Abstract

New data demonstrate oceanic underthrusting and enable construction of a model of trench inner slopes. Magnetic, earthquake, and deep-penetration reflection seismic data supplemented by data from JOIDES holes, dredge hauls, and piston cores leave little room for doubt that thrusts and folds are the primary structures on the landward slopes of trenches, and, in accord with plate tectonics theory, they are caused by underthrusting of the oceanic plate beneath continents or island arcs. The seismic data and magnetic anomalies document extension of the oceanic plate beneath the landward slope of the trench. These seismic data also show structural separation between the oceanic plate and overlying sediments and reveal that it is caused by thrusting and/or folding. The resultant uplift is documented by paleobathymetric and compaction studies of sediments from the slopes of several trenches and by surface motion associated with recent earthquakes.

This relatively simple basin picture is a model of the structuring of trench inner slopes at a particular point in time and may be complexly modified as geologic conditions change with the passage of time. The model can be used to interpret the volumes of sediments in trenches, the rate of continental accretion, the development of fan structure, the depositional environments of sediments expected beneath trench inner slopes, and the phenomenon of diapirism in these sediments.

Accretion occurs by operation of the model through time. The trench slope builds outward while behind it the products of progressively older trench slopes first subside and then are uplifted while being incorporated into the continent by a plutonic belt.