Low‐angle normal faults and seismicity: A review

Abstract

Abstract. Although large, low-angle normal faults in the continental crust are widely recognized, doubts persist that they either initiate or slip at shallow dips (<30ø), because (1) global compilations of normal fault focal mechanisms show only a small fraction of events with either nodal plane dipping less than 30 ø and (2) Andersonian fault mechanics predict that normal faults dipping less than 30 ø cannot slip. Geological reconstructions, thermochronology, paleomagnetic studies, and seismic reflection profiles, mainly published in the last 5 years, reinforce the view that active low-angle normal faulting in the brittle crust is widespread, underscoring the paradox of the seismicity data.

Observations from the Basin and Range Province (western United States) pertinent to the interpretation of regional detachment faults

“…1), and at known slide blocks elsewhere (Anders et al 2000(Anders et al , 2006Walker et al 2007), has dealt mainly with the characteristics of fault zones and with the discrimination of crustally rooted structures from features that are surficial (or rootless). Data in hand are not consistent with the long accepted interpretation of the Mormon Peak and associated detachments as rooted faults (Wernicke 1981(Wernicke , 1995Wernicke et al 1985Wernicke et al , 1989Axen et al 1990;Axen 1993Axen , 2004, and suggest instead that these structures relate to block-sliding with crustal extension accommodated entirely by high-angle normal faults (Anders et al 2006;Walker et al 2007; see also Cook 1960;Hintze 1986;Carpenter et al 1989;andAxen 2004 for a markedly different opinion).…”

“…The low westward dip (118) of the hypothesized detachment, its lateral continuity over 7000 km 2 , the purported offset of Mesozoic structures by as much as 47 km, the downward termination of high-angle normal faults within the Sevier Desert basin (SDB) and the involvement of sediments as young as Holocene provide seemingly unassailable evidence for large normal offset on a fault that could never have been appreciably more steeply inclined than it is today, and might still be active (Wernicke 1995;Niemi et al 2004).…”

“…The Mormon Peak, Tule Springs and Castle Cliff detachments (MPD, TSD and CCD, respectively) of SE Nevada and adjacent Utah have been interpreted on the basis of geological mapping to be crustally rooted and of regional extent, and to accommodate 54 + 10 km of Miocene extension (Figs 5 and 6; Appendix 2) (Wernicke 1981(Wernicke , 1995Wernicke et al 1985Wernicke et al , 1989Axen et al 1990;Axen 1993Axen , 2004. Research on these detachments, beginning in the late 1970s, was instrumental in arguing for the importance of low-angle normal faults and for high extensional strain in regions not associated with metamorphic core complexes.…”

“…Axen , 1999cf. Axen , 2004Scott & Lister 1992;Axen & Selverstone 1994;Wernicke 1995;Rietbrock et al 1996;Abers et al 1997;Westaway 1999;Sorel 2000;Collettini & Barchi 2002;Hayman et al 2003).…”

Low‐angle normal fault earthquakes and triggering

“…A paradox has persisted since the recognition that low-angle normal faults (LANFs) are important structures in extended regions [e.g., Wernicke, 1981]: large earthquakes on LANFs are rare in the seismological record ("low-angle" is generally defined as <30 ø, see reviews by Jackson [1987] and Wernicke [1995]). Some LANFs may creep aseismically, but glassy veins of frictional melt (pseudotachylyte) along others [e.g., John, 1987] record paleoseismicity.…”