Evidence for a developing plate boundary in the western Mediterranean

The study tests the prevailing diffuse-deformation model for the Africa–Eurasia plate boundary in the western Mediterranean, where slow convergence (4.5 ± 1 mm/yr) and widespread moderate seismicity have led to the interpretation of a broad, distributed plate boundary. Because much deformation occurs offshore on poorly constrained structures, the model remains unvalidated. The authors target two major offshore systems with prominent seafloor relief—the right-lateral Yusuf Fault System (YFS) and the oblique-reverse Alboran Ridge Fault System (ARFS)—to quantify their Plio-Holocene slip and evaluate whether they localize plate convergence into a discrete plate boundary. The research addresses whether a narrow, kinematically linked YFS–ARFS system has absorbed a substantial fraction of Africa–Iberia convergence, with implications for underappreciated seismic and tsunami hazard given potentially long earthquake recurrence intervals not captured by instrumental catalogs.

• Regional geodynamics: Rollback of Tethyan slabs opened western Mediterranean basins until latest Miocene, followed by Plio-Holocene contraction driven by Africa–Eurasia convergence (~4.5 ± 1 mm/yr). Seismicity is scattered across a ~300-km-wide zone, and GPS indicates complex block motions interpreted as a diffuse plate boundary.
• Crustal domains: Miocene processes created thin continental crust (West Alboran, Malaga sub-basins), magmatic arc basement (East Alboran Basin), oceanic crust (Algero-Balearic Basin), and North African continental crust beneath the South Alboran Basin. These inherited boundaries have been reactivated during Plio-Holocene contraction.
• Major faults: The YFS, ARFS, Al-Idrissi, and Carboneras systems are largest regional structures, but offshore ones are often overlooked in kinematic models. Prior work suggests Al-Idrissi is developing with limited documented slip, and Carboneras slip rates are 0.8–1.3 mm/yr—both likely absorbing only a small portion of plate convergence.
• Hazard context: Instrumental seismicity is moderate, but historical tsunamigenic events (e.g., 1522 Mw ~6.5, 1680 Mw ~7, 1804 intensity 8) indicate larger events. Submarine sources remain poorly identified.
• Motivation: To robustly quantify offshore deformation on YFS and ARFS to test the diffuse vs concentrated plate boundary models and reassess regional seismic/tsunami hazard.

Data acquisition and processing:

• Two multichannel seismic datasets: TOPOMED (2011; deep-penetration, streamer up to 6 km, G-II airguns) and EVENT-DEEP Leg 1 (2010; high-resolution, 600 m streamer, G-gun II airgun array). Processing included time-domain flows (deconvolution, multiple attenuation, SRME, Radon, migration) and depth-domain Pre-Stack Depth Migration (PSDM) with iterative velocity analysis and noise filtering.

Seismic interpretation:

• Stratigraphy correlated with regional seismostratigraphy and calibrated using ODP Leg 161 Sites 977/978 and HBB-1 well. Key markers: M horizon (5.3 Ma, Messinian) and Q1 (0.79 Ma). Moho identification based on characteristic reflectivity (validated by wide-angle data).

Structural analysis of YFS:

• Pull-apart basin architecture mapped with six units (S1–S6). Pre-kinematic unit S6 (early Messinian) is offset; S1–S5 show syn-kinematic thickening and tilting toward depocentre.
• Minimum slip from kinematic restoration: ~11 km gap of S6 across basin parallel to fault strike.
• Forward kinematic modeling (fbfFOR) of extensional faults F1–F5, sequentially activated, using pre-kinematic horizons to estimate cumulative extension: ~12.05 km. Recognized limitation: small faults below seismic resolution may contribute additional 25–60% extension, implying total 16–30 km.
• Independent strike-slip estimate using Rodgers’ elastic dislocation model for pull-apart basins: with ~3 km accommodation space, total offset ~20–30 km (10–15% depth/offset ratio).

Structural analysis of ARFS:

• PSDM profiles across Alboran Ridge show frontal thrust (ARFS-F1) and main splay (ARFS-F2). Growth strata indicate initiation in early Pliocene. Crustal-scale contrast across ARFS with Moho ~11–13 km (north) vs ~15 km (south); different volcanic basements on either side.
• Forward modeling with fault-bend-fold geometries tested various detachment depths; best-fit indicates detachment ~15 km and total slip 10–12 km, but models cannot reproduce all observed complexity (blind thrusts, multiple contemporaneous faults, heterogeneous rheology).
• Excess-Area method applied to four pre-kinematic horizons (restored across erosional gaps): linear regressions yield slip 12.7 ± 2.8 km (including Messinian horizon) and 16 ± 4.7 km (excluding Messinian); detachment depth ~7.6–8.4 km with large (>6 km) uncertainty.
• Fault-related folding method relates uplift (~2 km) to slip via detachment angle: for 5° angle, slip ~22.9 km; for 10°, ~11.5 km—consistent with Excess-Area range.

Thermomechanical constraints:

• Geothermal gradient estimated from heat-flow (mean 121 mW/m²), thermal conductivity (2.2 W/mK), and radiogenic heat production (0.002 mW/m³). Brittle–ductile transition depth derived by comparing frictional and creep stresses (quartz-diorite and plagioclase An75) indicates ~8.5–10 km below the southern flank, consistent within uncertainty with seismically inferred detachment depths and changes in basement reflectivity.

Assumptions and limitations:

• Plane-strain approximations in methods; if transpression is significant, true 3D slip could be larger. Deep fault traces not directly imaged; uncertainties in erosion, later deformation, and measurement of excess areas acknowledged. Syn-kinematic sediments excluded from forward models due to insufficient constraints.

