Copyright by Nysha Chaderton 2009 - The University of Texas at ...
Copyright by Nysha Chaderton 2009 - The University of Texas at ...
Copyright by Nysha Chaderton 2009 - The University of Texas at ...
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<strong>The</strong> highest point <strong>of</strong> the island, is 336 m above sea level, is loc<strong>at</strong>ed <strong>at</strong> Mount<br />
Hilla<strong>by</strong>, St. Thomas. At this loc<strong>at</strong>ion there are exposures <strong>of</strong> the Oceanic Form<strong>at</strong>ion. East<br />
<strong>of</strong> this loc<strong>at</strong>ion the Scotland Form<strong>at</strong>ion is exposed. According to work carried out on the<br />
limestone terraces on Barbados, the island is uplifting in the Qu<strong>at</strong>ernary <strong>at</strong> a r<strong>at</strong>e <strong>of</strong> 0.44<br />
mm/yr (Taylor and Mann, 1991). Assuming a constant r<strong>at</strong>e <strong>of</strong> rise <strong>of</strong> 0.44 mm/yr show<br />
th<strong>at</strong> simple uplift could have raised the Oceanic Form<strong>at</strong>ion 4,840 m since middle<br />
Miocene times. <strong>The</strong> vertical rise <strong>of</strong> the Barbados Ridge due to underpl<strong>at</strong>ing can account<br />
for emergence <strong>of</strong> the Oceanic Form<strong>at</strong>ion, with no need for l<strong>at</strong>eral transl<strong>at</strong>ion <strong>by</strong> thrust<br />
faulting.<br />
This is consistent with the failure to find nappe geometries in seismic reflection<br />
pr<strong>of</strong>iles in the Forearc Basin sediments (the Oceanic Form<strong>at</strong>ion). This observ<strong>at</strong>ion lends<br />
support to the hypothesis th<strong>at</strong> the Oceanic Form<strong>at</strong>ion was deposited in situ on top <strong>of</strong> the<br />
accretionary prism sediments.<br />
DATA<br />
<strong>The</strong> primary d<strong>at</strong>a used in this study are a 2D seismic d<strong>at</strong>a set measured on 10-km<br />
spacing <strong>of</strong> lines and traces, covering approxim<strong>at</strong>ely 450 km 2 (Figure 4.1). <strong>The</strong> d<strong>at</strong>a,<br />
which image up to 12 seconds <strong>of</strong> str<strong>at</strong>a, have a good signal-to-noise r<strong>at</strong>io and are the<br />
most complete seismic coverage <strong>of</strong> an accretionary forearc basin in the world. <strong>The</strong>y were<br />
obtained from two industry seismic surveys—one shot in 1979 <strong>by</strong> Mobil and the other<br />
shot in 1999 <strong>by</strong> ConocoPhillips. Parameters from the 1979 d<strong>at</strong>a are not known. <strong>The</strong> d<strong>at</strong>a<br />
measured in 1999 were shot using 38-inch air guns as the energy source <strong>at</strong> a depth <strong>of</strong> 6<br />
m, with a shot interval <strong>of</strong> 25 m. <strong>The</strong> streamer length was 6,000 m, and streamers were<br />
towed <strong>at</strong> a depth <strong>of</strong> 8 to 9 m below the vessel. <strong>The</strong> sample r<strong>at</strong>e was 2 milliseconds. D<strong>at</strong>a<br />
were acquired predominantly over the Barbados Ridge and the east part <strong>of</strong> the Tobago<br />
Basin (Figure 4.1). Also available for interpret<strong>at</strong>ion is a single, long, regional seismic line<br />
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