Research Focus: Significance of large-displacement, low-angle normal faults

Low-angle normal faults (LANFs or detachment faults; dip <30°) are a major class of faults that became widely accepted by earth scientists only in the late twentieth century. Continental LANFs were mapped for nearly 100 yr (e.g., Ransome et al., 1910) but typically were interpreted as odd thrust faults, mega-landslide faults, or nonconformities. This changed with the recognition of LANFs in widespread association with Tertiary metamorphic tectonites (e.g., Davis and Coney, 1979; Crittenden et al., 1980), and we now know that individual LANFs in the Basin and Range province slipped 5–50 km, and several LANFs exhumed midcrustal rocks (Anderson, 1971; Armstrong, 1972; Wright and Troxel, 1973; Wernicke, 1981; Wernicke et al., 1988). LANFs are globally significant; they occur in much of the geologic record (e.g., Holm, 1996; Axen et al., 1999), on most continents, in both extensional and contractional settings (e.g., Selverstone, 1988; Burchfiel et al., 1992), and at slow-spreading mid-ocean ridges (e.g., Tucholke and Lin, 1998). LANFs continue to be central topics at the Geological Society of America Penrose Conferences (Cordilleran metamorphic core complexes, 1977; Exhumation processes, 1996; Extending a continent, 2007) and were prominent in the U.S. National Science Foundation (NSF) MARGINS Rupturing Continental Lithosphere Science Plan (National Science Foundation, 2003).

Here, I review the significance of, and debates about, continental LANFs, which remain controversial because (1) they do not conform to current fault-mechanical theory, and (2) there is little strong evidence for major (M > 6) LANF earthquakes (Jackson and White, 1989; Collettini and Sibson, 2001). Nevertheless, several LANFs originated and slipped in the brittle crust as primary, gently dipping normal faults (Wernicke et al., 1985; Wernicke, 1995; Axen, 2004). This requires reevaluation of current fault mechanical theory that predicts only steep (∼60°) normal faults in the brittle crust (Anderson, 1942) and frictional lock up of existing normal faults at ∼30° dip (Collettini and Sibson, 2001), at which point new steep faults “should” form.

Some LANFs clearly were rotated from steep to gentle dips. Some rotated while slipping, with intervening blocks, like books falling over on a shelf (Morton and Black, 1975; Proffett, 1977; Chamberlin, 1983), and others rotated passively to low dips. Spencer (1984) recognized an isostatic mechanism that tilts LANFs: as the upper-plate load is withdrawn laterally, the footwall progressively rebounds and arches into a dome, rotating the LANF to gentler, even opposite, dips. This “rolling hinge” mechanism was later invoked to rotate steep normal faults to gentle dips (Wernicke and Axen, 1988; Buck, 1988), but compelling continental examples are unknown (Axen and Bartley, 1997).

Garcés and Gee (2007; this issue of Geology) argue, using paleomagnetic inclination data, for 50°–80° of dip decrease of oceanic detachment faults along the slow-spreading Mid-Atlantic Ridge, the first LANFs likely to have been rotated significantly by the rolling hinge mechanism (lack of declination data leaves some ambiguity). This implies fundamental differences between oceanic and continental LANFs, emphasizing the need for more study and comparison of both.

Normal faults that slip at low dips inform the heated debate about the strength of natural faults (e.g., Rice, 1992). LANF slip requires either low apparent fault friction relative to laboratory values (Byerlee, 1978) or rotation of the stress field around the faults. Sparse observations do not support the latter hypothesis (Reynolds and Lister, 1987; Axen and Selverstone, 1994), suggesting that LANFs are frictionally weak due to high fluid pressure or weak fault-zone materials. Like LANFs, the San Andreas fault appears weaker than laboratory measurements imply (Lachenbruch and Sass, 1980; Zoback et al., 1987), which motivates the SAFOD (San Andreas Fault Observatory at Depth) component of the NSF EarthScope Program.

Formation of brittle normal faults with low initial dips is an unsolved mechanical problem. It requires (1) rotation of the stress field as the LANF is approached (Yin, 1989, Spencer and Chase 1989; but compare Buck, 1990), and/or (2) other special and unusual(?) mechanical, boundary, and initial conditions (e.g., Westaway, 1999), or suggests that we do not understand how to apply laboratory results on the fracture of intact rocks to natural time and space scales (e.g., Anderson, 1942).

LANFs also are significant for understanding seismicity. Although controversial, some historical earthquakes probably did involve LANF slip (Axen, 1999; Abers, 2001), and strong evidence exists for LANF paleoseismicity (e.g., John, 1987; Axen et al., 1999). If such events do occur, then their low historical frequency suggests that LANF earthquakes may be rare but large (Wernicke, 1995). Alternatively, a trigger may be needed to overcome static frictional resistance to seismogenic slip (Axen, 1999). Either way, some major cities (e.g., Salt Lake City, Utah) may be at risk from LANF earthquakes. Also, it appears that special attention to seismic records is required to identify LANF earthquakes (see Axen, 1999). Of course, the instrumental seismic record simply may be too short to have captured an unambiguous LANF event. If LANFs are not seimogenic, then they are the first known class of faults to slip only by creep, and they may hold the key to frictional stability versus instability on natural faults.

The footwalls of large-slip LANFs record the history of the midcrust, both during and before extension. For example, isostatic LANF footwall rebound requires compensation by a medium of crustal, not mantle, density (Block and Royden, 1990; Wernicke, 1990). This confirmed the concept that crustal asthenospheric layers exist, and allowed characterization of their viscosity (Kruse et al., 1991, Wdowinski and Axen, 1992) and of the local flexural rigidity of the upper crust (Spencer, 1984; Buck, 1988). LANFs also hold important clues to the evolution and dynamics of the strongest part of the continental crust, the brittle-ductile transition (Brace and Kohlstedt, 1980), which is less well understood than the weaker over- and underlying brittle or ductile parts. Many LANFs evolved from ductile shear zones into brittle faults as removal of the insulating hanging walls cooled the footwall shear zones (Davis and Coney, 1979). These record shear-zone evolution in the brittle-ductile transition (Axen et al., 2001).

To date, most major scientific results from LANFs come from exposed continental examples, due to their easy access, but breakthroughs can be anticipated from the increasing study of LANFs at mid-ocean ridges (Garcés and Gee, 2007), where crustal structure and evolution are very different (and simpler) than in continental settings. The potential is great for data from oceanic LANFs to influence our understanding of mid-ocean ridge processes and dynamics, especially combined with knowledge of spreading rates and crustal structure, and with analogs from ancient ophiolites. Comparison of oceanic and continental LANF complexes should open new doorways to understanding the dynamics of crustal deformation and evolution.

S. Bilek and J. Selverstone provided helpful reviews.