Persistent elastic behavior above a megathrust rupture patch: Nias island, West Sumatra
We quantify fore-arc deformation using fossil reefs to test the assumption commonly made in seismic cycle models that anelastic deformation of the fore arc is negligible. Elevated coral microatolls, paleoreef flats, and chenier plains show that the Sumatran outer arc island of Nias has experienced a complex pattern of relatively slow long-term uplift and subsidence during the Holocene epoch. This same island rose up to 2.9 m during the Mw 8.7 Sunda megathrust rupture in 2005. The mismatch between the 2005 and Holocene uplift patterns, along with the overall low rates of Holocene deformation, reflects the dominance of elastic strain accumulation and release along this section of the Sunda outer arc high and the relatively subordinate role of upper plate deformation in accommodating long-term plate convergence. The fraction of 2005 uplift that will be retained permanently is generally <4% for sites that experienced more than 0.25 m of coseismic uplift. Average uplift rates since the mid-Holocene range from 1.5 to −0.2 mm/a and are highest on the eastern coast of Nias, where coseismic uplift was nearly zero in 2005. The pattern of long-term uplift and subsidence is consistent with slow deformation of Nias along closely spaced folds in the north and trenchward dipping back thrusts in the southeast. Low Holocene tectonic uplift rates provide for excellent geomorphic and stratigraphic preservation of the mid-Holocene relative sea level high, which was under way by ∼7.3 ka and persisted until ∼2 ka.
Additional InformationCopyright 2008 by the American Geophysical Union. Received 11 March 2008; revised 24 July 2008; accepted 17 September 2008; published 12 December 2008. We thank the Captain and crew of the Mentawai Indah. Thanks to Hong-Wei Chiang and Larry Edwards for the U-Th dates. Dick Peltier and Rosemarie Drummond kindly provided predictions from the ICE-5G (VM2) model, and Aron Meltzner generously provided tide model calculations. We benefited greatly from discussions with Mike Gagan, Jean-Philippe Avouac, Glenn Milne, Chris Goldfinger, Harvey Kelsey, Ben Horton, and Heidrun Kopp. We also thank Daniel Melnick, Alan Nelson, two anonymous reviewers, and Associate Editor Greg Moore for their reviews and discussion of our manuscript. This work was supported by NSF (EAR-0610078) and a grant from the Gordon and Betty Moore Foundation. This is Caltech Tectonics Observatory contribution 80.
Published - BRIjgrb08.pdf