Mode-of-Failure Transitions in High Porosity Sedimentary and Ignimbrite Deposits, and Implications for Fault-Zone Structure and Architecture
Abstract
Recent studies have demonstrated that fault-zone deformation processes, and thus structures and fault-zone architecture, vary with the petrophysical properties of the protolith. Factors controlling whether or not open fractures form within a given fault zone are of particular importance to understanding fault-zone impacts on fluid flow. We discuss three examples of mode-of-failure transitions from strike-slip and normal faults in sedimentary and volcanic protoliths. Each example illustrates a mechanism by which porosity was reduced over time, resulting in changes in the mechanical behavior of the faulted material. These temporal variations are recorded by overprinting relationships within fault-zone architectural elements. The Sand Hill normal growth fault cuts poorly lithified sediments of the Rio Grande rift, NM. Architectural elements include a fault core bound by tabular mixed zones, which are in turn bracketed by deformation-band damage zones. The mixed zones have been described in poorly lithified sediments only; they consist of material derived from adjacent beds during slip, which has been disaggregated and tectonically mixed. Structures developed during mixing and particulate flow include foliations defined by compositional bands and aligned grains and attenuated and disarticulated beds. These structures are cut by deformation bands in the footwall mixed zone. We interpret these relationships as recording a transition from bulk particulate flow (in normally consolidated sediment) to localized shear within deformation bands (in overconsolidated sediment) as the footwall of the fault was syntectonically exhumed. Mixed zones are also present in the San Gregorio fault zone, a branch of the San Andreas fault system. The San Gregorio fault was active through sedimentation and lithification of rocks currently exposed along the CA coast. Attenuated and boudinaged beds on the SW side of the fault are cut by fractures and veins. We interpret these features as recording overprinting of a mixed zone (unlithified sediment) by a fracture-rich damage zone (lithified sediment). Faults in non-welded units of the Bandelier tuff, NM also exhibit evidence of temporal changes in deformation processes. In one case, deformation bands are cut by a through-going slip surface. The deformation bands significantly reduced porosity in the tuff, which we believe allowed a mode-of-failure transition, leading to the development of a discrete slip surface.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2001
- Bibcode:
- 2001AGUFM.S41A0581G
- Keywords:
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- 5100 PHYSICAL PROPERTIES OF ROCKS;
- 5114 Permeability and porosity;
- 8010 Fractures and faults;
- 8045 Role of fluids