Volcanologic and tectonic evolution of the Kasuga seamounts, northern Mariana Trough: Alvin submersible investigations

Abstract

The Kasuga seamounts are active volcanoes situated wholly within the northern part of the backarc basin west of the active Mariana volcanic arc. They lie along a lineament that cuts across the northern portion of the Mariana arc from the trench axis to at least 60 km west of the volcanic front. The distribution of the major edifices probably represents a localized focusing of volcanic activity in the eastern backarc basin as a consequence of both cross-arc rifting and backarc basin formation. Structure of the edifices, distribution of satellite cones, and draping of the fault scarp by lava flows suggest continuing deformation of the volcanoes as a consequence of movement on the fault that bounds a graben to the east of the volcanic chain. Volcaniclastic deposits are common. Most are probably emplaced cold through the water column, but one of these deposits was probably emplaced hot. Hydrothermal processes are active at the summits of at least two of the volcanoes and produce compositionally distinctive fluids. The volcanoes have a broad range of lava compositions from basaltic to dacitic. Detailed studies of the volcanoes using the Alvin submersible show that the two southern edifices consist mainly of primitive, mantle-derived basalts, which range from relatively low-K calcalkaline basalts to high-K absarokites and shoshonites. The highest-K lavas lack many of the trace element characteristics of subduction-related volcanism (e.g. they have low Ba, Sr, Pb and B relative to other incompatible elements). We attribute this lack to derivation from old lithospheric sources during thinning of the arc as rifting proceeds. The intersection of cross-arc fractures related to forearc rifting and arc-parallel faults related to incipient backarc rifting permits primitive low-degree melts to erupt with minimal crustal-level differentiation. These melts preserve information about heterogeneities in the arc lithosphere.