Large-volume basaltic hyaloclastite eruption along a propagating land/lake lithosphere fracture at Lake Van (Eastern Anatolia): impact of volcanism on the evolution of Lake Van V

Abstract

The Incekaya hyaloclastite cone (eastern Anatolia, Turkey), the focal point along a major eruptive fissure, was the main source of an unusually large explosive basaltic eruption. The ca. 80 ka-old eruption began onshore with scoria cones from a 5 km N-S fracture propagating toward Lake Van (surface area of 3755 km²). At the intersection with the fault-bounded lake basin, a ca. 400-m-high subaerial hyaloclastite edifice formed, which can be crudely subdivided into a main lower massive bulk of hydrothermally altered lithic-rich hyaloclastites (CL) topped unconformably by a > 30-m-thick, well-bedded fallout tephra (CU). The CU tephras are correlated with (1) widespread onshore hyaloclastite fallout deposits mostly west-southwest of the cone and (2) a ca. 2-m-thick, ca 80-ka-old bedded hyaloclastite (V-60), part of a 220 m ICDP (International Continental Scientific Drilling Program) core, drilled in Lake Van, 27 km N of Incekaya. The hyaloclastite unit was seismically identified as being the most widespread and well-defined reflector throughout much of western Lake Van. A minimum volume of > 9 km³ fallout hyaloclastite tephra is estimated when the area of the seismic reflector is extrapolated to the coast and 2 km inland. Seismic reflectors also suggest at least two (hyaloclastite?) intralake cones rising up to 388 m above the lake sediment surface 1.5 km NW off Incekaya cone and were possibly erupted along the same fracture. The total volume of hyaloclastites includes (a) subaerial Incekaya cone, (b) the inferred subaqueous continuation of the cone(s), (c) the bedded intralake and onshore deposits, and, tentatively, (d) a widespread (seismically defined) mass flow deposits directly beneath Incekaya reflector of roughly 20 km³ and may represent the deposits of explosively erupted basaltic magma. Sideromelane shards, the main clast type, are dominantly angular, and most show ≪ 50 vol.% vesicles. Less common tachylite clasts are poorly vesicular (< 50 vol.%). Structural transitions and interlayering between tachylite and sideromelane are ubiquitous. Fluidal and pumiceous lapilli are present in the basal massive facies. Bulk rock and glass compositions indicate constant composition of the slightly evolved Al-rich basalt magma. Olivine (Fo_78–82) and plagioclase (An_70–80) microphenocrysts, many skeletal with growth features, and microlites make up < 1 vol.% and suggest rapid magma ascent. The high explosive energy of the eruption is interpreted to be due to (1) high magma discharge rates and shearing in the eruptive jet and (2) magma-water interaction conditions. Approximate temporal coincidences with the Incekaya eruption include the following: (a) an abrupt cessation in the supply of evolved tephra from the adjacent Süphan Volcano to the lake sediments, which ended abruptly for ca. 60 ky, (b) an extreme fall in lake level by ca. 150 m, and (c) a drastic increase in pore water salinity (Na⁺ and Cl⁻ (g/L)) and pH.