Mantle processes in strike-slip faults
The Earth’s many tectonic plates slide past each other in a combination of vertical and horizontal directions. A strike-slip fault lacks any downward component, and the movement is purely horizontal. The EU-funded RHEOMANTLE (Evaluation of mantle rheology in exhumed strike-slip faults) project studied strike-slip boundaries. Specifically, the team examined the factors governing the cycle of earthquakes in such faults. Given that the faults probably pass completely through the crust, researchers also examined whether the upper crust or the upper mantle controls deformation. The consortium showed that mantle-crust interactions affect deformation and mechanical properties of tectonic plates, and therefore control the earthquake cycle. The team studied deep sections of the lithosphere at strike-slip fault locations. They focused on upper mantle xenoliths from beneath the San Andreas fault in the United States, and on deeper rocks from the same fault in Mexico. Xenoliths are deep rocks that were brought to the surface by volcanic eruptions. Sampled rocks revealed the different strengths of rock layers at various depths, and how the strengths varied laterally along the fault. RHEOMANTLE also studied upper mantle rocks from the Bogota Peninsula shear zone (New Caledonia) and the Mavrovouni shear zone (Greece). The former represents the boundary between two oceanic plates, while the Greek site reveals localised deformation of the oceanic lithosphere. In both locations, the team studied lateral variation in the strengths of upper mantle rocks and the processes leading to deep-zone deformation. Results from the San Andreas fault indicated that the lithospheric strength remains constantly low throughout the depth profile. Thus, deformation must not be controlled by a single lithospheric layer. Upper mantle rocks from the Bogota Peninsula were wet, and showed a lateral increase in strength and strain. The variations decrease rock resistance to deformation, promoting strain localisation. Deeper rocks from the same site were weak like San Andreas fault rocks. The team’s lithospheric feedback model explains strike-slip fault systems in terms of the interaction of the faulting upper crust and a flowing upper mantle. The work thus revealed that interaction of lithospheric layers, and not their relative strengths, may drive the earthquake cycle.
Keywords
Strike-slip faults, earthquake cycles, lithospheric layers, RHEOMANTLE, upper mantle, xenoliths
