Vp=8.25-8.45 km/s), being normal or lower to the southwest (7.9-8.1 km/s). Based on all the available geological and seismic data in the area digital 3D model of the crust and lower lithosphere will be constructed. The knowledge of the crustal structure, Moho depth and upper mantle velocity in the area will be used by other seismic methods, such as P- and S-body wave tomography, surface waves, SKS-analysis and P- and S-receiver functions for recognition of the deeper structure of the lithosphere-asthenosphere system and upper mantle.

The seismic structure of the Variscan upper mantle will be determined down to lithosphere-asthenosphere boundary (LAB) using results from new passive seismic experiment PASSEQ. Extra (possibly a new experiment) passive seismic experiment with broad-band and short period instruments could be useful to cover the edge of EEC – important reference of Precambrian lithosphere. S-wave velocity models of the Variscan crust will be determined using forward and inverse techniques of receiver function. Deep mantle boundaries "410" and "670" km depth will be investigated using receiver function sections. All integrated seismic data will provide model of Variscan crust and mantle. Together with integrated gravity data for Central Europe they will be used for density modelling of the area. The present day structure of deep mantle, lithosphere-asthenosphere boundary and crust with its sedimentary basins and rift zones will be used in modelling the lithosphere-scale processes including its influence for neotectonics and topographic effects.

Bibliography

Bayer U., Grad M., Pharaoh T.C., Thybo H., Guterch A., Banka D., Lamarche J., Lassen A., Lewerenz B., Scheck M., Marotta A.-M., 2002. The southern margin of the East European Craton: new results from seismic sounding and potential fields between the North Sea and Poland. Tectonophysics, 360, 301-314.

Dadlez R., Grad M., Guterch A., 2005. Crustal structure below the Polish Basin: Is it composed of proximal terranes derived from Baltica? Tectonophysics, 411 (1-4), 111-128.

Grad M., Guterch A., Mazur S., 2002. Seismic refraction evidence for crustal structure in the central part of the Trans-European Suture Zone in Poland. In: J.A. Winchester, T.C. Pharaoh, J. Verniers (editors), Palaeozoic Amalgamation of Central Europe, Geological Society, London, Special Publications, 201, 295-309.

Grad M., Tiira T., ESC Working Group, 2009. The Moho depth map of the European Plate. Geophys. J. Int., 176, 279–292 doi: 10.1111/j.1365-246X.2008.03919.x

Gregersen S., Voss V., Shomali Z.H., Grad M., Roberts R.G., TOR Working Group, 2006. Physical differences in the deep lithosphere of Northern and Central Europe. In: Gee D.G. & Stephenson R.A. (eds), European Lithosphere Dynamics. Geological Society, London, Memoirs, 32, 313–322.

Hrubcová P., Środa P., Špičák A., Guterch A., Grad M., Keller G. R., Brueckl E., Thybo H., 2005. Crustal and uppermost mantle structure of the Bohemian Massif based on CELEBRATION 2000 data. J. Geophys. Res., 110, B11305, doi:10.1029/2004JB003080.

Majorowicz J.A., Čermak V., Šafanda J., Krzywiec P., Wróblewska M., Guterch A., Grad M., 2003. Heat flow models across the Trans-European Suture Zone in the area of the POLONAISE’97 seismic experiment. Phys. Chem. Earth., 28, 375-391.

Wilde-Piórko M., Geissler W.H., Plomerová J., Grad M., et al., 2008. PASSEQ 2006−2008: passive seismic experiment in Trans-European suture zone. Stud. Geophys. Geod., 52, 439−448.

Wilde-Piórko M., Świeczak M., Grad M., Majdański M., 2010. Integrated seismic model of the crust and upper mantle of the Trans-European Suture Zone between the Precambrian craton and Phanerozoic terranes in the Central Europe. Tectonophysics, 481, 108-115, doi: 10.1016/j.tecto.2009.05.002.

