Tectonic termination of oceanic detachment faults, with constraints on tectonic uplift and mass wasting related erosion rates

Abstract

New high-resolution bathymetric data from Atlantis Massif and surrounding seafloor (30°N, Mid-Atlantic Ridge) records avolcanic extension associated with the formation of the axial rift valley floor, following the tectonic truncation of an active corrugated oceanic detachment fault system. The truncated Atlantis detachment is tectonically uplifted by a high-angle valley-bounding normal fault, formed after westward migration of the ridge at ∼0.4 to 0.1 Ma. Detachment fault remnants, with preserved corrugations, lie within the present-day rift valley seafloor, and demonstrate that a ∼20 km ridge section in the immediate vicinity of the Atlantis Fracture zone has not recorded any recent volcanic activity. Avolcanic extension may thus occur locally at the slow-spreading Mid-Atlantic Ridge, albeit for limited periods of time (less than a few hundred thousand years). The new fault dissecting the detachment shows a throw of ∼2800 m, partly due to flexural uplift. Emplacement of the Lost City hydrothermal site occurred at a late stage post-dating the detachment truncation and avolcanic rift valley formation. From the inferred timing of the westward ridge axis shift we calculate uplift rates ≥ 7 mm/yr, possibly as high as 33 mm/yr, which are equivalent to or greater than the fastest vertical uplift rates of active normal faults measured to date on Earth (Gulf of Corinth). Geomorphologic observations also demonstrate that mass wasting efficiently reworks the seafloor topography. We obtain local incision and erosion rates ≥1-2 mm/yr locally, and as high as 4-8 mm/yr, depending on the assumed age for the rift bonding fault (0.4 vs. 0.1 Ma respectively). Our results suggest that (1) avolcanic extension may occur locally at the slow-spreading Mid-Atlantic Ridge, albeit for limited periods of time (less than a few 100s of kyrs), and (2) document that shifts in axial valley location related to the abrupt abandonment of detachment faults is a first-order process in the asymmetric accretion of slow-spread oceanic lithosphere.