Back to list of UTIG abstract submissions, Fall Agu 2003

Frederick W. Taylor, Institute for Geophysics, University of Texas at Austin, Austin, TX, USA
C. Frohlich, Institute for Geophysics, University of Texas at Austin, Austin, TX, USA
Paul Mann, Institute for Geophysics, University of Texas at Austin, Austin, TX, USA
G.S. Burr, NSF Accelerator Facility, Physics Department, University of Arizona, Tucson, AZ, USA
W. Beck, NSF Accelerator Facility, Physics Department, University of Arizona, Tucson, AZ, USA
R.L. Edwards, Dept. Geology and Geophysics, Univ. Minnesota Twin Cities, Minneapolis, MN, USA
David A. Phillips, SOEST, University of Hawaii at Manoa, Honolulu, HI, USA

Aseismic subduction of very young lithosphere may occur if the downgoing plate is too young and hot to support stick-slip cycles of interplate elastic strain accumulation and rupture. However, we have found a series of discrete episodic uplift events of 1-2 m each in the Western Solomons forearc where ~1-2 Ma Woodlark Basin lithosphere is subducting at rates on the order of 80-100 mm/yr. These uplift events produce average forearc Holocene uplift rates ranging up to 5-8 mm/yr on southern Rendova and Tetepare Islands about 25 km from the plate boundary in a forearc area lacking historical evidence for large forearc thrust events.

The best evidence of paleouplifts is in central eastern Rendova where the Holocene uplift rate is ~3 mm/yr. Here we find morphologically pristine emerged corals at ~2 m elevation that give a cluster of radiocarbon ages (783+38, 799+28, 800+34 yr BP) (calib. age: 279 - 463 yr BP) that match a local solution notch at ~2 m elevation. Additional distinct notches occur immediately above at ~3.5 and 5 m elevation. Corals at ~3.3 m give ages of 1286+43 yr BP (calib. age: 663 - 787 yr BP) and at ~ 4.75 m the age is 1436+39 yr BP (calib. age: 762 - 932 yr BP). Additional notches cut the lower 10 m of emerged Holocene reef that rises to 20 m maximum elevation in this area. Thus, late Holocene uplift appears to occur by discrete events on the order of 1-2 m that affect a large area of the outer forearc.

Maximum uplift rates located in a zone only ~25 km from the convergent plate boundary are best explained by subducting rugged seafloor topography causing full or at least partial locking of the interplate thrust zone from near the sea floor to ~10-20 km depth on the interplate thrust zone. A recurrence interval of several hundred years between ruptures on a locked thrust zone could explain why seismicity in this area is depressed. However, if the interplate thrust zone is aseismic, then perhaps these uplift events are generated by episodic non-seismic interplate slip. Another possibility is that locking at the shallow end of the interplate thrust zone is sufficient to delay slip at greater depths so that it occurs episodically rather than continuously.