Folds with axial traces parallel to the extension direction are a common
feature in continental detachment systems and metamorphic core
complexes. Yet, how they form has been puzzling for many decades. Here,
we show that the key to solving the conundrum lies in revising the
long-held single-scale view toward natural deformation and application
of kinematic models. We demonstrate that extensional folding can result
naturally from the partitioned stress field in competent layers in
plate-scale extension and transtension deformations. Competent layers
that develop extension folds should be regarded as rheological
inclusions in the lithosphere rather than infinitely extending plates
clamped at system boundaries and subjected to system boundary
conditions.