Seismicity, structure and strength of the continental lithosphere

Geological–geophysical transects of active and ancient plate margins as well as structural studies of exhumed fault rocks confirm the general validity of laboratory-based strength–depth profiles for the long-term (106–107 a) rheology of the continental lithosphere. The deep structure of orogens and rifted margins requires that a relatively strong, olivine-rich upper mantle underlies a weaker granitic to dioritic crust. The continental crust itself comprises one or more viscously deforming layers that are overlain by a stronger brittle layer. This view refutes recent speculation that because the thickness of the seismogenic zone within the crust varies with the effective elastic thickness of the lithosphere, the crust must be stronger than the upper mantle. We argue that seismicity is an ambiguous indicator of strength and propose that earthquakes are more reasonably interpreted as a manifestation of transient mechanical instability within shear zones. Shear zones are often long-lived zones of weakness in which viscous mylonitic creep is punctuated by ephemeral high-stress events involving fracture, frictional melting, and a temporary, local loss of cohesion. Seismicity may therefore be used to locate current zones of episodic decoupling between and beneath crustal blocks in active interplate fault systems.