Nathan L. Bangs

Megasplay faults, very long thrust faults that rise from the subduction plate boundary megathrust and intersect the sea floor at the landward edge of the accretionary prism, are thought to play a role in tsunami genesis. We imaged a megasplay thrust system along the Nankai Trough in three dimensions, which allowed us to map the splay fault geometry and its lateral continuity. The megasplay is continuous from the main plate interface fault upwards to the sea floor, where it cuts older thrust slices of the frontal accretionary prism. The thrust geometry and evidence of large-scale slumping of surficial sediments show that the fault is active and that the activity has evolved toward the landward direction with time, contrary to the usual seaward progression of accretionary thrusts. The megasplay fault has progressively steepened, substantially increasing the potential for vertical uplift of the sea floor with slip. We conclude that slip on the megasplay fault most likely contributed to generating devastating historic tsunamis, such as the 1944 moment magnitude 8.1 Tonankai event, and it is this geometry that makes this margin and others like it particularly prone to tsunami genesis.

We present results of a seismic reflection and refraction investigation of the Aleutian island arc, designed to test the hypothesis that volcanic arcs constitute the building blocks of continental crust. The Aleutian arc has the requisite thickness (30 km) to build continental crust, but it differs strongly from continental crust in its composition and reflectivity structure. Seismic velocities and the compositions of erupted lavas suggest that the Aleutian crust has a mafic bulk composition, in contrast to the andesitic bulk composition of continents. The silicic upper crust and reflective lower crust that are characteristic of continental crust are conspicuously lacking in the Aleutian intraoceanic arc. Therefore, if island arcs form a significant source of continental crust, the bulk properties of arc crust must be substantially modified during or after accretion to a continental margin. The pervasive deformation, intracrustal melting, and delamination of mafic to ultramafic residuum necessary to transform arc crust into mature continental crust probably occur during arc-continent collision or through subsequent establishment of a continental arc. The volume of crust created along the arc exceeds that estimated by previous workers by about a factor of two.

Log and core data document gas saturations as high as 90% in a coarse‐grained turbidite sequence beneath the gas hydrate stability zone (GHSZ) at south Hydrate Ridge, in the Cascadia accretionary complex. The geometry of this gas‐saturated bed is defined by a strong, negative‐polarity reflection in 3D seismic data. Because of the gas buoyancy, gas pressure equals or exceeds the overburden stress immediately beneath the GHSZ at the summit. We conclude that gas is focused into the coarse‐grained sequence from a large volume of the accretionary complex and is trapped until high gas pressure forces the gas to migrate through the GHSZ to seafloor vents. This focused flow provides methane to the GHSZ in excess of its proportion in gas hydrate, thus providing a mechanism to explain the observed coexistence of massive gas hydrate, saline pore water and free gas near the summit.

Many major geological terranes are interpreted as accretionary complexes, and there are several speculative models for their structure and mode of formation. The seismic reflection section across the Barbados Ridge complex at lat 16°12′N presented here shows, for the first time, the entire cross-sectional shape of a large accretionary wedge and its forearc basin. Atlantic oceanic crust underlies 122 km of the wedge and then passes beneath the crust of the forearc of the Caribbean plate, where it can be traced 15 km farther; it dips landward at 9°. The forearc basement dips seaward to meet the ocean crust. The maximum thickness of the wedge is about 10 km. A layer of sediments, 1 km thick, is drawn beneath the accretionary wedge on the surface of the oceanic crust, with little disturbance, for a distance of 70 km, and some sediments still appear to adhere to the ocean crust to where it passes beneath the forearc basement. It is not clear whether sediment is subducted deeper, but it appears probable. The principal resistance to landward motion of the accretionary wedge is provided by the weight of up to 6 km of forearc-basin sediments on the seaward-dipping forearc basement. Both the forearc sediments and the basement have been deformed as a consequence of the horizontal compression produced by the subduction of ocean crust.