Volcanic evolution of Gran Canaria reconstructed from apron sediments: Synthesis of Vicap Project drilling

Abstract

The ages, types, and compositions of volcaniclastic sediments, physical properties, and downhole logs obtained during Leg 157 drilling at Sites 953 and 954 north and 955 and 956 south of Gran Canaria, with a cumulative penetration of almost 3000 m and an overall recovery rate of 75%, correlate well compositionally and stratigraphically to major volcanic and nonvolcanic phases on the island. Volcaniclastic sediment contributions from neighboring islands are volumetrically insignificant. The highest rate of volcaniclastic sedimentation (>150 m/m.y.) corresponds to the middle Miocene basaltic late seamount stage of Gran Canaria, the lowermost deposits drilled being thick, graded, hyaloclastite tuffs and debris flows, dominated by poorly vesiculated, altered sideromelane clasts, suggesting eruption at a water depth of more than 500 m. Large basalt clasts, some of subaerial origin, at the base of the strongly graded deposits suggest synchronous subaerial activity. A lithic-rich debris-flow deposit (Site 956), at least 80 m thick, is interpreted to have been generated by a major collapse of southwestern Gran Canaria, thereby triggering tsunami waves, whose deposits are now represented by 2 m of amphibole, phlogopite, apatite, and Cr-spinel-rich sandstones overlying the debrite. Breccias, lapillistones, and tuffs containing vesicular shards record the transition from shallow submarine to emergent environment. The erosion of the older subaerial shield basalts in eastern Gran Canaria is reflected in hundreds of thin, fine-grained turbidites at Site 953, whereas submarine activity continued in the southwest (Site 956). Nearly identical Nb:Zr and Nb:Y ratios in all submarine and subaerial basalts suggest fairly constant source conditions. A precisely dated (13.9 Ma) rhyolitic to basaltic syn-ignimbrite turbidite separates the basal hyaloclastites at three sites from 50- to 10-m-thick, dominantly rhyolitic volcaniclastic turbidite series correlated to the Mogan Group (14-13.3 Ma) and 130- to 250-m-thick, dominantly trachyphonolitic volcaniclastic turbidite sections correlated to the Fataga Group (13.3-9.5 Ma). Near-unique mineral phases or assemblages, glass, feldspar, amphibole and clinopyroxene, and bulk-rock compositions represent robust criteria for unequivocally correlating at least seven syn-ignimbrite volcaniclastic turbidites between three sites, which are 160 km (Sites 953 and 956) and 170 km (Sites 953 and 955) apart from each other.