Emplacement of a fold-nappe, Betic orogen, southern Spain

Abstract

The Aguilon nappe, Sierra Alhamilla, southeast Spain, is a north-closing recumbent isoclinal fold with a greatly thinned lower limb. This large-scale geometry can be demonstrated using small-scale structures. The fold nappe is detached along the lower limb and lies on a carpet of mylonitic schist, mylonite, and cataclasite. Formation of the nappe must have involved northward relative transport—a point of current controversy in the Betic Zone. During emplacement, the nappe underwent extensive ductile deformation, accomplished partly by pressure solution of silica. In the highly deformed footwall rocks, however, quartz was deformed by crystal-plastic processes. The change in deformational mechanism suggests a downward increase in deviatoric stress, which is compatible with a gravitational mechanism of nappe emplacement. The core of the low-grade nappe is occupied by medium-grade schist. The abrupt change in grade suggests that the schist contact is an extensional fault that excised part of the original metamorphic zonation and was subsequently refolded in the nappe. Such relations are common in the Betic Zone and suggest extension by listric normal faulting, a characteristic feature of nappes emplaced by gravity spreading.