CYCLICAL BEHAVIOR OF THRUST WEDGES - INSIGHTS FROM HIGH BASAL FRICTION SANDBOX EXPERIMENTS

Scaled sandbox experiments with high basal friction, simulating the gr owth of accretionary wedges, display cycles alternating between fronta l imbricate thrusting and underthrusting of long, undeformed sheets, B y contrast, low basal friction experiments with otherwise similar and constant, initial conditions produce a classic frontal imbricate fan t hrough repeated failure along frontal thrusts, The cyclical behavior o bserved in high basal friction experiments is expressed by three quant ities: (1) the average spacing between frontal thrusts, (2) the advanc e and retreat of the deformation front, and (3) the frontal slope (a) of the actively deforming wedge, As a long sheet is underthrust, the f ront is steepened through slumping until the maximum critical angle is reached, Then frontal thrusting resumes and the accretion of imbricat e slices builds the wedge forward, thereby lowering the taper to the m inimum critical angle. At shallow tapers, a long unit is underthrust a nd subsequently uplifts, shortens, and steepens the overlying wedge th rough backthrust deformation, thus completing the cycle. Underthrustin g of long units offers a simple mechanism for underplating overlying u nits, It also provides a possible explanation for temporally and spati ally varying wedge geometries in nature, when basal frictions attain 8 0%-90% of the internal friction.