The nature of the physical processes responsible for the formation of conti
nental and oceanic metamorphic core complexes is widely debated. The contro
versy focuses primarily on whether the low-angle normal faults observed in
these environments formed and slipped at low angles or were rotated from an
original high-angle orientation after large offsets. We describe a self-co
nsistent numerical model for the extension of a brittle layer that can spon
taneously produce normal-fault structures. In our formulation, a fault or f
aults form because strength is locally reduced with increasing strain. If t
he reduction in fault strength is <similar to 10% of the total strength of
the layer, then faults lock after an offset smaller than the layer thicknes
s and new faults form. Larger strength reduction leads to single faults tha
t continue to slip no matter how large the fault offset. If the strength re
duction occurs by the loss of cohesion, then we see the unlimited offset fa
ults for layers <11-22 km thick for reasonable values of cohesion. The key
result of this study is that structures very similar to those observed in b
oth oceanic and continental core complexes are produced by rotation of the
inactive part of the model fault after very large offset.