Growth of Fault-Related Folds and Blind Thrusts

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High resolution seismic profile of compressive growth strata in the forelimb of a fault-related fold on the Island of Honshu, Japan. Image courtesy - Y. Sugiyama

Fault-related folds typically form in response to slip on seismogenic thrusts. These structures are thus important for assessment of seismic hazards where urban centers lie above active thrusts. Fault-related folds are also a key target in the exploration of oil and gas in thrust belts. Recent studies of active fault-related folds have developed much new understanding of how these structures grow, in particular during large earthquakes. Assumptions for deformation mechanisms have been confirmed by recent coseismic folding events, while other processes such as bedding parallel shear has been assessed in other well-constrained examples. More importantly, quantitative studies continue to define how mechanical stratigraphy governs growth of fault-related and other types of folds.

Fault-related folds hold the key to understanding the three dimensional growth of otherwise inaccessible blind thrust faults. Resolving how blind thrusts propagate upward and laterally thus depends on our ability to restore fault-related fold, typically based on analysis of synorogenic sediments. My research interests in active folding center on using these interesting structures as a proxy for the sllip history and growth of active faults, both from a seismogenic perspective and from the viewpoint of a structural geologist. Some of the most interesting opportunities lie in using very well dated sequences of synorogenic strata to characterize faulting behavior over periods of hundreds to tens of thousands of years. Of interest is whether rates of shortening in a particular thrust belt are expressed by constant rates of slip on long-lived thrust faults, or whether shorter term oscillations in slip can be extracted from growth strata. In addition, I hope to explore whether thrusts that propagate upward from the base of the seismogenic crust grow in ways that can be related to changes in rock strength and confining pressure, and sedimentation in overlying basins. An ideal opportunity to test models of fault growth lies in Japan, where very abundant volcanic tephra, and fine-grained shallow-water depositional environments combine to create extraordinarily well dated sequences of synorogenic strata. In addition, very high quality seismic data such as the example shown above, provides a means of restoring long-lived fault propagation folds that form above blind thrusts. Using new tools for unraveling bed-by-bed, or reflector-by-reflector strain will form a key part of this analysis, and can also be applied to trench scale studies where fault-propagation folds grow incrementally during large thrust-type earthquakes.