HR: 16:15h
AN: T24A-02    [Abstracts]
TI: Shortening Distribution In Accretionary Wedges, Examples From Nankai And Cascadia.
AU: * Gonzalez-Mieres, R A
EM: ragonzal@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08540 United States
AU: Suppe, J
EM: suppe@princeton.edu
AF: Princeton University, Department of Geosciences, Princeton, NJ 08540 United States
AB: We analyze the shortening and shortening history of the frontal structure of two active accretionary wedges [1] Nankai Trough offshore Japan and [2] Cascadia front in offshore Oregon, US. We show that take up only a small fraction of the plate shortening and that these structures are relative long lived, implying that the deformation front propagated slowly. Nankai trough a low taper wedge, characterized at the most frontal part by a pure shear fault bend fold and a low amplitude detachment fold. The shortening consumed by the frontal shear fault bend fold is at the top 390 m (Suppe et al., 2004). The detachment fold consumes only 77 m. In addition an equivalent shortening is taken up by horizontal compaction (Henry et al., 2003). Analysis of the history of shortening of this detachment fold, constrained by thickness relief (Gonzalez-Mieres & Suppe, 2004), shows that growth strata begin ~ 150 m below ocean floor at an estimated age of 0.22 Myr, base on magneto-stratigraphy and paleontological data (Moore G. F. et al., 2001). Ratio of sedimentation to shortening (δH/δS=2) constrained by the analysis gives shortening rate (δS/δT) of 0.33 mm/yr and sedimentation rate (δH/δT) of 0.67 mm/yr. The frontal shear fault bend fold has less well defined growth strata; however analysis of the sequence permits an estimate of the initiation of deformation between 0.28 Myr to 0.14 Myr base on a sedimentation rate of 0.8 mm/yr, which yield a shortening rate of 1.4 mm/yr to 2.8 mm/yr. Therefore, both of the frontal detachment fold and shear fault bend fold are active at the same time and the shear fault bend fold led the deformation. Using these rates of deformation and the known plate motion of ~ 40 mm/yr, we have the detachment fold consuming approx 2 % the plate motion while the shear fault bend is taking up 5 %, therefore the other 94 % of plate motion has to be distributed in the more internal structure. The Cascadia front is similar to Nankai showing a low amplitude detachment fold at the frontal structure followed by a series of imbricate thrusts. The detachment fold is more complicated and showing two detachment levels working together to produce the simple fold. The lower detachment consumes 84 m of displacement and this is located at the interface between the oceanic crust and sediment. The upper the detachment is developed 1.2 km over the first and this takes most of the deformation 171 m. The frontal thrust has an estimated slip of ~3 km and its level of detachment is the same as the upper detachment of the detachment fold. The thickness relief analysis shows that δH/δS has been constant ~6 for most of his history but it has decrease recently to 1.1, which implies that shortening rate has increased. Based on a background sedimentation rate of ~1 mm/yr for the frontal part (Moore C et al., 1995) deformation started ~ 0.9 Myr ago and the shortening rate was 0.16 mm/yr until 0.2 Myr when it increased to 1 mm/yr. In addition, analysis of growth strata on the frontal thrust ramp shows that the beginning of growth is similar to the detachment fold. The shortening rate for the frontal ramp is estimated at ~ 3 mm/yr. Thus the detachment fold and the frontal thrust together consume approximately 10 % of the plate motion, which is 40 mm/yr. Therefore both Cascadia and Nankai have low frontal shortening rates relative to total plate convergence and relatively slow forward propagation of the deformation front.
DE: 3060 Subduction zone processes (1031, 3613, 8170, 8413)
DE: 8104 Continental margins: convergent
DE: 8107 Continental neotectonics (8002)
DE: 8108 Continental tectonics: compressional
DE: 8169 Sedimentary basin processes
SC: Tectonophysics [T]
MN: Fall Meeting 2005