• YFS structure and slip:

  • ~160-km-long dextral strike-slip system forming a pull-apart basin with up to ~2.7 km syn-tectonic sediment thickness and ~3 km accommodation space.
  • Minimum cumulative slip constrained by restoration: ~11–12 km (11 km gap of S6; ~12.05 km from forward modeling of F1–F5 faults).
  • Accounting for sub-seismic faults increases total extension to ~16–30 km. Rodgers model with ~3 km subsidence implies 20–30 km strike-slip.
  • YFS marks a lithospheric boundary with abrupt crustal thickness change: Moho ~8–9 s TWTT (~15 km, <10 km crust) north vs ~11–12 s TWTT (~18–20 km, >13 km crust south), deepening to ~30 km toward N. Africa.

• ARFS structure and slip:

  • Largest regional structure, 130 km long with ~2 km seafloor relief; kinematically linked laterally to YFS.
  • Crustal-scale thrust separating thinner magmatic arc crust (north) from thicker North African continental crust (south); Moho ~11–13 km north vs ~15 km south.
  • Initiation of shortening in early Pliocene; active frontal thrust (ARFS-F1) and main splay (ARFS-F2).
  • Slip estimates:
    • Excess-Area method: 16 ± 4.7 km (excluding Messinian horizon); 12.7 ± 2.8 km (including Messinian horizon). Detachment depth estimate ~7.6–8.4 km with >6 km uncertainty.
    • Forward models suggest detachment ~15 km and 10–12 km slip but cannot capture all complexities.
    • Fault-related folding: slip ~11.5–23 km for detachment angles 10°–5°.
  • Brittle–ductile transition depth ~8.5–10 km; detachment likely rooted near brittle–ductile transition and possibly at Moho level (~10–14 km) beneath ARFS.

• Plate convergence budget and boundary development:

  • Plio-Holocene Africa–Iberia convergence ~24 ± 5 km (<5.3 Ma at 4.5 ± 1 mm/yr).
  • YFS minimum 12 km (potentially 16–30 km) and ARFS ~16 ± 4.7 km imply the linked system has absorbed ≥50% of convergence, plausibly most of it, indicating a discrete, narrow plate boundary rather than diffuse deformation.

• Seismic and tsunami hazard:

  • Maximum magnitude estimates (continental scaling): Mw ~7.4 (YFS), Mw ~8.0 (ARFS); joint rupture Mw ~7.7 (strike-slip) to Mw ~8.8 (reverse).
  • Long recurrence intervals likely; these structures are underrepresented or absent in current hazard maps, implying underappreciated seismic and tsunamigenic risk.

The quantified Plio-Holocene slips on the YFS and ARFS demonstrate that deformation is concentrated along a ~300-km-long, kinematically compatible system that localizes plate convergence, contradicting the diffuse plate boundary model. Structural, stratigraphic, and crustal-scale seismic evidence show YFS as a lithospheric-scale strike-slip boundary and ARFS as a major thrust/oblique-thrust with a lower-crustal to Moho-rooted detachment. The cumulative slip budget (≥12 km on each system; ARFS ~16 ± 4.7 km; YFS likely 16–30 km when accounting for sub-seismic deformation) indicates that at least half, and potentially the majority, of Africa–Iberia convergence since ~5.3 Ma has been taken up by this narrow boundary. This localization is favored by inherited contrasts between magmatic arc and continental crustal domains. Consequently, regional seismic hazard is greater than inferred from moderate instrumental seismicity: the dimensions and depth extent of the ARFS/YFS imply capability for large to great earthquakes and tsunamis, consistent with historical accounts. Other regional faults (e.g., Carboneras, Al-Idrissi, Eastern Betics) collectively accommodate a minor share of convergence, reinforcing the central role of the YFS–ARFS in present-day tectonics.

This work provides the first quantitative, multi-method slip estimates for the offshore Yusuf and Alboran Ridge fault systems and shows they form a discrete, developing plate boundary that has absorbed ≥50% of Plio-Holocene Africa–Iberia convergence. The ARFS has accommodated ~16 ± 4.7 km of shortening along a lower-crustal to Moho-rooted detachment, while the YFS has at least ~12 km and likely 16–30 km of dextral slip associated with a major pull-apart basin. These findings overturn the prevailing diffuse boundary paradigm and imply substantial, underappreciated seismic and tsunamigenic hazard, with potential maximum events up to Mw ~8 for ARFS and even larger for combined rupture scenarios. Future work should integrate the YFS–ARFS into regional kinematic models and probabilistic seismic hazard analyses, acquire higher-resolution and 3D seismic datasets to better image deep fault geometry and sub-seismic faulting, refine slip-rate estimates, and characterize earthquake recurrence and tsunami potential.

• Deep fault traces of ARFS/YFS are not directly imaged; detachment depth estimates from Excess-Area have large uncertainties (>6 km; >45% average error in detachment depth estimation).
• Forward modeling assumes plane strain, single active fault at a time, homogeneous rheology, and cannot incorporate minor/secondary and contemporaneous faults or thick-skin complexities; syn-kinematic sediments excluded due to limited constraints.
• Excess-Area method requires restoration of eroded/tilted layers and assumes detachment folding; erosion and later deformation introduce measurement uncertainty.
• Strike-slip estimates from pull-apart geometry (Rodgers model) are first-order and depend on simplified elastic assumptions and basin completeness.
• Possible transpressional 3D deformation means 2D methods may underestimate total slip.
• Seismic resolution limits detection of small-offset faults, potentially underestimating cumulative slip; improved imaging and 3D modeling are needed.