THE PROCESSES OF UNDERTHRUSTING AND UNDERPLATING IN THE GEOLOGIC RECORD: STRUCTURAL DIVERSITY BETWEEN THE FRANCISCAN COMPLEX (CALIFORNIA), THE KODIAK COMPLEX (ALASKA) AND THE INTERNAL LIGURIAN UNITS (ITALY)

Existing studies on active subduction margins have documented the wide diversity in structural style between accretionary prisms, both in space and time. Together with physical boundary conditions of the margins, the thickness of sedimentary successions carried by the lower plate seems to play a key role in controlling the deformation and fluid flow during accretion. We have tested the influence of the subducting sedimentary section by comparing the structural style and fluid-related structures of four units from three fossil accretionary complexes characterized by similar physical conditions but different subducting sediment thicknesses: (1) the Franciscan Complex of California, (2) the Internal Ligurian Units of Italy and (3) the Kodiak Complex, Alaska. Similar comparisons have been commonly reported in literature but essentially for modern margins.

Subducting plates bearing a thick sedimentary cover generally result in coherent accretion through polyphase deformation represented by folding and thin thrusting events, while underplating of sediment-starved oceanic sections results in diffuse deformation and mélange formation. These two structural styles can alternate through time in a single complex with a long record of accretion such as Kodiak.

The parallel analysis of the selected analogues show that although the volume of sediments carried by the lower plate determines different structural styles, deformation is strongly controlled by injection of overpressured fluids during underthrusting and accretion. Transient hydrofracturing occurs through development of a system of dilatant fractures grossly parallel to the décollement zone.