MacLeod et al. (2002) made a strong case that subhorizontal corrugated surfaces adjacent to the slowly spreading Mid-Atlantic Ridge near 15°45′N are exhumed fault surfaces that have been uncovered by normal faulting associated with oceanic-lithosphere divergence (see also Cann et al., 1997; Tucholke et al., 1998; Blackman et al., 1998). They identified submarine “rock outcrops in the form of elevated ridges tens of meters wide, hundreds of meters long, and <10 m in relief elongated parallel to the spreading direction” with smooth surfaces “covered with centimeter-scale striations, also parallel to spreading direction” (MacLeod et al., 2002, p. 880). Dredge and drill core samples included mylonitic fault rocks “formed primarily of talc, chlorite, and tremolite-actinolite, and minor serpentine” “with strong foliations dipping subparallel to the striated surfaces.” (p. 880). At deeper structural levels, “most serpentinized peridotites” contain “relict undeformed olivine” (p. 880–881) and other features indicating lack of high-temperature crystal-plastic deformation. Fault zone thickness was estimated to be ≤100 m.
Because they did not identify evidence of penetrative crystal-plastic deformation of nonhydrous minerals in these rocks, the authors infer that the detachment fault is a brittle, shallow structural feature unrelated to a deeper and hotter “brittle-ductile transition.” They “propose that the detachment soled out along an alteration front horizon at relatively shallow depth within the brittle lithosphere, a rheological boundary distinct from the brittle-ductile transition” and suggest that the fault was localized along “a profound rheological contrast between a serpentine-free, stronger, lower lithosphere and a weaker, slightly serpentinized upper lithosphere” (MacLeod et al., 2002, p. 882).
This concept of a strength contrast with stronger below and weaker above is inconsistent with the great continuity of footwall grooves. A weak hanging wall could not mold, shape, or cut a strong footwall into grooves as it was tectonically exhumed. However, a strong hanging wall could mold a weak footwall. A molding process whereby the cool and strong side of an irregular slip surface molds the hot and weak side as it cools and acquires a grooved form has long been proposed for extrusion of grooved lavas with remarkably continuous, straight, and parallel grooves (e.g., Nichols, 1938; Marocco, 1980; Chadwick et al., 1999). Such grooves are much smaller but otherwise resemble multibeam images of deep-ocean corrugations (e.g., images in Feininger, 1978). This molding process is used by industrial metal fabricators to shape products with a continuous linear form, and is known as “continuous casting” (e.g., Tselikov, 1984). Continuous casting at tectonic rates and scales has been proposed for corrugation genesis at mid-ocean ridges and continental metamorphic core complexes (Spencer, 1999), and for an enormous grooved surface on Venus (Spencer, 2001).
In tectonic continuous casting, irregularities in the initial shape of a normal fault (e.g., Ferrill et al., 1999) are inferred to be responsible for the grooved footwall form, with such a weak footwall and a strong hanging wall that the groove-forming protrusions on the underside of the hanging wall are not worn away. Such a strength contrast would affect mid-ocean ridge settings where footwall rocks are weak, serpentinized ultramafic rocks, and hanging-wall rocks are olivine-poor gabbro or diabase not greatly weakened by fracturing. Recent experimental evidence that 10% to 15% serpentinization of peridotite can cause strength reduction to that of pure serpentine (Escartín et al., 2001) shows that such a strength contrast could develop fairly readily where water has access to warm peridotite. Serpentinized ultramafic rocks can be so weak, in fact, that they form dike-like intrusions and subaerial extrusions (e.g., Dickinson, 1966).
MacLeod et al. (2002) recognized serpentine in their sampled footwall rocks, but did not give an indication of whether serpentine was so sparse as to be consistent with their proposal that the footwall was the strong side of the fault. Furthermore, they did not identify any samples of the supposedly weaker and strongly serpentinized hanging wall. It seems possible, given their data, that irregularities on the underside of strong, gabbroic or diabasic hanging-wall rocks deformed weak, serpentinized footwall rocks as they were exhumed from beneath a mid-ocean-ridge normal fault, leaving grooves in the footwall. This is the reverse of the strength contrast across the fault surface envisioned by MacLeod et al. (2002) and highlights the importance of footwall serpentinization in the genesis of grooved fault surfaces in magma-poor oceanic rift environments. MacLeod et al. (2002) envision a different groove-forming process, but it is not clear from their article what that process is. It is also unclear why they are so confident in the applicability of this process that they completely ignore continuous casting, a widely used industrial process that has been proposed as the causative process for groove genesis in diverse geologic settings with a variety of materials, scales, and deformation rates.