Central role of detachment faults in accretion of slow-spreading oceanic lithosphere

Abstract

Enhanced seismicity is probably generated along detachment faults accommodating a sizeable proportion of the total plate separation. In contrast, symmetrical segments have lower levels of seismicity, which concentrates primarily at their ends. Basalts erupted along asymmetrical segments have compositions that are consistent with crystallization at higher pressures than basalts from symmetrical segments, and with lower extents of partial melting of the mantle. The large fields of detachment surfaces recently identified in oceanic crust formed along the slow spreading MAR and ultra-slow spreading South-West Indian Ridge (SWIR) 3,6 demonstrate the involvement of these structures in the accretion of a larger portion of the oceanic lithosphere than previously inferred from seafloor corrugated planes alone 7 . The resulting seafloor morphology and lithospheric structure on the flanks of the ridge axis are strongly asymmetrical 3 and differ from the more regular and roughly symmetrical axis-parallel abyssal hill fabric believed to characterize 'normal' slow-spreading seafloor. The abyssal hill morphology is caused by ridge-parallel, highangle faulting of volcanic seafloor 8 (Figures 1a-c). In contrast, detachment-related terrain is caused by long-lived steep, normal faults initiated beneath the rift valley floor that rotate to low angles as their footwalls are exposed 7,8 . Distinctive narrow ridges with steep outward-facing slopes that are often curved in plan view develop near exposed detachments at the seafloor, and bound deep swales 7 (Figures 1d-e), producing blocky and chaotic terrain 7,9 . The asymmetric nature of accretion in the presence of detachments is also observed in the overall lithospheric structure, composition and geophysical character wherever data are available 3,4,10 . The MAR lacks the broad ridges only found along the melt-poor SWIR, likely a manifestation of detachment faulting that is different from striated fault planes and associated structure 3 . There is an excellent correlation between mode of accretion and seismicity at the ridge axis. This section of the MAR was hydroacoustically monitored between January 1999 and September 2003 11 . The hydroacoustic catalogue is complementary to the >30 year teleseismic catalogue, as it records smaller magnitude events (magnitude of completeness of 3 and 5, respectively 12 ), over a shorter period of time (<5 vs. >30 years). Both seismic catalogues show that detachment-dominated, asymmetrical ridge sections host ~75% more hydroacoustic events and ~65% more teleseismic events than 4 symmetrical segments (Figure 2b and c). The concentration of seismicity at segments shown to have active detachment faults (Figures 1d-e), such as the Logachev massif south of the Fifteen-Twenty Fracture zone and the TAG detachment fault near 26°N 6,7,13 , is thus a general pattern. Active detachments also control the zones of sustained seismicity, which lack shock-aftershock sequences that were previously identified along the northern MAR 14 . Differences between the hyd...

Diverse styles of submarine venting on the ultraslow spreading Mid-Cayman Rise

“…Type 3 is the most recently identified, from the Lost City site, 30°N MAR (10) and also involves serpentinization of ultramafic rock but yielding substantially lower fluid temperatures exiting the sea floor (∼40-90°C). The Lost City style of venting is associated with long-lived detachment surfaces that play an important role in extension along all slow spreading ridges (22). Although plume data for Lost City are not available, the vent fluids contain CH 4 and H 2 at levels comparable to Type 2 vents but with much lower dissolved metal concentrations (10,23).…”

A short electromagnetic profile across the Kane Oceanic Core Complex

“…[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.…”

Composition of the Oceanic Crust

“…Oceanic core complexes now appear to be common along slow-and ultraslow-spreading ridges such as the MAR and SWIR. Indeed, Escartin et al (2008) suggested that detachment faulting and oceanic core complexes occur along almost half the MAR between 12 30 0 N and 35 N.…”