Geology of the Mariana Trough

Abstract

its present position, although occasional outbreaks of volcanism occur in the western portion of the basin along boundaries offault blocks. A block-faulted region approximately 60 km wide exists along the western margin of the Mariana Trough reflecting the early stage of rifting of the basin. The central spreading region of the Mariana Trough represents the mature spreading section of the basin. The seafloor morphology is typical of slow spreading, with a series of north-south trending valleys and ridges similar to abyssal hills offset by transform fault Valleys. Volcanism is predominantly confined to the spreading center. Graben and horst structures at the margin of the abyssal hill fabric in the western part of the basin represent the early phase of opening of the basin. Preexisting structural elements that cut across the arc system persist throughout both the early rifting and the later spreading stages of the basin. Chains of submarine volcanoes (volcanic cross-chains) extend into the backarc basin along some of these lineaments, pennitting the leakage of arc magmas into the backarc basin. The southern basin platfonn is a shallow (less than 3 km), relatively low relief region, except where it is separated from the active volcanic front and from the remnant arc by deep, narrow troughs. Perturbations of the typical spreading center morphology occur where the active spreading zone of the platfonn intersects the volcanic front at about 13°N, 144°E in the southernmost portion of the backarc basin. This position coincides with a major north-south trending fault that crosscuts the forearc, arc, and backarc basin. It has influenced the spreading axis and the eastern margin of the trough. The distribution of volcanism within the basin varies according to the developmental stages of the spreading center. Volcanism and hydrothennal activity are also influenced by cross-arc and along-arc defonnation of the Mariana platelet. Most lavas from the backarc basin spreading ridges show systematic differences from mid-ocean ridge basalt as a consequence of infusion of the source v>,ith subduction-related constituents. The composi- tional variations of lavas within the basin indicate complex local tectonic control over magma genesis and an intricate interplay of mixing from several sources with crystalliza- tion at various pressures.