CRITERIA FOR IDENTIFYING STRUCTURES RELATED TO TRUE CRUSTAL EXTENSIONIN OROGENS

Faults and shear zones in orogens may be related to crustal shortening or extension or both. True extensional structures should ultimately i ntersect the Earth's surface when traced out in a direction opposite t o that of hangingwall transport; the present dip of a fault in an orog en, and its relation to local layering, are unreliable guides. The obs ervation of low-pressure metamorphic rocks structurally above high-pre ssure metamorphic rocks is not sufficient to diagnose crustal extensio n. A valid metamorphic criterion is the difference in pressure-time hi stories between footwall and hangingwall: if the footwall was decompre ssing faster than the hangingwall during shear zone movement, then the shear zone was truly extending the crust. The pressure-time history c an be constrained via the temperature-time and pressure-temperature hi stories. Ideally, crustal extension inferred from structural data shou ld be confirmed using diagnostic PTt information.