EXPERIMENTAL MODELING OF FORE-ARC BASIN DEVELOPMENT DURING ACCRETIONARY WEDGE GROWTH

We present results of three sand-box experiments that model the associ ation between tectonic accretion and sedimentation in a forearc basin. Experimental sedimentation occurs step by step in the forearc basin d uring shortening of the sand wedge. In each experiment, the developmen t of the accretionary wedge leads to the formation of a major backthru st zone. This major deformation zone accounts for the thickening in th e rear part of the wedge. In natural settings this tectonic bulge dams sediments that are transported toward the trench from mountainous ter rain behind the forearc. We test the variation of friction along the d ecollement and note the following: (1) shortening of a low-friction we dge involves a mechanical balance between forethrusts and backthrust p ropagation and this balance is recorded by the sedimentary sequence tr apped in the forearc basin. Indeed, if most of the movement occurs alo ng the backthrust, the deepening of the basin will be larger and conse quently the thickness of the sedimentary sequence will be greater. (2) Such balance does not exist in the case of a high-friction wedge. (3) Variation of friction along the decollement during shortening of the sand wedge leads to modification in the forearc basin filling. Thus, f or similar increments of convergence, the sequence deposited in the fo rearc basin shows relatively larger thickness when the wedge is shorte ned above a high-friction decollement. We suggest that contraction and thickening in the rear part of the wedge is an efficient mechanism to initiate and develop a forearc basin. Thus, this kind of basin occurs in convergent settings, without collapse related to local extension o r tectonic erosion. They represent a sedimentary trap on a passive bas ement, bounded by a tectonic bulge. The Quaternary Hikurangi forearc b asin, southeast of the North Island of New Zealand, is bounded by two actively uplifting ridges. Thus, this basin is considered to be a poss ible example of the basins modelled in our experiments, and we suggest that the limit between the basin and the wedge could be a complex bac kthrust zone.