Abrupt Cessation of Subsidence and Initiation of Uplift on the Western Solomons and Central New Hebrides Forearcs: Possible Tectonic Mechanisms

In both the Central New Hebrides and the Western Solomons forearcs, fossil coral ages show that hundreds of meters of ongoing rapid uplift were preceded by similar amounts of subsidence. In the central New Hebrides, new 230Th/238U ages from Araki Island provide tight constraints on local tectonic history and show that tens of meters of subsidence occurred after MIS 5e (~120-130 ka). However, by MIS 5c (~105 ka) or soon after, uplift began and continues at a mean rate of ~2.3 mm/yr. Total convergent plate motion has been ~10 km at the Central New Hebrides since uplift began. In the Western Solomons, the change from subsidence to uplift is younger and occurred while plate convergence has been <5 km. In both settings, uplift began on time scales too brief and across areas too large to simply reflect the volumetric displacement of subducted bathymetry. Rather, we interpret the abrupt initiation of uplift as related to sudden strong interplate coupling triggered by impingement of seamounts or other prominent bathymetric features on the downgoing plate that abruptly contact the edge of the forearc or some deeper strong area along the interplate thrust zone. The subducting topographic features must tear or cut their way along the base of the upper plate to advance. Interplate stress rises to much higher than normal levels while the upper plate deforms by shortening and uplift to accommodate part of the plate convergence. We hypothesize that the seamount-forearc contact area of extreme stress creates a damage zone ahead of the seamount that ultimately leads to relief of the stress and subsidence of the forearc as interplate slip resumes. We predict that once coupling weakens again that much of the ongoing uplift will be removed by the next period of subsidence.