The Western Solomons Forearc: Independent Inner and Outer Forearc Paleo-Uplift Histories and Relationship to Megathrust Rupture

Abstract:

The Western Solomons forearc has undergone repeated uplifts that probably were coseismic and similar to that during the 1 April 2007 Mw 8.1 megathrust rupture that raised the outer forearc as much as 2.5 m. A parallel swath of the inner forearc subsided ~0.5 m during the 2007 event. The Western Solomons is ideal for crustal motion measurements because both the outer forearc above the seismogenic zone and the inner forearc are occupied by reef-fringed islands enabling land-based measurements of vertical displacements. U-series and ¹⁴C dating of uplifted corals and microatolls has provided the ages for a number of paleo-uplift events that we infer to have been coseismic. While the outer forearc has uplifted rapidly in late Holocene time at rates from ~1 to 8 mm/yr, the inner forearc has risen at only 0 – 1.1 mm/yr. It is notable that the inner and outer forearcs are separated by an arc-parallel boundary along which there is little or no net uplift. However, although both the inner and outer forearcs have late Quaternary histories of relatively rapid net uplift, they appear to have quite different uplift histories. The outer forearc has uplifted several times over the past 1000 years by as much as several meters in each event. Although the inner forearc subsided during the 2007 event and should subside during all events that ruptured the seismogenic zone beneath the outer forearc, it also has undergone abrupt late Holocene uplifts that do not correspond to those of the outer forearc. But the most surprising difference between the inner and outer forearcs is that 8-9 ka corals in growth position and emerged solution notches were found in a number of inner forearc sites adjacent to corals of the ~6 ka mid-Holocene high sea level that was a little higher than present sea level. Because sea level at 8-9 ka was 10-20 m lower than present in this region, 10-20 m of uplift is required between 8-9 ka and 6 ka during one or more tectonic events. Some of the 9 ka corals have morphologies indicate abrupt emergence that may have been coseismic. Much slower uplift after ~8 ka allowed rising sea level to almost overtake the 8-9 ka corals. Fortunately they remained slightly above subsequent sea level so that we could discover them. Our presentation will show the evidence and propose hypotheses to explain the early Holocene interval of great uplift inferred from the 8-9 ka corals.