Fault- and fluid-systems in supra subduction zone ophiolites

During International Ocean Discovery Program Expedition 352, a section through the volcanic stratigraphy of the outer fore-arc of the Izu-Bonin-Mariana system was drilled in order to trace the processes of magmatism, tectonics, and crustal accretion associated with subduction initiation. This study in turn has implications for understanding the origin of the many ophiolites that are now believed to form in this setting, and the expedition provides a good opportunity to test this supra-subduction zone ophiolite model. Previous studies have established a stratigraphy in which peridotites, gabbros, and sheeted dikes are overlain by fore-arc basalt (FAB) and then in turn by boninites. The expedition provided (1) a high-fidelity record of magmatic evolution during subduction initiation; (2) a test of the hypothesis that FAB tholeiites lie beneath boninites; (3) a record of the chemical gradients within these units and across their transitions; (4) information on how mantle melting processes evolve during subduction initiation from early decompression melting of fertile asthenosphere to late flux melting of depleted mantle, providing key empirical constraints for realistic subduction initiation geodynamic models; and (5) a test of the hypothesis that fore-arc lithosphere created during subduction initiation is the birthplace of supra-subduction zone ophiolites. Two important hypotheses to be tested by drilling are (1) that subduction initiation produces a consistent volcanic stratigraphy; and (2) that this sequence was originally stacked vertically before erosion and therefore represents an in situ analog for sections through many supra-subduction zone ophiolites.

During International Ocean Discovery Program (IODP) Expedition 352, a section through the volcanic stratigraphy of the outer fore-arc of the Izu-Bonin-Mariana (IBM) system was drilled in order to trace the processes of magmatism, tectonics, and crustal accretion associated with subduction initiation. The drilled cores provided structures that mainly result from brittle deformation and comprise fault zones with various kinematics, distinct faults and cataclastic shear zones, and extensional fractures as well as veins with precipitated minerals mainly derived from hydrothermal fluids (see project description below). The study results in general reveal the evolution of the IBM forearc system from its formation to its subsequent tectonic evolution, with implications for the hydrologic system, fluid-rock interaction within oceanic lithosphere, and the deformation history in supra-subduction zone (SSZ) settings in general. This was tested within the Troodos ophiolite in order to confirm the hypothesis that this ophiolite represents a fossil SSZ analogue. Forearc basalts (FAB) are the first products of subduction initiation and form by higher degrees of partial melting being related to upwelling asthenosphere, compared to normal Mid Ocean Ridge Basalts (MORB). This was followed by late proto-forearc spreading boninite magmatism. The source for the latter was a hydrous melt derived from partial melting of subducted oceanic crust under amphibolite-facies conditions. The published magmatic ages from the IBM forearc basement, isotope data and the biostratigraphic record from the cover sediments revealed a ~15 Ma hiatus between the rapidly forming near-trench seafloor after subduction initiation around 52 Ma, and subsequent tectonic forearc extension. This 15 Ma time gap can be explained by on-going rollback of the Pacific Plate that triggered upper plate extension. In the area of IODP Expedition 352 drilling sites, the IBM forearc was stretched by ~16 - 19% at the 'boninite sites' and up to 55% at 'FAB sites', at strain rates in the range of 10-16s-1 to 10-15s-1. Blocky vein microtextures with host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Rare earth element and isotopic (18O, 13C, 87Sr/86Sr, 47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB-hosted low-T (< 30 C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35-33 Ma and ~22 Ma indicating the age of vein formation and precipitation. Boninite-hosted vein calcites precipitated from seawater that was modified by fluid-rock interactions. The overall IBM forearc architecture and development as inferred from this study may serve as a reference setting, in particular with regard to post-magmatic rift-related structures and a sedimentary cover, and will help with the identification and interpretation of a SSZ origin of ophiolites worldwide.