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Tomographic images of the mantle below the Hercynian belt of Brittany display strong anomalies of P-wave traveltimes that likely represent variations in composition, and whose trends change above and below 130 km. A quantitative strain analysis of surface geological structures has shown that the crust in the area was affected by regional-scale distributed simple shear. The change of trends of tomographic anomalies below and above 130 km is largely removed by extending the simple shear model down to 130 km. From this, we infer lithospheric-scale wrenching, and therefore deep-seated horizontal decoupling, possibly at the lithosphere-asthenosphere boundary. Simple shear restoration further provides arguments for the occurrence of a steeply dipping slab. Geological data support its interpretation as a remnant of oceanic lithosphere subducted prior to the Hercynian collision. Beyond regional implications, the study underlines the interest of combining geology and tomography for the understanding of lithosphere deformation. Keywords: Hercynian lithosphere, mantle deformation, seismic tomography, shear belt, central Brittany.

Teleseismic tomography across the Chinese Tien Shan shows that seismic wave speeds in the lithosphere beneath central Tien Shan are high and therefore the lithosphere is not weaker than that beneath the adjacent undeformed Tarim and Junggar basins. There is evidence for significant velocity contrasts within the lithosphere that are presumably inherited from the Palaeozoic collision history. The high-velocity, thick Yili block observed underneath the northern Tien Shan is a clue for shortening by a intracontinental subduction. The observed geometry is consistent with a simple model of intracontinental subduction and suggests that, during orogeny, the lithosphere has remained heterogeneous and has deformed along existing planes of weakness rather than by homogeneous thickening of a particularly weak lithosphere. [ABSTRACT FROM AUTHOR]

Seismic tomography across the Altyn Tagh fault, at the north edge of the Tibetan Plateau, reveals a low P-wave velocity anomaly below the fault down to 140 kilometers. This anomaly probably reflects strike-slip shear in the lithosphere. Slip-partitioning may also induce a wedge of crust from the Tarim Basin to plunge into the mantle.