Upper Crustal Structure of Superfast‐Spread Oceanic Crust Exposed at the Pito Deep Rift: Implications for Seafloor Spreading. (22nd February 2023)

• Record Type: Journal Article
• Authors: Karson, J. A.; Chutas, L. A.; Hayman, N. W.; Hey, R. N.; Horst, A. J.; Hurst, S. D.; Klein, E. M.; Naar, D. F.; Varga, R. J.
• Is Part Of: Geochemistry, geophysics, geosystems. Volume 24:Number 3(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2023-02-22
• Subjects: seafloor spreading -- oceanic crust -- sheeted dikes -- ophiolites -- faulting; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 550.5
• DOI: 10.1029/2022GC010527
• Languages: English
• ISSNs: 1525-2027

Abstract

Abstract: A tectonic window into the upper 2, 000 m of oceanic crust generated at the superfast spreading (∼142 mm/yr) southern East Pacific Rise exposes a continuous layered structure of basaltic lavas and sheeted dikes over gabbroic rocks. This relatively simple structure is in accord with expectations for crustal accretion at a very fast spreading rate and high magma budget where magmatic construction keeps pace with plate separation. Detailed observations show that basaltic lava flows dip progressively more steeply inward (toward the spreading axis where they were erupted). Underlying sheeted dikes are faulted and tectonically rotated to dip steeply outward . These structures are interpreted in terms of subsidence beneath the axis of the southern East Pacific Rise during crustal construction that allowed the lava unit to thicken to >400 m without creating comparable relief at the spreading center. Transitional units above and below the sheeted dike complex show that the thickness of upper crustal rock units is modified by tectonic and intrusive processes during accretion. The crustal structure shows that even approaching the superfast spreading end‐member of seafloor spreading, crustal accretion involves dramatic tectonic processes that are not obvious from the surface geology of spreading centers. Plain Language Summary: Studies of mid‐ocean ridge spreading centers document the surface expression of spreading axes where new oceanic crust is constructed. Where major fault scarps cut into the oceanic crust, the structure and composition of that crust can be viewed in the third dimension. The Pito Deep Rift (PDR) in the SE Pacific exposes the upper 2, 000 m of the oceanic crust that was created by seafloor spreading at the southern East Pacific Rise at a rate of >140 mm/yr, one of the fastest spreading parts of the global Mid‐Ocean Ridge system. The geologic structure of lava flows, dikes that feed them, and magma‐chamber rocks shows how these units were assembled beneath the spreading center. At high spreading rates, new oceanic crust is thought to be constructed mainly by the eruption of lava flows and the intrusion of dikes above a magma chamber, with little faulting. The pattern of tilting and fracturing of lava flows and dikes at the PDR shows that even at a very high spreading rate, building the upper crust is more complicated than suggested by surface studies alone. Key Points: Upper oceanic crust of the superfast‐spreading southern East Pacific Rise is exposed along escarpments at the Pito Deep Rift (SE Pacific) The upper crustal units include continuous layers of lavas, sheeted dikes, gabbroic rocks, and transitional units Tilted and fractured lava flows and dikes show that subaxial subsidence and brittle deformation occurred during superfast crustal accretion