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

New earthquake focal mechanism and centroid depth estimates show that the deformation style in the forelands of the Andes is spatially correlated with rift systems that stretched the South American lithosphere in the Mesozoic. Where the rifts trend sub‐parallel to the Andean range front, normal faults inherited from the rifts are being reactivated as reverse faults, causing the 30–45 km thick seismogenic layer to break up. Where the rift systems are absent from beneath the range front, the seismogenic layer is bending and being thrust beneath the Andes like a rigid plate. Force‐balance calculations show that the faults inerhited from former rift zones have an effective coefficient of static friction μ ′ < 0.2. In order for these frictionally weak faults to remain seismogenic in the lower crust, their wall rocks are likely to be formed of dry granulite. Xenolith data support this view, and suggest that parts of the lower crust are now mostly metastable, having experienced temperatures at least 75–250° C hotter than present. The conditions in the lower crust make it unlikely that highly pressurized free water, or networks of intrinsically weak phyllosilicate minerals, are the cause of their low effective friction, as, at such high temperatures, both mechanisms would cause the faults to deform through viscous creep and not frictional slip. Therefore pre‐existing faults in the Andean forelands have remained weak and seismogenic after reactivation, and have influenced the style of mountain building in South America. However, the controls on their mechanical properties in the lower crust remain unclear.