Active Extension Above the Peruvian Flat Slab: Segmentation, Slip-Rate Variability, and Paleoseismicity of the ∼300-km-Long Cordillera Blanca Fault Zone

Benavente C.; Rosell L.; Palomino A.; Wimpenny S.; Rosell J.; García B. · Tectonics (2026) · DOI: 10.1029/2026tc009634

Resumen

The ∼300-km-long Cordillera Blanca Fault Zone (CBFZ) is the longest and fastest-slipping active normal fault system in the central Andes, yet its paleoseismic record has remained constrained to a few sites since Schwartz (1988, https://doi.org/10.1029/jb093ib05p04712). We combine geomorphic mapping, 101 fault-scarp profiles, and six paleoseismic trenches spanning ∼230 km across four structural segments. Our mapping extends the active fault trace ∼90 km northward to Huamachuco, expanding the documented domain of upper-plate extension within the Peruvian flat-slab region. The 17 surface-rupturing earthquakes identified since ∼12.6 ka produced per-event vertical displacements of 0.5–2.3 m, consistent with Mw 6.5–7.0 events. Bayesian modeling reveals tentative evidence for a temporal cluster at ∼3.5–3.1 ka across sites spanning ∼220 km, although adjacent trenches indicate the paleoseismic record is incomplete. Throw rates vary by more than an order of magnitude among segments—from 0.1–0.3 mm/yr at the northern tip to 1.9–3.7 mm/yr in the center—and co-vary with footwall relief and the geometry and thickness of the inherited mylonitic shear zone of the Cordillera Blanca Batholith, suggesting that inherited crustal structure may modulate fault kinematics. Low slip rates and immature scarp morphology in the northernmost ∼90 km may reflect northward propagation of the fault tip into the flat-slab domain. The CBFZ can sustain single-segment ruptures of Mw 6.5–7.0 and multi-segment cascades of Mw 7.1–7.2 based on our preferred scaling relation (up to ∼Mw 7.9 across alternative scaling relations), providing critical source parameters for seismic hazard assessment of ∼500,000 people living within 20 km of the fault.

Instituciones: Universidad Nacional de San Antonio Abad del Cusco