Dilational fault zone architecture in a welded ignimbrite: The importance of mechanical stratigraphy

Analysis of a population of dilational faults within a densely welded ignimbrite layer reveals fault zone geometries that vary greatly within a single fault and between faults, but does not correlate with displacement. Within an individual fault the thickness of the fault core can differ by up to an order of magnitude along dip. Similarly, joint density adjacent to faults varies along fault dip but does not increase with displacement. A correlation does exist however, between joint density and the degree of ignimbrite welding, which can vary vertically within an ignimbrite layer. Previous work has shown that welding increases ignimbrite strength: non-welded ignimbrites form deformation bands and densely welded ignimbrites form discrete fractures. We observe zones of densely welded ignimbrite with high joint density, while less-welded zones have lower joint density. In turn, high joint densities correlate with narrow fault cores and low joint densities with wide fault cores. We propose a joint based model for dilational fault initiation and growth. Faults initiate on precursory joints and grow by entraining joint bound slabs, hence the correlation between high and low joint density (thin and thick slabs) and narrow and wide fault cores respectively. Ultimately joint density and consequently fault zone architecture are controlled by subtle variations in mechanical strength within the ignimbrite layer.

► Dilational normal fault population characterized in a welded ignimbrite.
► Conceptual model of dilational fault initiation and growth.
► Subtle variation in layer mechanical strength controls fault growth and structure.