Distinguishing between Rooted and Rootless Detachments: A Case Study from the Mormon Mountains of Southeastern Nevada

Abstract

Rooted detachment faults and detachments beneath rootless slide blocks exhibit many similar structural characteristics. However, while rooted detachments are thought to penetrate into the midcrust and to accommodate significant crustal extension, rootless detachments break to the surface downdip and are not directly involved in such extension. Distinguishing between these two mechanically different kinds of structure is central to the assessment of extension magnitude. Here we examine deformation along the Mormon Peak detachment, a feature that has been cited as an example of both a rooted and a rootless structure. Located in the Mormon Mountains of southeastern Nevada, this detachment has been interpreted as one of three low-angle normal faults of regional scale that together are thought to have accommodated more than 50 km of Basin and Range extension. For the most part, however, the Mormon Peak detachment is expressed as a series of isolated exposures where Paleozoic rocks are in brittle fault contact with nonmylonitized underlying rocks. Individual blocks contain high-angle normal faults that terminate downward at their respective detachment surfaces, yielding a geometry common to both modes of emplacement. In order to test between these competing interpretations, we studied deformational characteristics close to the detachment surface, reasoning that a seismogenic fault ought to differ fundamentally from a surficial slide block, particularly if the slide block was emplaced in a single event rather than by protracted or episodic creep. An examination of the contact mapped as the Mormon Peak detachment reveals that the character of deformation is indistinguishable from that of known gravity-driven slide blocks and is fundamentally different from that associated with seismically cycled faults. Moreover, the orientation of kinematic indicators observed at detachment surfaces is consistently close to the downdip direction, which in many places diverges strongly from the expected direction of movement in the rooted detachment model. We conclude that outcrops of the inferred upper plate of the Mormon Peak detachment represent an assemblage of individual rootless gravity-driven slide blocks and not the erosional remnants of a formerly contiguous extensional allochthon. If similar misidentifications have been made elsewhere in the Basin and Range Province, total Cenozoic extension may have been significantly overestimated. Implications for the interpretation of extensional geology in general are far-reaching.

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

“…Available evidence is consistent with the rapid surficial sliding of blocks downhill, away from the crest of the Mormon Mountains, as it existed in Miocene time (see Appendix 2) (Anders et al 2006;Walker et al 2007). Polymictic conglomerate, similar to that observed at the MPD and recognized as basal layers and dykes within a wide range .…”

“…8; 2558+108) , and at many locations correspond approximately with the modern downdip direction of the detachment surface (Anders et al 2006;Walker et al 2007;cf. Axen 2004, p. 60).…”

“…Following earlier work by , a challenge to the generally accepted rooted interpretation of these detachment faults has emerged recently from studies of the character and kinematics of deformation associated with the MPD (Appendix 2) (Anders et al 2006;Walker et al 2007). As recognized long ago by , the MPD is in many places characterized by a 0.1-1m-thick layer of polymictic conglomerate (Fig.…”

“…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 detachment era (1977–1982) and its role in revolutionizing continental tectonics

“…Although most geologists today, including the author, remain comfortable drawing reconstructions that include low-angle normal faults with large displacements, over the last quarter century a 'loyal opposition' has persistently challenged evidence in support of active slip on low-angle planes (e.g. Jackson & White 1989;Anders & Christie-Blick 1994;Anders et al 2006;Wong & Gans 2008). These authors have properly stressed that an inventory of earthquake focal mechanisms on a par with that of low-angle thrust faults, strike-slip faults, and moderate-to high-angle normal faults has yet to materialize, and that slip on low-angle normal faults is in any event extremely difficult mechanically (e.g.…”