Accretionary complexes that form at subduction zones develop a spectrum of styles of deformation that range between coherent forms in which the processes of thrusting predominate and incoherent forms in which melanges, formed by such processes as mud diapirism, are the dominant constituent. This thesis examines processes that control the development of these accretionary styles by comparing geophysical observations of the modern Barbados Ridge accretionary complex, with ancient subaerially exposed examples in Sabah, N. Borneo and W. Timor.A structural and lineament map of the offshore regions of the Barbados Ridge complex has been constructed using GLORIA. Seabeam and seismic data. It reveals marked changes in the surface structure of the accretionary wedge, particularly where basement ridges associated with oceanic fracture zones underthrust it. It also documents the presence of large numbers, of mud diapirs in its southern regions. The mud diapirs appear to be associated with the accretion of thick sequences of turbidites and their distribution is proposed to relate to the subcretion or underplating of underconsolidated material to the base of the complex. Fieldwork in Sabah and Timor concentrated on describing features associated with currently active mud diapirs, and potential ancient examples. A classification of the various forms of diapiric activity has been erected as part of a general discussion on the importance, genesis and emplacement of mud diapirs. In addition, the general structural development of the accretionary complex in W. Sabah was found to broadly resemble that of the frontal regions of a particularly thin part of the Barbados Ridge complex studied during DSDP Leg 78A and ODP Leg 110.A series of principal controls or boundary factors appear to control the general development of accretionary complexes. During the course of this study the importance of the sedimentary input, critical taper (or balance between boundary stresses and gravitational body forces) and subducting basement topography has been made particularly clear.