Solid Earth

The topical issue has been derived from Symposium GD2.4/SM4.1/TS10.2 The Lithosphere-Asthenosphere Boundary (LAB) Dilemma during the EGU 2011 Assembly. We encourage scientists to submit their results achieved in interdisciplinary studies of the LAB, especially petrological and dynamical modelling including asthenosphere-lithosphere interaction in geodynamic processes. In the volume we will bring together scientific achievements from all fields in Earth Sciences to incorporate the current status and advancements in the studies of the LAB. We especially invite contributions from authors who were not at EGU to submit papers to the Special Issue on LAB and to the lithosphere and asthenosphere structures.

The lithosphere-asthenosphere boundary is the most extensive and active plate boundary on Earth. However, it is a relatively cryptic boundary 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. 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 "LAB" by the colleagues from the particular fields in Earth Science.

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 questions, 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.

Subduction drives plate tectonics, forms continents, and transports surface material to the deep Earth. Therefore, it is arguably the most important geodynamical and geochemical phenomenon on Earth. In addition, the direct association with devastating earthquakes and active volcanism signifies a need for deep understanding of all aspects of this process. Recent studies of subduction processes have been able to build on results of decades of research and we consider it timely, therefore, to bring together studies of subduction zone dynamics at all scales from the surface to the lower mantle.

For this special issue we invite contributions from a wide range of disciplines, including, and certainly not limited to, geodynamics, modelling, geochemistry, mineralogy, petrology and seismology. Topics could include:

• the initiation of subduction;
• the role of phase transitions and the migration of fluids and melts;
• controlling parameters for slab dip angle;
• the role of near-surface processes in controlling slab dynamics;
• the fate of subducted crust, sediments and volatiles;
• the influence of the mantle wedge and the back-arc on slab dynamics;
• slab break-off;
• subduction of seamounts, LIPs and ridges;
• continental collision;
• links between subduction and the development of mineral deposits;
• imaging of present-day subduction zones and surrounding mantle flow.

Black shales have captured the interest of geoscientists since at least the 19th-Century. A black shale is usually considered an organic-rich, fine-grained sedimentary rock irrespective of lithology and other petrologic considerations. It was recognised early on that black shale horizons are more than just of local significance, and in fact be of global importance. With the advent of the Deep Sea Drilling Project and the recovery of black shale horizons from ocean basins—coeval to those previously known from land sections—geoscientists fully recognised the supraregional significance of these horizons. In 1976, time intervals in the Mesozoic characterised by widespread distribution of black shales were termed oceanic anoxic events in recognition of their discrete interbasinal significance, but not implying total global anoxia. Whilst the term was originally intended to include a number of Mesozoic black shale horizons, the term is actually valid for any Phanerozoic time interval associated with perturbations of the global carbon cycle and widespread black shale deposition.

Black shales are unique in a variety of aspects. The requirement of a multidisciplinary approach to their study distinguishes them from the majority of other sedimentary rocks. The contributions to this special issue reflect that multidisciplinarity, and represent the current views of black shale deposition and environments. This special issue also represents a holistic approach to black shales, rather than just considering them as the carriers of particular biogeochemical signals. Thus, the sedimentology and stratigraphy of black shale facies associations are at the core of this special issue.