Growth of accretionary wedges and pulsed ophiolitic melange formation by successive subduction of trench-parallel volcanic elevations

Fault-bounded coherent belts alternating with belts of melanges are common in accretionary wedges and are usually interpreted as a result of imbrication along subduction zone megathrusts. Using the Neoproterozoic/early Cambrian Blovice accretionary complex (BAC), Bohemian Massif, as a case example, we present a new model for the origin of alternating belts through the repetition of several cycles of (1) off-scraping and deformation of trench-fill sediments to form the coherent units, interrupted by (2) arrival and subduction of linear, trench-parallel volcanic elevations. The latter process leads to an increase in the wedge taper, triggering mass-wasting and formation of olistostromes. At the same time, ophiolitic melanges form by disruption of an upper part of the volcanic ridge and incorporation of the disrupted oceanfloor succession into the olistostromes. Specifically, the BAC represents a complete section across an accretionary wedge and records three such major pulses of ophiolitic melange formation through subduction of an outboard back-arc basin.