Mantle Beneath the Gibraltar Arc from Receiver Functions

nair mohan;

P and S receiver functions (PRF and SRF) from 19 seismograph stations in the Gibraltar Arc and the Iberian Massif reveal new details of the regional deep structure. Within the high-velocity mantle body below southern Spain the 660-km discontinuity is depressed by at least 20km. The Ps phase from the 410-km discontinuity is missing at most stations in the Gibraltar Arc.A thin (∼50km) low-S-velocity layer atop the 410-km discontinuity is found under the Atlantic margin. At most stations the S410p phase in the SRFs arrives 1.0–2.5s earlier than predicted by IASP91 model, but, for the propagation paths through the upper mantle below southern Spain, the arrivals of S410 pare delayed by up to+1.5s.The early arrivals can be explained by elevated Vp/Vs ratio in the upper mantle or by a depressed 410-km discontinuity. The positive residuals are indicative of a low (∼1.7 versus ∼1.8 in IASP91) Vp/Vs ratio. Previously, the low ratio was found in depleted lithosphere of Precambrian cratons. From simultaneous inversion of the PRFs and SRFs we recognize two types of the mantle: ‘continental’ and ‘oceanic’. In the ‘continental’ upper mantle the S-wave velocity in the high-velocity lid is 4.4–4.5kms−1, theS-velocity contrast between the lid and the underlying mantle is often near the limit of resolution (0.1kms−1), and the bottom of the lid is at a depth reaching 90– 100km. In the ‘oceanic’ domain, the S-wave velocities in the lid and the underlying mantle are typically 4.2–4.3 and ∼4.0kms−1, respectively. The bottom of the lid is at a shallow depth (around 50km), and at some locations the lid is replaced by a low S-wave velocity layer. The narrowS–N-orientedbandofearthquakesatdepthsfrom70to120kmintheAlboranSeaisin the ‘continental’ domain, near the boundary between the ‘continental’ and ‘oceanic’ domains, and the intermediate seismicity may be an effect of ongoing destruction of the continental lithosphere.