Introduction to special section: Active Fault‐Related Folding: Structural Evolution, Geomorphologic Expression, Paleoseismology, and Seismic Hazards

[1] Folds are one of the obvious and common manifestations of deformation of the continental crust, and attempts to understand the origins of these structures extend back to the very beginnings of geology as a discipline within the Earth Sciences. In particular, the relationship between faults and folds, and the manner in which fault-related folds grow and evolve through time, has been the subject of intense interest within the structural geology and tectonics communities for more than a century. Indeed, it did not take long after geologists understood the basic principles of stratigraphy and geological mapping [Lyell, 1830] to realize that subsurface structures could be delineated with some accuracy from surface observations, through the use of simple geometric rules [e.g., Rogers, 1856]. The fact that folding is associated with thrust faulting and mountain building was also recognized in the early days of structural geology, together with the necessity for large tangential displacements at the Earth’s surface across fold-thrust belts [Rogers, 1856; Moesch, 1867; Heim, 1878; Muller, 1878; Willis, 1891; Chamberlin, 1910; Argand, 1924; Rich, 1934]. Moreover, geologists quickly realized how to relate quantitatively folding and horizontal shortening by assuming conservation of mass [Chamberlin, 1910], and soon began investigating the mechanics of folding and the relationships between fold growth and detachment faulting and thrusting through the use of analogue experiments [e.g., Willis, 1891] (Figure 1). The necessity of low friction along the basal detachments of fold-thrust systems was also noticed early on [Hubbert and Rubey, 1959]. [2] Over the past several decades, the ever increasing availability of observations from natural exposures and geophysical exploration (much of the latter conducted by the petroleum industry) has led to tremendous progress in our understanding of the kinematics and mechanics of faultrelated folding, and the relationship between fault slip and fold growth, at scales ranging from individual structures to entire orogens [e.g., Price, 1981; Boyer and Elliott, 1982; Suppe, 1983; Davis et al., 1983; Dahlstrom, 1990; Suppe and Medwedeff, 1990; Erslev, 1991; Allmendinger, 1998; Allmendinger and Shaw, 2000]. Collectively, these advances have facilitated modeling and quantitative analysis of the origin and evolution of fault-related folds and have fostered an understanding of fold-thrust belts as integrated mechanical systems. That understanding how fold-andthrust systems evolve is not only of academic and economic significance has been amply demonstrated by the recent occurrence of a number of highly destructive thrust earthquakes (e.g., 1980 Ms 7.3 El Asnam, Mw 6.5 Coalinga, 1994 Mw 6.7 Northridge, 1999 Mw 7.6 Chi-Chi, and 2005 Mw 7.6 Kashmir) [King and Vita-Finzi, 1981; Yielding et al., 1981; Philip and Meghraoui, 1983; Stein and King, 1984; Stein and Ekstrom, 1992; Scientists of the USGS and SCEC, 1994; Yu et al., 2001; Avouac et al., 2006]. These events have led to the growing recognition of the hazards posed by such structures to numerous urban centers around the world and highlight the need to understand the relationship between seismic slip on thrust faults and the resulting fold growth. These issues have also been the focus of much recent research by structural geologists, particularly those interested in understanding the mechanisms and rates of fold growth at timescales shorter than those provided by most exhumed examples of fold-thrust belts. [3] Motivated by all of these issues, there has been a dramatic increase in recent research focused on active fold growth and associated fault slip. Many of these recent studies take advantage of technical advances in a number of disciplines, including (1) development of new methods to unravel the history of folding from analysis of growth strata (i.e., strata deposited above and adjacent to growing folds) [e.g., Suppe et al., 1992, 1997; Shaw and Suppe, 1994, 1996; Hardy et al., 1995; Storti and Poblet, 1997; Novoa et al., 2000; Shaw et al., 2005], (2) recognition that the kinematics of active folding and thrusting is recorded by alluvial, fluvial, and marine geomorphologic markers [e.g., Rockwell et al., 1984, 1988; Stein and King, 1984; Atwater et al., 1990; DeCelles et al., 1991; Dolan and Sieh, 1992; Avouac et al., 1993; Bullard and Lettis, 1993; Molnar et al., 1994; Burbank et al., 1996; Jackson et al., 1996; Benedetti et al., 2000; Van der Woerd et al., 2001; Ishiyama et al., 2004; Bennett et al., 2005], (3) advances in our general understanding of how tectonic uplift and climatic factors influence geomorphologic processes such as alluvial deposition and river entrenchment [e.g., Weldon, 1986; Bull, 1991; Schumm et al., 2000; Burbank and Anderson, 2001; Poisson and Avouac, 2004], and development of new JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B03S01, doi:10.1029/2007JB004952, 2007 Click Here for Full Article