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Recent investigations of deep crust-mantle sections exhumed in orogenic belts, as well as the study of crustal and mantle xenoliths brought to the surface by volcanic activity provide new perspectives on the nature of the crust-mantle transition. These studies indicate that the geophysical Moho is not a simple feature, with variations at different scale lengths that preclude a single, universally applicable interpretation. A key challenge is to unravel the structural relationship between the lower crust and underlying lithospheric mantle. The crust-mantle transition is often characterized by intermixing of mantle and crustal rocks, sometimes forming mylonites and mélanges. Tectonic slices of mantle rocks within the crust often occur in association with trans-lithospheric shear zones, both formed during continental lithospheric extension that may precede continental breakup, and also during plate convergence and continent-continent collision. These processes cause the interlayering of peridotites and high-grade metamorphic rocks on scales from a few km to tens of km. If involved in melting processes, such heterogeneous crust-mantle transitions, typical of polycyclic orogenic belts, could represent a suitable source region for exotic magma types and the ideal site for mass transfer between crustal and mantle reservoirs.

At greater depths, the lithosphere-asthenosphere boundary (LAB) represents a first-order structural discontinuity that accommodates differential motion between the tectonic plates and the underlying mantle. This boundary is also irregular and the detection of its location and morphology is complex. For this reason, the seismic proxies must be complemented by petrological investigations of mantle-derived melts and exhumed mantle xenoliths in order to constrain the physical and chemical gradients across the LAB that create differences in mechanical strength. A particular focus of interest is also the thermo-chemical erosion of the lithospheric mantle and its possible delamination and recycling via convective removal of its lower parts.

GSA Special Paper 526, which includes 10 chapters, encompasses contributions from a wide spectrum of earth science disciplines, including geophysics, geodynamics, geochemistry, and petrology, to provide an overview of the nature and evolution of the crust-mantle and lithosphere-asthenosphere boundaries in different tectonic settings, combining studies that exploit different types of data and interpretative approaches. The integration of geochemical, geophysical, and geodynamic datasets and their interpretation will provide a state-of-the-art summary of current understanding, and will serve as a blueprint for future research activities.

Acknowledgments

Each of the contributions in this volume has been reviewed by two or more external reviewers. We are grateful to the suggestions and criticism provided by Maurice Colpron, Jaroslav Dostal, David Eaton, Gary Ernst, Yener Eyuboglu, Andrew Frederiksen, Dominique Frizon De Lamotte, Claude Herzberg, Hannan Hughes, David Jousselin, Veronique Le Roux, Jaroslaw Majka, Maurizio Mazzucchelli, André Michard, Maria Rosaria Renna, Ben-Xun Su, José María Tubía, Kuo-Lung Wang, and Gerhard Wörner.

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