Slow-Spreading Oceanic Crust Formed By Steady-State Axial Volcanic Ridges

Oceanic crust originates at mid-ocean spreading ridges (MORs), covers almost three quarters of the earth's surface and dominates the global magmatic flux. Axial volcanic ridges (AVRs) are almost ubiquitous features of orthogonal slow-spreading ridges, which account for three quarters of the global mid-ocean spreading ridge system today. Typically 3-6 km wide, 200-500 m high and 10-20 km long, AVRs are the loci of recent volcanic activity and form the most prominent topography rising above the otherwise flat-lying Median Valley floor. Previous studies indicate that AVRs, and their related crustal magma reservoirs are episodic, on a time scale of 150-300 ka. Yet their near ubiquitous occurrence at slow-spreading ridge segments provides us with a paradox: if AVRs have a life cycle of formation and degradation, does their near ubiquitous presence at slow spreading ridges imply their life-cycles are synchronised? In this contribution, we report the findings from a high-resolution study of a well-developed axial volcanic ridge (AVR) at 45°N on the Mid-Atlantic Ridge (MAR). Here, the MAR is typical of most slow-spreading ridges: it spreads generally symmetrically and orthogonally, at a full rate of 23.6 mm per year, has second and third-order segmentation, and contains a typical AVR. Using a combination of detailed micro-bathymetry, sidescan sonar, visual surveying and petrology, we suggest that the AVR is the product of quasi-steady state volcanotectonic processes. Small volume lava flows, originating at or near the crest and with short run-out lengths, form ~60 m high hummocky pillow-lava mounds that dominate the construction of the AVR. The lavas are the product of moderate degrees of mantle melting that are typical for normal mid-ocean ridge basalt. Synchronous with these eruptions the flanks of the AVR subside forming a structural horst. Subsidence is partially accommodated by a series of outward-facing volcanic growth faults that step-down and away from the AVR crest and towards the Median Valley floor. Here, much larger volume, yet less frequent, effusions of massive lava flows erupt rapidly from large flat-topped seamounts, found almost exclusively outside of the AVR. The sheet-flows have run-out lengths of up to several kilometres, a combined thickness sufficient to bury the hummocky topography of the AVR flanks, producing smooth flat-lying seafloor typical of the Median Valley floor and its uplifted flanks. These lavas are relatively enriched geochemically and are characteristic of small melt fractions from the mantle. Thus it appears that the volcanic crust at slow-spreading ridges is formed through a continuous process of small volcanic eruptions along AVRs that evolve through syn-volcanic subsidence and episodic burial by large volume massive lava eruptions. From this, we conclude that AVRs have neither a particular life cycle nor are they synchronised along the global mid-ocean ridge system. Rather, they approximate steady-state features in which subsidence plays as large a part in their origin as volcanic construction.