Lister and Davis (1989) is a seminal article for core complexes and detachment faults, cited well over a thousand times. The authors describe geometry and kinematics of core complexes, cropping out in the USA, which result from lithosphere extension. The paper carefully describes the main detachment fault separating lower plate from upper extending plate and discusses the role of granitoid intrusion and of the folding of the detachment fault during extension. Detachment faulting is seen as a progressive exposure of crustal rocks through denudation of progressively deeper levels. This is beautifully shown in many structures and sketches of the article as, for example, in Fig. 1 below (Axen, 2019), where 20 Ma volcanic rocks in the upper plate rest directly in low-angle fault contact on 1400 Ma crystalline rocks in the lower plate.
Fig. 1 Photographs of the Whipple detachment fault (WDF) and minidetachments after Axen (2019). Above: View perpendicular to transport direction in north-eastern Whipple Mountains. Width of view is ∼4 km. Below: Exposure of Whipple detachment fault showing structural sequence and detail view of foliated upper-plate gouge and top of footwall.
The “Whipple” detachment fault in southeastern California has clear, unvegetated exposures. This detachment dips less than 10º, with a knife-sharp contact between the upper plate and lower plate (Fig. 1). Rocks of the lower plate are commonly transformed into mylonites and cataclasites, in turn recording their lower-plate provenance and their transport up to the surface from depths of 10 to 15 km. The faulted upper plate rocks include Miocene volcanic and sedimentary rocks. Formation of these microbreccias is the natural result of the friction led by kilometers of slip as the footwall was translated toward the surface. Early ductile fabrics may be overprinted by lower-grade mineral assemblages, but they are not extensively annealed (Fig. 1). The interesting sequence of faulting in the splays of the master detachment fault reduces the size of the upper plate and allows exhumation of deep portions of the crust.
The problem remains: How are faults nucleating at very low angles mechanically feasible? In the 80s and 90s there was a considerable debate in which analogue experiments played a major role (Brun et al., 1994, Scott and Lister, 1995). Were low angle normal faults nucleating at 60º and then rotating to lower angle during progressive extension? Are simple models considering detachment faults as a representation of the brittle-ductile transition correct?
The debate increased when observations from the USA were used to discuss the mechanics of rifted margins. The application of detachment faulting in rifted margin communities is supported by low angle normal faults observed in cross-sections across rifted margins exposed in mountain belts (Manatschal, 2004). The importance of this kind of faults to accommodate lithospheric extension is indisputable, despite considerable debate still ongoing about how lithospheric stretching is achieved in different contexts (e.g. Brun et al., 2017).
Fig. 2. Block diagram depicting the Whipple detachment fault in relationship to upper plate and lower plate rocks. Note the Whipple detachment surface and the underlying ledge of microbreccias. Normal faults in the upper plate terminate in the Whipple detachment fault. The dome is a mylonitic arch of lower plate rocks, exposed as a tectonic window. (From Davis et al., 2011).
Gordon Lister commented on Reddit showing disappointment about the fact that modellers continue to ignore the basic observations presented in the article. For him, the most important factor is the superimposition of low-angle surfaces excising the bottom of tilt-blocks. He suggested reading a review article Axen (2004) and his abstract at EGU 2019 (Lister and Forster, 2019). He considers that the condition of maximum moment yield can explain how low angle normal faulting might be mechanically explicable.
Gianluca Frasca, David Fernández-Blanco, Pan Luo, and the TS Must Read team
References
Axen, G. (2004) 3. Mechanics of Low-Angle Normal Faults. Rheology and Deformation of the Lithosphere at Continental Margins, edited by Garry D. Karner, Brian Taylor, Neal W. Driscoll and David L. Kohlstedt, New York Chichester, West Sussex: Columbia University Press, pp. 46-91. https://doi.org/10.7312/karn12738-004
Axen, G. J. (2019) How a strong low-angle normal fault formed: The Whipple detachment, southeastern California. GSA Bulletin. 132 (9-10), pp.1817–1828. doi: https://doi.org/10.1130/B35386.1
Brun, J.-P., Sokoutis, D., Van Den Driessche, J. (1994) Analogue modeling of detachment fault systems and core complexes. Geology, 22 (4), pp. 319–322. doi: https://doi.org/10.1130/0091-7613(1994)022<0319:AMODFS>2.3.CO;2
Brun J.-P., Sokoutis D., Tirel C., Gueydan F., Van Den Driessche J. and Beslier M.-O., 2018. Crustal versus mantle core complexes. Tectonophysics, 746, pp. 22-45. https://doi.org/10.1016/j.tecto.2017.09.017.
Davis, G.H., Reynolds, S.J., Kluth, C.F. (2011) Structural geology of rocks and regions, John Wiley & Sons
Lister, G.S., Davis, G. (1989) The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A. Journal of Structural Geology 11, pp. 65–94. https://doi.org/10.1016/0191-8141(89)90036-9
Lister, G., Forster, M. (2019) – The operation of Tethyan detachment faults and the maximum moment failure criterion. EGU Geophysical Research Abstracts, 21, p 1-1.
Manatschal, G. (2004) New models for evolution of magma-poor rifted margins based on a review of data and concepts from West Iberia and the Alps. Int. J. Earth Sci., 93, pp. 432-466.
Scott, R.J., Lister, G.S., Brun, J.-P., Sokoutis, D., Van Den Driessche, J. 1995. Analogue modeling of detachment fault systems and core complexes: Comment and Reply. Geology, pp. 287-288.
