Observations and dynamical implications of active normal faulting in South Peru

Abstract

SUMMARY Orogenic plateaus can exist in a delicate balance in which the buoyancy forces due to gravity acting on the high topography and thick crust of the plateau interior are balanced by the compressional forces acting across their forelands. Any shortening or extension within a plateau can indicate a perturbation to this force balance. In this study, we present new observations of the kinematics, morphology and slip rates of active normal faults in the South Peruvian Altiplano obtained from field studies, high-resolution DEMs, Quaternary dating and remote sensing. We then investigate the implications of this faulting for the forces acting on the Andes. We find that the mountains are extending ∼NNE–SSW to ∼NE–SW along a normal fault system that cuts obliquely across the Altiplano plateau, which in many places reactivates Miocene-age reverse faults. Radiocarbon dating of offset late Quaternary moraines and alluvial fan surfaces indicates horizontal extension rates across the fault system of between 1 and 4 mm yr–1—equivalent to an extensional strain rate in the range of 0.5–2 × 10−8 1 yr–1 averaged across the plateau. We suggest the rate and pattern of extension implies there has been a change in the forces exerted between the foreland and the Andes mountains. A reduction in the average shear stresses on the sub-Andean foreland detachment of ≲4 MPa (20–25 per cent of the total force) can account for the rate of extension. These results show that, within a mountain belt, the pattern of faulting is sensitive to small spatial and temporal variations in the strength of faults along their margins.

Weak, Seismogenic Faults Inherited From Mesozoic Rifts Control Mountain Building in the Andean Foreland

“…It is likely that many parts of the Andes are close to the state of F b ≈ F f , as the highest portions of the mountain range have relatively flat, plateau‐like topography (Lamb, 2006). In addition, in the most rapidly deforming areas of the high Andes, such as the Cordillera Blanca in central Peru and in the Altiplano of southern Peru, seismicity rates and fault slip rates imply that deviations from F b = F f are ≲0.5–0.7 × 10 12 N per meter along‐strike, which is ≲10%–25% of F b (Wimpenny et al., 2020). Therefore, to place a bound on the forces acting through the forelands F f , I calculated the buoyancy forces F b in eight different regions of the Andes using the method described in Copley and Woodcock (2016), with the range of parameters given in Table 2.…”

Differential Exhumation of the Eastern Cordillera in the Central Andes: Evidence for South‐Verging Backthrusting (Abancay Deflection, Peru)

“…Also, this preliminary study can neither validate nor invalidate potential tectonic-driven exhumation. The observed seismicity for the Apurimac fault system area could be linked and/or connected with either Subandean flat-ramp thrust systems or undocumented active internal backthrusts, or even normal faulting as currently occurring in the Altiplano (Wimpenny et al, 2020). Potential tectonic drivers responsible for building the Abancay Deflection and particularly the Eastern Cordillera area remain unknown.…”

Characterizing the seismic response and basin structure of Cusco (Peru). Implications for the seismic hazard assessment of a World Heritage Site

“…Besides the existence of fresh scarps and offset moraines that constitute good geomorphological markers (Benavente Escobar et al, 2013;Cabrera et al, 1991;Rosell et al, 2023) and allow us to estimate a current rate of deformation of the order of 1 to 4 mm/year (Wimpenny et al, 2020), palaeoseismic trenches revealed events of moment magnitude (Mw) greater than 6 over the last 10,000 years (Cabrera and Sébrier, 1998;Palomino Tacuri et al, 2021;Rosell et al, 2023). Finally, the installation of a temporary seismological network to the north of Cusco recorded localized seismicity along the Qoricocha and Tambomachay fault planes (Fig.…”