Focal Mechanisms for Subcrustal Earthquakes Beneath the Gibraltar Arc

Intermediate deep earthquakes are usually associated with active subduction, and show mostly dip-slip faulting mechanisms aligned with the downgoing oceanic lithosphere. Forty-two new focal mechanisms from 50-to 100-km depth beneath the Gibraltar Arc and Alboran Sea show different characteristics. The most abundant solutions are strike-slip mechanisms, in agreement with relative plate motion between Nubia and Eurasia. Additional reverse faulting mechanisms indicate compression in direction of absolute plate motion, reproducing the basal drag of the mantle on the hanging lithosphere. In turn, no signature of ongoing subduction was found. Migrated sections of P wave receiver functions suggest that a significant part of intermediate deep seismicity is produced within the stalled remnants of Jurassic age, oceanic lithosphere that once formed the connection between the Alpine Tethys and the central Atlantic and later has been buried beneath the Gibraltar Arc. Plain Language Summary Earthquakes below 50-km depth are usually associated with active subduction, and the direction of faulting is aligned with the orientation of the subduction zone. Faulting in 42 earthquakes beneath the Gibraltar Arc and Alboran Sea shows different characteristics. The most abundant solutions show horizontal slip, in agreement with relative plate motion between Africa and Europe. Further solutions are associated with shortening and suggest compression from the basal drag of the Earth's mantle on the moving plates. In turn, no signature of active subduction was found. Images of the Earth's interior from teleseismic waves suggest a relation between the earthquakes and a stalled remnant of ~150-Ma-old oceanic material that once formed the connection between two oceans and later has been buried beneath the Gibraltar Arc.