Can vertical compaction within wedges promote accretion by backthrusts?

Friday, 19 December 2014
Jessica McBeck, Michele L Cooke, Justin W Herbert and Elizabeth H Madden, University of Massachusetts Amherst, Amherst, MA, United States
In natural subduction zones, frontal accretion dominantly occurs via the propagation of forethrusts, whereas accretion via backthrusts has been observed in only a few active subduction zones, including the Cascadia margin. Similarly, in most analog experiments of accretionary wedges deformation is accommodated by forethrusts or backthrust/forethrust pairs, except for some experiments with a layer of silicone below sand, which can produce accretionary backthrusts. Vertical deflection of the detachment caused by the lateral flow of the silicone layer could promote the propagation of backthrusts in these analog experiments. Alternatively, the high Holocene sediment input in parts of the Cascadia margin could produce vertical compaction deep within the wedge that promotes backthrusting. To explore the effect of vertical compaction and deflection of the detachment on fault development in accretionary prisms we use the Boundary Element Method modeling tool Growth by Optimization of Work (GROW) to predict the vergence of faults in a deforming wedge. GROW predicts fault growth by propagating faults in the direction that maximizes the efficiency of the system, or minimizes the external work of the system. We simulate vertical compaction with compliant elements and observe that the addition of these elements deep in the wedge or along the detachment promotes backthrusting rather than forethrusts. Similarly, local downward deflection of the detachment promotes backthrust development over that of forethrusts. These numerical model results suggest that vertical compaction or local deflection of the megathrust may account for backthrust development in parts of the Cascadia margin.