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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.

The seismic phase PKiKP, reflected from the inner core, and its coda give information on the short scale heterogeneities at and below the inner core boundary (ICB) of the Earth. We have collected PKiKP recorded at short distances (<45°) at broadband stations in Australia and at the Warramunga seismic array (WRA). Despite potential complications from a dual-passage through D″, PKiKP is frequently observed on single traces in the frequency band 1–5 Hz. PKiKP usually has a sharp onset, but sometimes its waveforms have multiple pulses separated by times of the order of 1 s. At WRA, the coda of PKiKP initially decays very rapidly after the main pulse. Thereafter, its amplitude is nearly constant and more than three times smaller than PKiKP with duration longer than 200 s. Our observations differ from those of [Nature 404 (2000) 273–275], at rather greater distances, who found that the coda is larger than PKiKP and increases slowly. We suggest that the PKiKP coda is generated near the inner core boundary by complex reverberation effects, rather than scattering through the volume of the inner core. [Copyright &y& Elsevier]

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.