HR: 1340h
AN: T43B-1404    [Abstracts]
TI: Crustal Structure, Gravity Anomalies and Flexure of the Continental Margin Offshore North-Eastern Brazil
AU: * Rodger, M
EM: matthewr@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences, Parks Road, Oxford, Oxo OX13PR United Kingdom
AU: Watts, A B
EM: tony@earth.ox.ac.uk
AF: University of Oxford, Department of Earth Sciences, Parks Road, Oxford, Oxo OX13PR United Kingdom
AU: Greenroyd, C J
EM: christopher.greenroyd@durham.ac.uk
AF: University of Durham, Department of Earth Sciences, South Road, Durham, Dur DH13LE United Kingdom
AU: Peirce, C
EM: christine.peirce@durham.ac.uk
AF: University of Durham, Department of Earth Sciences, South Road, Durham, Dur DH13LE United Kingdom
AU: Hobbs, R W
EM: r.w.hobbs@durham.ac.uk
AF: University of Durham, Department of Earth Sciences, South Road, Durham, Dur DH13LE United Kingdom
AB: During November/December 2003 we carried out a marine geophysical survey of the continental margin offshore north-eastern Brazil onboard RRS Discovery. The survey included 340 line km of coincident multi-channel seismic reflection and wide-angle seismic refraction data that were acquired along a transect of the margin in the region of the Amazon deep-sea fan. The wide-angle data, which were recorded by 16 ocean bottom instruments deployed at ~ 15 km intervals, have been used to determine a P wave velocity model of the crust and upper mantle. The model shows that the middle Amazon fan is underlain by > 12 km of sediments which can be divided into an upper and lower layer with seismic velocities between 1.6-4.0 and 3.6-4.6 km s^-1 respectively. The sediments overlie a relatively thin, ~5 km thick, oceanic crust with seismic velocities between 5.0-7.5 km s^-1. We have used the velocity model, together with existing commercial seismic reflection data, to estimate the distribution of sediments beneath the adjacent upper and lower Amazon fan. The data show two main sediment depocentres: an early one associated with the Cretaceous to mid-Miocene margin and a later one associated with the mid-Miocene to Recent superposition on the margin of the Amazon fan. Our data show that the maximum sediment thickness of the later depocentre is displaced by ~ 50 km seaward of the early depocentre. We have used a sediment thickness grid, together with combined 3-D flexural backstripping and gravity anomaly modelling techniques, to estimate the temporal and spatial distribution of the elastic thickness of the lithosphere, T_e, in the Amazon fan region. Preliminary results show that the early margin sediments loaded a relatively weak lithosphere with a T_e ~ 10 km while the later Amazon fan sediments loaded a relatively strong lithosphere with a T_e ~ 35 km. We discuss here the significance of these results and their implications for the structure, evolution and thermal and mechanical properties of rifted margins.
DE: 1219 Gravity anomalies and Earth structure (0920, 7205, 7240)
DE: 3010 Gravity and isostasy (1218, 1222)
DE: 3025 Marine seismics (0935, 7294)
DE: 8105 Continental margins: divergent (1212, 8124)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: Fall Meeting 2005