P3: Os and Hf isotope and High Field Strength Element systematics and high-precision Ar-Ar ages of Tertiary alkaline lavas from the Central European Volcanic Province by Stefan Jung (Hamburg) and Jörg A. Pfänder (Freiberg) State-of-the-Art of Research The planned project aims at an evaluation of the Os and Hf isotope systems and the High Field Strength Element (Nb-Ta-Zr-Hf) budget of Tertiary alkaline lavas in order to constrain the source components (lithospheric vs. asthenospheric) and their geochemical evolution linked to geodynamic processes. High-precision Ar-Ar dating will be used to asses the timescales and rates of these processes. The Re-Os isotope system is an excellent tool to distinguish between mantle heterogeneity and crustal contamination because the subcontinental lithospheric mantle (SCLM) has characteristically unradiogenic (subchondritic) 187Os/188Os ratios which are distinct from both typical asthenospheric signatures and the 187Os/188Os ratios of most ocean island basalts (Walker et al., 1989). The continental crust develops variable but consistently higher 187Os/188Os ratios than those typical for the mantle (Asmerom and Walker, 1998). The variability of the Hf isotope composition of continental basalts may also place constraints on contamination processes especially when high-Lu/Hf contaminants are involved but may additionally be used to infer partial melting processes within the garnet or spinel peridotite stability fields (Bizimis et al., 2004). Variations of high-field strength element (HFSE) ratios (Zr/Hf, Nb/Ta) in terrestrial reservoirs are critical for understanding crust-mantle differentiation and mantle and lithosphere evolution (Pfänder et al., 2007). Previous work Early studies, based on major and trace element and Sr, Nd and Pb isotope data, concluded that the isotope chemistry of the basalts is the result of partial melting of a heterogenous, lower mantle source (Wedepohl and Baumann, 1999) whereas subsequent work emphasized the role of lithosphere contributions and crustal contamination, even in so-called „primitive“ alkaline volcanic rocks (Bogaard and Wörner, 2003, Haase et al., 2004; Jung and Masberg, 1998; Jung and Hoernes, 2000; Jung et al., 2005; 2006). There is also some consensus that most of the lavas come from the TBL (thermal boundary layer). In central Europe, the TBL broadly coincides with the LAB (lithosphere/asthenosphere boundary). Work plan It is planned to obtain (collect) a representative data set of Os and Hf isotopes and HFSE abundances as well as high-precision Ar-Ar ages on a number of Tertiary alkaline volcanic centers. These include (from E to W) the Hocheifel, Siebengebirge, Westerwald, Vogelsberg and Rhön areas. This is reasonable because the PI (S. Jung) has worked previously in these areas and major and trace element abundances and Sr, Nd and Pb isotope data and also some HFSE data are available. This study will focus on well-characterized samples and no further sampling is required. It is planned to analyse 10-15 primitive samples per volcanic center excluding the Quarternary Eifel and some other volcanic center further to the east of the Rhön area, because from these centers, an equivalent set of Nd, Sr and Pb isotope data is not available (the Ohre rift may be an exception). A possible perspective could be an extension of this method to the highly differentiated volcanic rocks from the above mentioned centers. Since it has been previously established that the differentiated lavas are contaminated with crustal material, this approach would allow to constrain the composition of the lower crust beneath these centers which in turn has implications for the eruption history of the Tertiary volcanism in time and space. Methods Methods will include measurement of Re and Os elemental concentrations and isotope abundances with isotope dilution by N-TIMS (negative thermal ionisation mass spectrometry). This work will be done in co-operation with M. Brauns (Curt-Engelhorn-Zentrum�für Archäometrie; Mannheim). Lu and Hf concentrations and isotope abundances

