Stress distribution at the transition from subduction to continental collision (northwestern and central Betic Cordillera)

Abstract

[1] We analyze focal mechanisms of shallow-intermediate earthquakes in a NW-SE transect along the western Betic Cordillera and Alboran Sea, and deep earthquakes located in the central Betics to constrain the state of stress at the Gibraltar Arc slow convergence area. Shallow earthquakes (<40 km) are preferably clustered at the mountain front. A general NW-SE horizontal compression is compatible with the convergence, and NW plunging compression axes are in agreement with frontal thrust activity. Toward the Alboran Sea the earthquakes are progressively deeper, reaching intermediate depths (40-120 km) near the coast and supporting the present-day activity of subduction only in this area. The Iberian continental crust concentrates most of the intermediate seismicity and is forced to partially sink into the mantle, probably through the pull of the oceanic slab. This downdip pull together with the buoyancy of the Iberian continental crust produces the slab curvature, downdip extension in the external arc of the continental slab, and downdip compression in the inner arc. T axes highly dipping to the southeast at 90-120 km depth occur at the oceanic/ continental transition. Deep earthquakes (>620 km) show very similar focal mechanisms, fitting the general slab behavior of resistance to further descent at the 660 km discontinuity. Seismicity features evidence the present-day stress distribution in a context of transition from subduction to continental collision.

Seismicity and active tectonics in the Alboran Sea, Western Mediterranean: Constraints from an offshore‐onshore seismological network and swath bathymetry data

“…However, calculations suggest that any crustal seismicity is related to depth <25 km. We therefore favor the interpretation that intermediate depth earthquakes are related to a subducting slab hanging below the Gibraltar arc and West Alboran Basin [e.g., Sparkman and Wortel , ; García‐Castellanos and Villaseñor , ; Pedrera et al ., ; Ruiz‐Constán et al ., ].…”

“…The most recent large earthquake was the MSK ~ VIII Adra earthquake of 1910 with an estimated moment magnitude of Mw = 6.1 [ Stich et al ., ]. In the western Alboran Sea, intermediate depth earthquakes (40–150 km depth) are most likely related to a remaining slab [ García‐Castellanos and Villaseñor , ; Ruiz‐Constán et al ., ; Bezada et al ., ] of stalled subduction [ Chertova et al ., ].…”

Shallow frontal deformation related to active continental subduction: structure and recent stresses in the westernmost Betic Cordillera

“…However, other researchers support subduction models (Doglioni et al ., ,b) – based on seismic tomography studies that imaged an oceanic subducted slab with SW–NE to N–S orientation (Blanco and Spakman, ; Gutscher et al ., , ; Piromallo and Morelli, ; Spakman and Wortel, ; Bezada et al ., ; Palomeras et al ., ) and E to SE‐dip – including slab roll‐back (Lonergan and White, ; Pedrera et al ., ) with partial or total slab break off and/or tearing (Blanco and Spakman, ; Spakman and Wortel, ; Garcia‐Castellanos and Villaseñor, ; van Hinsbergen et al ., ). Eastward (Gutscher et al ., , ) and south‐eastward (Ruiz‐Constán et al ., , , ) dipping slabs have been considered. The Alboran Basin may be interpreted as a backarc basin, although related to different proposed subduction slabs (Doglioni et al ., ; Gueguen et al ., ; Galindo‐Zaldivar et al ., ; Gueguen et al ., ; Doglioni et al ., ,b; Pedrera et al ., ; Carminati et al ., ; van Hinsbergen et al ., ).…”

Tearing of the mantle lithosphere along the intermediate‐depth seismicity zone beneath the Gibraltar Arc: The onset of lithospheric delamination

“…Crustal earthquakes are well distributed over the entire plate boundary region with concentration in the Betic-Rif Cordilleras, in the Alboran Sea along the Trans-Alboran Shear Zone (TASZ) and in the Gulf of Cadiz (Figure 1a). Beneath central-western Betic and Alboran Sea, significant intermediate-depth seismicity (IDS) occurs in an intra-continental setting with earthquake focal depth ranging from ~50 to ~120 km beneath the western Alboran Sea [Buforn et al, 1991;Ruiz-Constán et al, 2011]. The IDS is focused along a narrow elongated zone with no detectable linear trend in focal depth distribution.…”