Direct evidence for dynamic magma supply fossilized in the lower oceanic crust of the Troodos ophiolite

Abstract

Temporal and spatial variabilities of mantle upwelling and melt supply in mid‐ocean ridges (MORs) have long been documented. Such variabilities span a range of scales and have a profound effect on the structure as well as the composition of the oceanic crust. Previous seismic and gravity studies have suggested that the lower oceanic crust plays a major role in accommodating these changes in melt supply. Here we report the first direct evidence for a sharp transition from coherent sub‐horizontal to near vertical magma flows frozen in the lower oceanic crust of the Troodos ophiolite at the segment edge near a fossil ridge‐transform intersection. We constrain the preferred petrofabric lineation directions at 13 gabbroic sites using anisotropy of magnetic susceptibility (AMS) verified by electron backscatter diffraction. Pre‐emplacement accretion‐related rotations were corrected using magnetic remanence directions. We identify two provinces of nearly uniform susceptibility directions (principal axes) and attribute them to two magmatic episodes. A more focused mantle upwelling and melting episode near the segment midpoint may have resulted in lower crustal lateral magma flows along the fossil segment‐edge, whereas uniform mantle upwelling and melt supply along the entire axis may have resulted in vertical magma flows at the segment‐edge. Overall, our data verify the vital role of the lower oceanic crust in accommodating changes in mantle upwelling and melt supply beneath MORs.

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