The depth of the Lithosphere-Asthenophere boundary beneath the world oldest ocean: the Ionian plate case-study

Abstract:

The depth of the Lithosphere-Asthenosphere boundary (LAB) beneath the ocean is widely accepted to increase with the age of the oceanic plate. In particular, in the first 30-50 My, the thickness of the Lithosphere increases almost linearly to about 60-70 km, for a number of models of the LAB formation. Observations from the Pacific plate generally support this hypothesis (e.g. Schmerr, 2012). However, measures of the LAB depth beneath old oceanic plates seem to indicate an almost constant depth of about 70-80 km for oceanic plate older than 50-60 Ma (e.g. Kumar and Kawakatsu, 2011), in dis-agreement with most of the earliest LAB model (e.g. half-space cooling). In this study, we present the first measurement of the LAB depth beneath a 250Myr old ocean, the Ionian plate, obtained from the analysis of teleseismic waveforms recorded at two Ocean Bottom Seismometers (OBS) deployed on the ocean floor. From such recordings, we compute two independent data-set of S receiver function (S-RF), a widely used tool for estimating the depth of the LAB. The S-RF data sets indicate the presence of a negative S-wave velocity discontinuity (i.e. where Vs decreases with depth) at 75 +/- 10 km depth, which is interpreted as the LAB. Insights into the evolution of the oceanic LAB during subduction are obtained by comparing the LAB depth found from the analysis of the S-RF data-sets, with the LAB depth beneath the Calabrian arc, obtained from the analysis of a huge P receiver function data-set.