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2013-03-21 – Seminar: Nikolai Bagdassarov

21 March 2013Seminar

When: 4pm on Thursday, 21 March 2013
Where: DIAS, Geophysics Section, 5 Merrion Square, Dublin 2, (library)

Speaker: Nikolai Bagdassarov (Goethe University, Frankfurt, Germany)
Title: Lithosphere heterogeneity from petrology and electrical conductivity of xenoliths

Abstract:

The question about a correlation between geodynamics and electric properties of lithosphere is still a matter of discussion in geodynamic and geophysical communities. Three distinct situations of continental geodynamics: rift zones, stable cratons, and mobile orogenic belts, represent three differing patterns of the lithosphere electric conductivity. This presentation deals with the example of Tien Shan (Kirgysia) orogenic belt. In this study xenolith petrology and laboratory measurements of their electrical properties were combined to give an interpretation of magnetotelluric data from the region and the state of the lithosphere beneath Tien Shan. The results may be summarized as follows:

1. The upper mantle beneath south Tien Shan mostly consists of spinel lherzolites with composition which varies from a primitive to a slightly depleted with a subsequent enrichment of the most incompatible elements. This region of the upper mantle has been affected by 6-10% of partial. Subsequently spinel lherzolites have been re-enriched by carbonate and silicate melts from metasomatic processes. Thus, spinel lherzolite xenoliths were derived from the lithospheric mantle and from the depth close to the crust-mantle boundary.

2. The enrichment of spinel lherzolite xenoliths with LREE, LILE is an indicator of an intensive mantle silicate metasomatism prior
to volcanic activity in the region for at least 70 Ma ago which in turn may be caused by a local plume. A fluid composition
probably was not sufficiently rich in alkali and CO2 in order to provide a carbonate metasomatism alone.

3. 70-60 Ma ago the heat flux value was significantly larger (~ 80-85 mW/m2) than the observed today heat flux in the region ~ 50- 60 mW/m² (Vermeesh et al., 2004). This geologic time corresponds to a hot spot type volcanic activity in the region. The crustal thickness of south Tien Shan was about 35-40 km with a hotter and weaker crustal lithosphere basement. The temperature on the Moho discontinuity was c. 750-800°C, i.e. 100°C higher than today.

4. Modern thickness of the crust beneath Tien Shan is ~55 km (Roecker, 2001). The collision event of India with Eurasia caused a propagation of crustal shortenings and resulted in a crustal thickening of South Tien Shan for about 20-30 km. The temperature at Moho discontinuity now is c. 720-750°C. The integrated strength of the lithosphere beneath the South Tien Shan is 4 – 8×1012 N/m at the strain rate 3×10-15– 10-14 sec-1.