4-D Lithosphere Mapping: methodology, applications and implications

Suzanne Y. O'REILLY (Key Centre for the Geochemical Evolution and Metallogeny of Continents (GEMOC), School of Earth Sciences, Macquarie University, Sydney, 2109, Australia and W.L. GRIFFIN (GEMOC and CSIRO Explor. and Mining, North Ryde, 2113, Australia

4-D lithosphere mapping is a xenolith-based methodology drawing together many strands of geochemical, geophysical, tectonic and geochronological information. Together these allow a synthesis of the nature of the lithosphere to depths that can be accessed directly by xenoliths and mineral debris of deep-seated rock types and extended by geophysical data. This information can be used to construct sections of the lithospheric stratigraphy and physical state. These sections can constrain the interpretation of geophysical models and geophysical data can then be used to map the lateral extent of known mantle domains.

Lithosphere Mapping represents a methodology that we and others have developed over the last 10 years; this summary serves to introduce the Special Symposium on Lithosphere Mapping. Lithosphere Mapping is a holistic approach to understanding the composition, stratigraphy and thermal state of the lithosphere, the nature and significance of its important boundaries (eg the crust-mantle boundary and the lithosphere-asthenosphere boundary) and its evolution.

The basis of Lithosphere Mapping is the direct evidence for the petrology and geochemistry of the lower crust and upper mantle provided by xenoliths and xenocrysts of deep-seated rock types entrained in basaltic, kimberlitic and lamproitic magmas. These materials are generally transported to the surface in 10-30 hours, too fast for alteration or significant re-equilibration to occur. They yield the compositions and locations of specific rock types in the underlying crust-mantle section, and large specimens can be used to determine the petrophysical characteristics (density, acoustic velocity, magnetic properties, electrical and thermal conductivity, heat production) of the rock types existing at given depths.

The key technique of Lithospheric Mapping is the use of suites of xenoliths and xenocryst minerals to establish the (paleo-) geotherm at specific localities. This information is then used to place individual samples (for which temperature can be calculated) in their original stratigraphic relationship, and thus give a picture of the distribution with depth of rock types and (with geochemical data) of processes such as metasomatism. These data can be combined with geophysical surveys to provide a 3-dimensional picture of the composition, structure and thermal state of the lower crust and upper mantle. Interlocking strands of information from mantle-derived material sampled at different times by volcanic episodes of different ages that can be used to give a comprehensive interpretation of the evolutionary processes of the lithosphere in four dimensions (time as well as space).

Information on the nature of the lithospheric mantle and the processes that have affected it also has direct impact on our understanding of the mechanisms and timing of large-scale tectonic events in the crust because of the demonstrated coupling of the crust and lithospheric mantle during such episodes. Integration of geophysical data and modelling, combined with all possible information from the nature and location of volcanism and the fragments of the lithospheric mantle and deep crust brought to the surface, is the key to reconstructing the mechanisms and stages in the evolution of Earth's crust and mantle.