and Zr and Ta abundances are measured with isotope dilution using a multicollector mass spectrometer with an inductively coupled plasma source (MC-ICP-MS). Nb abundances will be calculated from the Nb/Zr ratio. E. Scherer (University of Münster) and/or C. Münker (University of Köln) will assist in obtaining these data. Ar-Ar ages will be performed by the co-PI J. Pfänder who runs a newly established Ar-Ar isotope laboratory in Freiberg equipped with a multi-collector Argus mass spectrometer and furnace and thermal laser devices for step heating experiments. The Argus mass spectrometer in Freiberg is particularly powerful in dating young volcanic rock samples owing the static Faraday collector assembly. Main questions and deliverables Since it is highly likely that most of the primitive alkaline volcanic rocks come from the TBL and the TBL coincides with the LAB, the main questions raised in this project are (i) what can we learn about partial melting depths, processes and sources involved during petrogenesis based on isotope and HFSE compositions of so-called „primitive“ alkaline volcanic rocks, (ii) what are the effects of crustal contamination and crystal fractionation on the isotope and HFSE composition of the volcanic products (iii) is the locus of partial melting within the lithosphere or asthenosphere or both and (iv) is there any correlation between age and location or chemical/isotope composition of the volcanic rocks? Results will help to define the chemical nature of the lithosphere/asthenosphere boundary. Furthermore, the data are essential for geophysical groups to gain knowledge about the nature and depth of the LAB. Geophysical data that constrain the LAB are in turn needed later to refine suggestions about lithospheric vs. asthenospheric mantle sources using coupled Sm-Nd/Lu-Hf isotope systematics. References: Asmerom, Y., Walker, R. J. 1998. Geology 26, 359-362. Bizimis M. et al. 2003. Contrib. Min. Petrol. 145, 281-300. Bogaard, P.J.F, Wörner, G. 2003. J. Petrol., 44, 569-602. Haase, K. M. et al. 2004. J. Petrol., 45, 883-905. Jung, S., Masberg, P. 1998. J. Volc. Geotherm. Res. 86, 151-177. Jung, S., Hoernes, S. 2000. J. Volc. Geotherm. Res., 99, 27-53. Jung, S. et al. 2005. Contrib. Mineral. Petrol. 150, 546-559. Jung, C. et al. 2006. J. Petrol. 47, 1637-1671. Pfänder, J. A et al. 2007. Earth Planet. Sci. Lett.. 254 (2007) 158–172. Walker, R.J. et al. 1989. Geochim. Cosmochim. Acta, 53, 1583-1595. Wedepohl, K.H., Baumann, A. 1999. Contrib. Mineral. Petrol., 136, 225-239. P4: The Shield regions – The Helsinki group: Shield(s)(Baltic, Canadian, etc) could be another test site. The definition of LAB below the shields is even more cryptic than below Cenozoic regions. And whatever the definition of LAB maybe it has to apply to the mature parts of the continental plates, the shields, as well. The shields could add a few additional questions, such as

1. How does a plate with large topographic LAB variations glide on the asthenosphere? 2. Why do the roots of the shield appear to be deeper than the lithosphere if only

lithosphere is moving? 3. How are tranform faults/plate boundaries affected by convection? What role do

transform boundaries play in the LAB?

To our understanding are the convection and plate tectonic theories merging quite rapidly. The only part of the plate tectonic theory that the convection theoretical models cannot produce are the torroidal component of the transforms. LABPAX could shed some light on that. The compilation of the seismic data acquired across Scandinavia over the last couple of years, including the recent LAPNET passive experiment can be seen as a first step for an analysis on the LAB across a shield region. This could then be compared with similar efforts for the Canadian shield. P5: Thermodynamically coupled thermo-mechanical models of mantle-lithosphere intereactions by E. Burov These models will use petrology data bases to predict rock properties such as composition, density, elastic parameters directly out of thermo-mechanical models of lithosphere-mantle interactions. For these models one can compute synthetic tomography models and compare them with the observations and vice-versa. Scientists involved in the project preparation are: Michael Abratis, Jena, Germany Vladislav Babuska, Prag, Czech Republic Evgeni Burov, Paris, France Dapeng Chao, Tokio, Japan Marek Grad, Warsaw, Poland Horst Kämpf, Potsdam, Germany Annakaisa Korja, Helsinki, Finnland Elena Koslovskaja, Oulu, Finnland Thomas Meier, Kiel, Germany Jörg Pfänder, Freiberg, Germany Victor Raileanu, Bukarest, Romania Lothar Viereck-Götte, Jena, Germany Monika Wilke, Warsaw, Poland