Brittle breaks, ductile distortion
The near-vertical fractures in this photo of the Miocene Monterey Formation at Jalama Beach, California, were folded following their infilling by calcite mineralization.Two pieces of evidence indicate the fractures formed while the strata were still compacting.The fractures experienced ptygmatic folding, which is a result of compression of a stiff beam or layer, like a mineralized fracture, within a medium that is undergoing shortening, like non-lithified sediment.In this case the shortening is orient vertical, in response to vertical compaction of the strata.Secondly, based on the concave-up geometry exhibited by the strata between fractures, it is apparent the strata midway between fractures has compacted significantly more than the material near the fractures.The fractures have behaved as struts, resisting compaction.The magnitude of compaction is significant – on the order of 20% based on line-length of folded fractures.
The strata in this scene exhibit two seemingly contrasting material-property behaviors.In order to develop discrete fractures the strata behaved elastically, which we usually equate with a relatively strong (cohesive) material.However, the fact that this same material shows evidence of distributed compactional strain suggests the strata were relatively plastic and thus undergoing permanent strain.By definition, elastic behavior is the opposite of plastic behavior.Here we see clear empirical evidence of elasticity followed by plasticity in these strata.This is seemingly contrary to our usual expectation, that strata become increasingly brittle during the burial process (note that these strata were not buried to great depth, where heating might induce rock to behave plastically). The evidence suggests elements of both elastic and plastic behavior, i.e. elastoplasticity.