Frictional Heterogeneities Along Carbonate Faults

Abstract:

The understanding of fault-slip behaviour in carbonates has an important societal impact as a) a significant number of earthquakes nucleate within or propagate through these rocks, and b) half of the known petroleum reserves occur within carbonate reservoirs, which likely contain faults that experience fluid pressure fluctuations. Field studies on carbonate-bearing faults that are exhumed analogues of currently active structures of the seismogenic crust, show that fault rock types are systematically controlled by the lithology of the faulted protolith: localization associated with cataclasis, thermal decomposition and plastic deformation commonly affect fault rocks in massive limestone, whereas distributed deformation, pressure-solution and frictional sliding along phyllosilicates are observed in marly rocks. In addition, hydraulic fractures, indicating cyclic fluid pressure build-ups during the fault activity, are widespread.

Standard double direct friction experiments on fault rocks from massive limestones show high friction, velocity neutral/weakening behaviour and significant re-strengthening during hold periods, on the contrary, phyllosilicate-rich shear zones are characterized by low friction, significant velocity strengthening behavior and no healing. We are currently running friction experiments on large rock samples (20x20 cm) in order to reproduce and characterize the interaction of fault rock frictional heterogeneities observed in the field. In addition we have been performing experiments at near lithostatic fluid pressure in the double direct shear configuration within a pressure vessel to test the Rate and State friction stability under these conditions. Our combination of structural observations and mechanical data have been revealing the processes and structures that are at the base of the broad spectrum of fault slip behaviors recently documented by high-resolution geodetic and seismological data.