Deep Seismic Expression of an Ancient Plate Boundary in Europe

Seismic evidence for water deep in Earth's upper mantle

Water in the deep upper mantle can influence the properties of seismic discontinuities in the mantle transition zone. Observations of converted seismic waves provide evidence of a 20- to 35-kilometer-thick discontinuity near a depth of 410 kilometers, most likely explained by as much as 700 parts per million of water by weight.

Seismic refraction evidence for crustal structure in the central part of the Trans-European Suture Zone in Poland

The results of seismic investigations obtained for the Trans-European Suture Zone (TESZ) show the presence of relatively low velocity rocks (Vp < 6.1 kms−1), of sedimentary, metamorphic or volcanic origin, down to a depth of 20 km; high velocity (Vp = 6.8–7.3 kms−1) lower crust, the Moho at a depth of approximately 30–33 km; and a high-velocity (Vp > 8.3 kms −1) uppermost mantle. The transition of the crustal structure is seen across a 200 km wide zone. The three-layered crystalline crust of Baltica changes over this distance into the two-layered crust of Palaeozoic (Variscan) Europe, due to the disappearance of the lowest layer (Vp ∼ 7.1 kms−1) and tapering off of the Baltican/cratonic wedge. The seismic profiles suggest that the lower crust (Vp ∼ 7.1 kms−1) in the transition zone represents the attenuated Baltica margin underthrust towards the SW beneath the Avalonian accretionary wedge. The latter corresponds to the low-velocity upper crust (Vp < 6.1 kms−1) characteristic of the German-Polish Caledonides. Consequently, the high-velocity reflective lower crust of Baltica affinity extends approximately 200 km to the SW of the Teisseyre-Tornquist Zone within the basement of the Palaeozoic Platform. The Avalonian upper/middle crust is confined in the SW against the WNW-ESE trending Dolsk Fault. To the SW of the Odra Fault, a typical Variscan crust is detected which shows two-layer structure and relatively low P-wave velocities. The WNW-ESE Odra Fault, approximately parallel to the Dolsk Fault, splits the Variscan domain into the Variscan externides buried beneath the Palaeozoic Platform in the NE and the Variscan internides of the Sudetes in the SW. We interpret both the Odra and Dolsk Faults as dextral strike-slip features that cross cut the NE termination of the Variscan Orogen parallel to the Teisseyre-Tornquist Zone. In a relatively small area, they juxtapose three crustal domains representing, successively, the Variscan internides, externides and the Variscan foreland.

The Seismic 8degrees Discontinuity and Partial Melting in Continental Mantle

Strong, scattered reflections beyond 8 degrees (8degrees) offset are characteristic features of all high-resolution seismic sections from the continents. The reflections identify a low-velocity zone below approximately 100 kilometers depth beneath generally stratified mantle. This zone may be caused by partial melting, globally initiated at equal depth in the continental mantle. Solid state is again attained at the Lehmann discontinuity in cold, stable areas, whereas the zone extends to near the 400-kilometer discontinuity in hot, tectonically active areas. Thus, the depth to the Lehmann discontinuity may be an indicator of the thermal state of the continental mantle.