Abstract
Abstract. New global maps of the depth to the boundary between the lithosphere and the asthenosphere are presented. The maps are based on updated global databases for heat flow and crustal structure. For continental regions the estimates of lithospheric thickness are based on determinations of subcrustal heat flow, after corrections for contributions of radiogenic heat in crustal layers. For oceanic regions the estimates of lithospheric thickness are based on the newly proposed finite half-space (FHS) model. Unlike the half-space cooling (HSC) and the plate models the FHS model takes into account effects of buffered solidification at the lower boundary of the lithosphere and assumes that the vertical domain for downward growth of the boundary layer have an asymptotic limit. Results of numerical simulations reveal that theoretical values derived from the FHS model provide vastly improved fits to observational data for heat flow and bathymetry than can be achieved with HSC and plate models. Also, the data fits are valid for the entire age range of the oceanic lithosphere. Hence estimates of depths to lithosphere- asthenosphere boundary (LAB) based on the FHS model are believed to provide more reliable estimates than those reported in previous thermal models. The global maps of depths to LAB derived in the present work reveal several features in regional variations of lithosphere thicknesses that have not been identified in earlier studies. For example, regions of ocean floor with ages less than 55 Ma are characterized by relatively rapid thickening of the lithosphere. Also there is better resolution in mapping the transition from oceanic to continental lithosphere, as most of the latter ones are characterized by lithospheric thickness greater than 150 km. As expected the plate spreading centers in oceanic regions as well as areas of recent magmatic activity in continental regions are characterized by relatively thin lithosphere, with LAB depths of less than 50 km. On the other hand, the areas of continental collisions and Precambrian cratonic blocks are found to have lithosphere thicknesses in excess of 250 km. Regional variations of lithosphere thickness in the interiors of continents are found to depend on the magnitude of subcrustal heat flux as well as the tectonic age of crustal blocks.
Highlights
The concepts of lithosphere and asthenosphere are fundamental components of plate tectonic theory, according to which the lithosphere-asthenosphere boundary (LAB) separates the upper rigid part from the underlying upper mantle in both oceanic and continental regions
We examine the fit of the finite half-space (FHS) model to oceanic heat flow data as part of the attempt to map regional variations in the depth of lithosphere-asthenosphere boundary (LAB) in oceanic regions
The maps are based on updated global databases for heat flow (Vieira and Hamza, 2010) and crustal structure (Mooney et al, 1998)
Summary
The concepts of lithosphere and asthenosphere are fundamental components of plate tectonic theory, according to which the lithosphere-asthenosphere boundary (LAB) separates the upper rigid part from the underlying upper mantle in both oceanic and continental regions. In oceanic regions large scale variations in heat flow and bathymetry has often been interpreted as indicative of systematic increase in the depth of lithosphere-asthenosphere boundary with distance from the spreading centers (see for example: Parsons and Sclater, 1977). Results of geothermal methods, based on observational data and thermal models, allow determination of temperatures in the lithosphere and this information may be employed in mapping LAB with relatively improved degree of reliability. The procedure employed allows determination of the thickness of the layer in which temperatures lie below the mantle adiabat and where the heat transport is mainly conductive This part of the lithosphere is usually identified as the thermal boundary layer (TBL) that overlie the mantle convection system. We examine the differences in the processes that determine regional variations in the thickness of the TBL in oceanic and continental regions
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