Publication 154

The lithosphere-asthenosphere system beneath Ireland from integrated geophysical-petrological modeling - I: Observations, 1D and 2D hypothesis testing and modeling

Jones, A.G., J.C. Afonso, J. Fullea, and F. Salajegheh

Abstract

Modeling and inferring the continental lithosphere’s physical properties, especially its depth extent, must be done within a self-consistent petrological-geophysical framework, not from only one or two data types that may easily lead to inconsistencies and erroneous interpretations. Using the LitMod approach for hypothesis testing and first-order modeling, we show how assumptions made about crustal information and likely compositions of the lithospheric and sub-lithospheric mantle affect various observables, particularly surface topographic elevation. The critical crustal parameter is crustal density, leading to approx. 600 m error in topography for 50 kg m-3 imprecision. The next key parameter is crustal thickness, and errors in its definition lead to around 4 km of LAB error for every 1 km of Moho variation. Possible errors in the other assumed crustal parameters introduce a few kilometers of variation in the depth to the LAB.
We use Ireland as our natural laboratory for demonstrating the approach. From first-order arguments and given reasonable assumptions, to yield a topographic height in the range of 50 – 100 m, which is the average height of Ireland, the lithosphere-asthenosphere boundary (LAB) beneath most of Ireland must lie in the range 90-115 km. A somewhat shallower (to 85 km) LAB is permitted, but the crust has to be thin (<29 km) to compensate. Particularly inconsistent with the observations, especially topography, is a prior suggestion based on interpretation of S-to-P receiver functions that the LAB thins from 85 km in southern Ireland to 55 km in central northern Ireland over a distance of <150 km. Such thin lithosphere would yield over 1000 m of uplift, and such rapid thinning by 30 km over less than 150 km would yield significant north-south topographic elevation change, Bouguer anomaly change, and geoid height change, none of which are observed. Even juxtaposing the likely most extreme depleted composition for the lithospheric mantle beneath southern Ireland against the most extreme likely fertile composition beneath northern Ireland only allows some 20 km of LAB variation at most; any more and there would be effects that are well beyond those observed.
One model that satisfies almost all the extant data to first-order comprises a spinel peridotite upper lithospheric mantle layer to 85 km in southern Ireland and to 55 km in northern Ireland, transitioning over 150 km. Below this in southern Ireland is a garnet peridotite layer to 115 km, and in northern Ireland a refertilized layer to 95 km. The mid-lithospheric discontinuity (MLD) at the base of the spinel peridotite may explain the observed S-to-P (Sp) receiver functions.

Source

Lithos, 189, 28–48, doi:10.1016/j.lithos.2013.10.033. [PDF]

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Alan G Jones / 15 February 2014 / alan-at-cp.dias.ie