A common deep source for upper-mantle upwellings below the Ibero-western Maghreb region from teleseismic P-wave travel-time tomography

Abstract

Upper-mantle upwellings are often invoked as the cause of Cenozoic volcanism in the Ibero-western Maghreb region. However, their nature, geometry and origin are unclear. This study takes advantage of dense seismic networks, which cover an area extending from the Pyrenees in the north to the Canaries in the south, to provide a new high-resolution P-wave velocity model of the upper-mantle and topmost lower-mantle structure. Our images show three subvertical upper-mantle upwellings below the Canaries, the Atlas Ranges and the Gibraltar Arc, which appear to be rooted beneath the upper-mantle transition zone (MTZ). Two other mantle upwellings beneath the eastern Rif and eastern Betics surround the Gibraltar subduction zone. We propose a new geodynamic model in which narrow uppermantle upwellings below the Canaries, the Atlas Ranges and the Gibraltar Arc rise from a laterally-propagating layer of material below the MTZ, which in turn is fed by a common deep source below the Canaries. In the Gibraltar region, the deeply rooted upwelling interacts with the Gibraltar slab. Quasi-toroidal flow driven by slab rollback induces the hot mantle material to flow around the slab, creating the two low-velocity anomalies below the eastern Betics and eastern Rif. Our results suggest that the Central Atlantic plume is a likely source of hot mantle material for upper-mantle upwellings in the Ibero-western Maghreb region.

Thermal Nature of Mantle Upwellings Below the Ibero‐Western Maghreb Region Inferred From Teleseismic Tomography

“…The model shows a pronounced positive velocity anomaly corresponding to the location of the subducted Gibraltar slab (d V S ≈ 0.3 km/s in its core), which dips near vertically in the upper mantle. The shape, dip, and amplitude of the slab are consistent with the P wave results (Civiero et al, ). We also observe a prominent high‐velocity anomaly below western Iberia and an EW ‐elongated high‐velocity feature beneath the southern Pyrenees, both of which extend through the upper mantle, similar to what was imaged in IBEM‐P18.…”

“…Instead, we compare the P wave and S wave relative arrival‐time residuals for common earthquake‐station pairs, thus avoiding difficulties associated with the amplitude recovery of anomalies, potential smearing, spatial resolution dependence on station distribution, and parameterization/regularization. The P wave residuals are those of Civiero et al (). Figure shows the S wave relative residuals plotted as a function of the P wave relative residuals, for stations deployed in the three different areas where we image subvertical low‐velocity anomalies extending within the MTZ: A1 (Canaries), A2 (Atlas Ranges), and A3 (Gibraltar Arc).…”

“… P wave (from Civiero et al, ) versus S wave relative traveltime residuals (with associated errors) for common earthquakes and stations, for stations located in regions A1–A3 (Figure ). The values of the slopes, a S,P , fall within the thermal range for all the selected regions (A1 = ~2.5 ± 0.1; A2 = ~3.8 ± 0.2; A3 = ~2.1 ± 0.1).…”

“…The low‐velocity anomalies imaged below the Atlas Ranges and the Betic‐Rif system by previous tomographic studies have been attributed to passive mantle upwellings due to edge‐driven convection/small‐scale convection (e.g., Kaislaniemi & Van Hunen, ; Missenard & Cadoux, ; Ramdani, ) and lithospheric delamination (e.g., Bezada et al, ; Levander et al, ) or active mantle upwellings connected to the Canary plume (Civiero et al, ; Duggen et al, ; Miller et al, ). In particular, our previous P wave model IBEM‐P18 allowed us to propose two main mechanisms for the generation of the low‐velocity anomalies: (1) The mantle upwellings found below A2 and A3 rise from ~700‐km depth and are connected to A1 below the MTZ , with all three being fed by a broad lower‐mantle plume (the central Atlantic plume) imaged in global inversions (e.g., Simmons et al, ); and (2) quasi‐toroidal mantle flow, induced by the Gibraltar slab, drags the hot mantle material from the subslab upwelling (which is sourced below the MTZ) laterally and upward around either lateral slab edge, to the eastern Rif and Betics.…”

“…The 3D starting model for the crust and upper mantle was built by converting the PRISM3D P wave model (Civiero et al, ) into an S wave velocity model assuming a V P / V S ratio calculated from the 1D ak135 P wave and S wave speed models as a function of depth (Figures S2a and S2b). The initial S wave velocities in the topmost lower mantle (660‐ to 800‐km depth) are those converted from the LLNL global P wave tomographic model of Simmons et al (, Figure S2c).…”