Episodic development of fault-zone architecture investigated using small displacement strike-slip faults.

摘要

Fault-zone architecture records valuable information about fault mechanics and imposes a structural influence on petrophysical and geophysical properties within fault zones. In this study, fault perpendicular and downdip variations in fault-related deformation are evaluated using cross-sectional exposures of small displacement strike-slip faults. The selected faults originated as dilational fractures in laumontite-cemented Pliocene sandstones and represent subsidiary deformation associated with the San Jacinto fault zone in the western Salton Trough region of southern California. The geometry of damage-zone deformation within these small displacement fault zones resembles a structural mesh, with cemented and open extensional fractures that are polymodal and mutually abutting. Integrated field and microstructural data provide evidence of episodic dilational fracturing, fluid flow, and cataclasis associated with phases of seismic activity. Spatial variability in petrophysical behavior is observed at multiple scales within the fault zones and results from the heterogeneity of host rocks, the irregular distribution of intergranular cement, and the variable distribution and localized effects of deformation features associated with alternating cycles of dilation, cementation, and cataclasis.;The paleo-damage zone model is introduced to account for observed fault-parallel damage-zone domains and provides a framework to evaluate contrasting models for the chronological and spatial pattern of damage-zone deformation. Step-wise decreases in fracture density with increasing perpendicular distance from the fault cores define relatively sharp domain boundaries that indicate the outer extent of deformation associated with unique slip events. The number of slip events on each fault exceeds the number of domains, and it is likely that the domain architecture was established early in the slip histories and preserved during subsequent events. Two of the fault zones exhibit downdip segmentation at the outcrop scale: vertically restricted, steeply dipping fault segments step over along mechanically weak stratigraphic units that accommodate bedding-parallel slip. Similar stepover geometries are observed within relocated seismicity data from the southern San Jacinto fault zone; this correspondence suggests that downdip segmentation may be common within strike-slip fault zones over a wide range of scales.