The Western Mediterranean basin has been formed by Miocene back-arc extension and is underlain by a thin and young lithosphere. This young lithosphere is warm, as testified by an overall elevated offshore heat flow. Heat flow within the Western Mediterranean is, however, highly variable and existing data are unevenly distributed and poorly studied in the central part of the Liguro-Provençal and Algero-Balearic basins. This central part is floored by a young oceanic crust, bordered by different continental margins, cut by transform faults, and filled by up to 8 km of sediments. We present a total of 148 new heat flow data collected during the MedSalt and WestMedFlux cruises in 2015 and 2016 and aligned along seven regional profiles that show an important heat flow variability on the basin-scale, but also locally on the margins.

A new heat flow map for the Western Mediterranean outlines the following regional features: (1) a higher average heat flow in the Algero-Balearic basin compared to the Liguro-Provençal basin (94 ± 13 mW/m2 and 78 ± 16 mW/m2, respectively), and (2) a regional thermal asymmetry in both basins, but with opposed heat flow trends. Up to 20% of this heat flow difference can be explained by sediment blanketing, but age and heterogeneity of ocean crust due to an asymmetric and polyphased opening of the basins are believed to have given the major thermal imprint. Estimates of the age of the oceanic crust based on the new heat flow suggest a considerably younger West Algerian basin (16–23 Ma) compared to the East Algerian basin and the West Sardinia oceanic floor (31–37 Ma).

On the margins and ocean-continent transitions of the Western Mediterranean the new heat flow data point out the existence of two types of local anomalies (length scale 5–30 km): (1) locally increased heat flow up to 153 mW/m2 on the Gulf of Lion margin results from thermal refraction of large salt diapirs, and (2) the co-existing of both low (<50 mW/m2) and high (>110 mW/m2) heat flow areas on the South Balearic margin suggests a heat redistribution system. We suspect the lateral heat advection is resulting from a regional fluid circulation in the sediments associated to the widespread Plio-Pleistocene volcanism on the South Balearic margin.