Role of melt supply in oceanic detachment faulting and formation of megamullions

Abstract

Normal faults are ubiquitous on mid-ocean ridges and are expected to develop increasing offset with reduced spreading rate as the proportion of tectonic extension increases. Numerous long-lived detachment faults that form megamullions with large-scale corrugations have been identifi ed on magma-poor mid-ocean ridges, but recent studies suggest, counterintuitively, that they may be associated with elevated magmatism. We present numerical models and geological data to show that these detachments occur when ~30%-50% of total extension is accommodated by magmatic accretion and that there is signifi cant magmatic accretion in the fault footwalls. Under these low-melt conditions, magmatism may focus unevenly along the spreading axis to create an irregular brittle-plastic transition where detachments root, thus explaining the origin of the enigmatic corrugations. Morphological and compositional characteristics of the oceanic lithosphere suggested by this study provide important new constraints to assess the distribution of magmatic versus tectonic extension along mid-ocean ridges.Keywords: mid-ocean ridge, detachment fault, megamullion, oceanic core complex, oceanic magmatism, numerical modeling. INTRODUCTIONThere is ongoing debate about the role of magmatism during normal faulting at mid-ocean ridges (MORs), particularly for very long-lived (~1-2 m.y.) normal faults, which we here term detachment faults. Plate separation at MORs is accomplished by both magmatic intrusion and slip on normal faults. Intrusion of dikes can occur when the difference between maximum and minimum stresses is lower than that required for fault slip, and intrusion further reduces the stress difference (Rubin, 1995). This implies that major normal faults should form only when there is little or no magma supply. Some numerical models also suggest that detachments can form during amagmatic extension if the lithosphere is thin and cohesion decreases with fault displacement (Lavier et al., 2000). These ideas are consistent with the observation that most detachment faults occur at the ends of spreading segments where melt supply is usually low compared to segment centers (e.g., Lin et al., 1990;Tucholke et al., 1998). Furthermore, large tracts of unusually smooth seafl oor, interpreted as successive fault surfaces, have been observed on the ultraslow-spreading Southwest Indian Ridge, where magmatism is extremely limited (Cannat et al., 2006).In contrast, some recent observations suggest that magmatism may be important during the evolution of detachment faults. Because of their long-lived slip, these faults can exhume tens of kilometers of lower ocean crust and upper mantle. To maintain isostatic equilibrium, the footwalls typically roll over (Buck, 1988) and form megamullions characterized by domed shapes and by large (to hundreds of meters), enigmatic, fault-surface corrugations as well as smaller mullions and striations that parallel fault-slip direction (Tucholke et al., 1998) (Fig. 1A). Three megamullions have been drilled, two on the Mi...

Oceanic Core Complex Development in Modern and Ancient Oceanic Lithosphere: Gabbro‐Localized versus Peridotite‐Localized Detachment Models

“…However, recent deep drilling of two core complexes thought to have developed during reduced magma supply, the Atlantis Massif and the 15Њ45ЈN, has revealed long sections (∼1400 and ∼200 m, respectively) of gabbroic rocks in the footwalls of both massifs Kelemen et al 2007). The recovery of gabbroic rocks in these drill cores has prompted revision of the amagmatic models of OCC development (Ildefonse et al 2007), leading to new models that suggest that core complexes form when 30%-50% of the total extension is accommodated by magmatic accretion, requiring significant magma intrusion into the footwalls of OCCs during detachment faulting (Tucholke et al 2008). In light of these new models, we present two end-member models of core complex development, neither of which require amagmatic extension; instead, they differ in (1) the relative proportions of gabbro and peridotite rocks in the footwall and (2) the nature of fabric development within their respective detachment-fault shear zones.…”

“…At spreading rates of !55 mm/yr, magmatism and volcanism are spatially and temporally variable along ridge segments because of the ephemeral nature of the axial magma chamber. This fluctuating magma supply may lead to increased tectonic partitioning of sea-floor spreading and significant thinning and denudation of oceanic crust that promote the formation of core complexes (Sinton and Detrick 1992;Tucholke et al 1994Tucholke et al , 2008Blackman et al 1998;Buck et al 2005). The interplay of tectonic deformation and magmatism at these mid-ocean ridges results in lithospheric architecture that is distinct from the layered, Penrose-type ophiolite sequence (cf.…”

The Trans-Atlantic Geotraverse hydrothermal field: A hydrothermal system on an active detachment fault

“…We now know that detachment faulting, which exposes lower crust and upper mantle rocks, accounts for crustal accretion/extension along as much as 50% of the Mid-Atlantic Ridge (Smith et al, 2006;Escartin et al, 2008;Tucholke et al, 2008;MacLeod et al, 2009). In addition, detachment faults host seven of the eleven confirmed active hydrothermal sites on the northern Mid-Atlantic Ridge, suggesting that most of the hydrothermal flux in the northern Atlantic is generated in terrain that was thought to form in regions with low magmatic activity, a prerequisite for detachment formation (Tucholke and Lin, 1994;Tucholke et al, 1998).…”

A short electromagnetic profile across the Kane Oceanic Core Complex

“…[3] Competing models of OCC formation and evolution have been proposed, including the creation of low angle detachment faults [e.g., Cannat et al, 1997;Escartín et al, 2003;Buck et al, 2005;Tucholke et al, 2008]. In general, OCCs are thought to form when the reduction in magma supply crosses a threshold that triggers detachment formation.…”

“…[2] Oceanic core complexes (OCCs), are widespread features along the mid-Atlantic Ridge and other slow and ultra-slow spreading ridges [e.g., Escartín et al, 2008;Tucholke et al, 2008], where they are involved in the accretion of large areas of the seafloor [Smith et al, 2006;Cannat et al, 2006]. The low-angle detachment faults exposed at the seafloor unroof and expose sections of lower crust and mantle, accommodating elevated strains over long periods of time, sometimes in excess of 1 Myrs, possibly promoting serpentinization within these structures.…”