ACCELERATING LATE QUATERNARY UPLIFT OF THE NEW-GEORGIA-ISLAND-GROUP (SOLOMON-ISLAND ARC) IN RESPONSE TO SUBDUCTION OF THE RECENTLY ACTIVE WOODLARK SPREADING CENTER AND COLEMAN SEAMOUNT

The New Georgia Island Group of the Solomon Islands is one of four pla ces where an active or recently active spreading ridge has subducted b eneath an island are. We have used coral reef terraces, paleobathymetr y of Neogene sedimentary rocks, and existing marine geophysical data t o constrain patterns of regional Quaternary deformation related to sub duction of the recently active Woodlark spreading center and its overl ying Coleman seamount. These combined data indicate the following vert ical tectonic history for the central part of the New Georgia Island G roup: (1) subsidence of the forearc region (Tetepare and Rendova Islan ds) to water depths of similar to 1500 m and deposition of marine turb idites until after 270 ka; (2) late Quaternary uplift of the forearc t o sea level and erosion of an unconformity; (3) subsidence of the fore arc to similar to 500 m BSL and deposition of bathyal sediments; and ( 4) uplift of the forearc above sea level with Holocene uplift rates up to at least 7.5 mm/yr on Tetepare and 5 mm/yr on Rendova, In the nort heastern part of the New Georgia Island Group, our combined data indic ate a slightly different tectonic history characterized by lower-ampli tude vertical motions and a more recent change from subsidence to upli ft. Barrier reefs formed around New Georgia and Vangunu Islands as the y subsided >300 m, By 50-100 ka, subsidence was replaced by uplift tha t accelerated to Holocene rates of similar to 1 mm/yr on the volcanic are compared with rates up to similar to 7.5 mm/yr in the forearc area of Tetepare and Rendova. Uplift mechanisms, such as thermal effects d ue to subduction of spreading ridges, tectonic erosion, or underplatin g of deeply subducted bathymetric features, are not likely to function on the 270-ka period that these uplift events have occurred in the Ne w Georgia Island Group. A more likely uplift mechanism for the post-27 0-ka accelerating uplift of the forearc and volcanic are of the New Ge orgia Island Group is progressive impingement of the Coleman seamount or other topographically prominent features on the subducting plate. R egional effects we relate to this ongoing subduction-related process i nclude: (I) late Quaternary (post-270 ka), accelerating uplift of the Rendova-Tetepare forearc area in response to initial impingement of th e Coleman seamount followed by exponentially increasing collisional co ntact between the forearc and seamount; (2) later Quaternary propagati on of uplift arcward to include the volcanic are as the area of collis ional contact between the forearc and seamount increased, and (3) larg e-wavelength folding that has produced regional variations in late Hol ocene uplift rates observed in both forearc (southern Rendova, Tetepar e) and volcanic are (New Georgia Island) areas. We propose that the do minant tectonic effect of Coleman seamount impingement is horizontal s hortening of the forearc and are crust that is produced by strong coup ling between the subducting seamount and the unsedimented crystalline forearc of the New Georgia Island Group. The horizontal forces due to mechanical resistance to subducting rugged ridge and seamount topograp hy may have terminated spreading of the Woodlark spreading center ente ring the trench (Ghizo ridge) and converted it to a presently active s trike-slip fault zone. (C) 1998 Elsevier Science B.V. All rights reser ved.