Widths of modern arc-trench gaps proportional to past duration of igneous activity in associated magmatic arcs

Arc-trench systems along convergent junctures between plates of lithosphere include subduction zones related to trenches and volcano-plutonic orogens built by the igneous activity of the magmatic arcs. Between trenches and arcs are elongate belts termed arc-trench gaps with transverse limits defined by the volcanic front of the arc on the inner side and by the trench slope break above the inner wall of the trench on the outer side. Thick sequences of undeformed sediments derived largely from the arcs accumulate within arc-trench gaps in elongate structural basins created by subsidence beneath arc-trench gaps coupled with probable uplift of deformed rock masses in the subduction zones beneath and beyond the trench slope breaks. Widths of arc-trench gaps in modern arc-trench systems are proportional to the past duration of magmatic activity in the associated arcs as inferred from the ages of igneous rocks exposed in the arcs. This correlation apparently implies progressive growth in the width of the arc-trench gap during the evolution of an arc-trench system through time. This growth can be ascribed to prograde or outward migration of the trench and subduction zone away from crustal rocks in the central region of the arc-trench gap plus a similar but opposing retrograde or inward migration of the magmatic axis of the arc away from this central region. Of the two influences on the growth of the arc-trench gap, the former has been discussed most and finds a ready explanation in terms of the accretion of crustal materials in the subduction zone of the trench region. However, the latter influence may be dominant in common cases and implies some systematic change with time in the conditions controlling the genesis of magmas at depth beneath the arc region.