4-D Lithosphere Mapping: the integration of geological and geophysical datasets

Suzanne Y. O'Reilly (GEMOC, Earth Sciences, Macquarie Univ., Sydney 2109, Australia)

W L Griffin (GEMOC, Earth Sciences, Macquarie Univ., Sydney 2109, and CSIRO-EM, Box 136, NSW 2113, Australia)

4-D Lithosphere Mapping is a xenolith-based methodology drawing together many strands of geochemical, geophysical, tectonic and age 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 laterally by geophysical data. This information can be used to construct sections of the lithospheric stratigraphy and physical state, which can constrain the interpretation of geophysical models; geophysical data can then be used to map the lateral extent of individual mantle domains. Lithosphere Mapping is an integrated 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 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 samples 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 rocks at given depths.

The key technique of Lithospheric Mapping is the construction of empirical (paleo)geotherms at specific localities by use of xenoliths and xenocrysts. This information is then used to place individual samples (for which temperature (T) can be calculated) in their original vertical sequence, and thus give the distribution with depth of rock types and mantle processes such as metasomatism. The thermal state of a lithospheric column also influences geophysical characteristics: T determines density and thus affects seismic velocities and gravity; magnetic responses are confined to rocks above the Curie isotherm. Mantle-derived material sampled by volcanic episodes of different ages allow interpretation of the evolution of the lithosphere in four dimensions (ie time as well as space).