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Geological Setting

Site 1256 (6°44.2'N, 91°56.1'W) lies in 3635 m of water in the Guatemala Basin on Cocos plate crust formed at ~15 Ma on the eastern flank of the EPR (Fig. F7). The site sits astride the magnetic Anomaly 5Bn–5Br transition in magnetic polarity (Fig. F8). This crust accreted at a superfast spreading rate (~200–220 mm/yr full rate) (Wilson, 1996) and lies ~1150 km east of the present crest of the EPR and ~530 km north of the Cocos-Nazca spreading center. Cocos plate crust of 23- to 25-Ma age is being subducted along the Central American Trench ~830 km to the northeast. The trace of the Cocos/Pacific/Nazca triple junction passes ~100 km to the southeast, with the elevated bathymetry of the Cocos Ridge, recording the trail of the Galapagos plume, farther to the southeast (~500 km) (Fig. F7). The Guatemala Basin has relatively subdued bathymetry, and the immediate surroundings of Site 1256 (~300 km) are relatively unblemished by major seamount chains or large tectonic features high enough to penetrate the sediment cover (~200–300 m).

Site 1256 formed at an equatorial latitude (Fig. F9) within the equatorial high-productivity zone and initially endured very high sedimentation rates (>30 m/m.y.) (e.g., Farrell et al., 1995). Reconstruction Figure F9 is in the reference frame fixed to the Antarctic plate, which is nearly fixed relative to the spin axis and to hotpots. Motion of the Pacific plate relative to Antarctica is from Cande et al. (1995), and motion of the Cocos plate relative to the Pacific plate is from Wilson (1996). Velocities relative to Antarctica are similar to the present for the Pacific plate but are much faster than present for the Cocos plate.

As is described in greater detail below in "Site Survey Results," Site 1256 sits atop a region of smooth basement topography (<10 m relief) and has a seismic structure reminiscent of typical Pacific off-axis seafloor. Upper Layer 2 velocities are 4.5–5 km/s and the Layer 2–3 transition is at ~1200–1500 m subbasement. The total crustal thickness at Site 1256 is estimated at ~5–5.5 km. Farther to the northeast of Site 1256 (15–20 km) a trail of ~500-m-high circular seamounts rise a few hundred meters above the sediment blanket (Fig. F10). Basement topography is more pronounced to the southwest of Site 1256 and in the grid 2 area (Fig. F11), where subparallel narrow ridges (1–2 km) and wider troughs (4–5 km) display ~100–150 m of basement relief.

Site Survey Results

The site survey cruise for this leg took place in March and April 1999 aboard the Maurice Ewing, led by D. Wilson, A. Harding, and G. Kent. At the urging of the Architecture of Oceanic Lithosphere Program Planning Group, the original plan for four sites in the Guatemala Basin was modified to instead cover three sites there and a separate site near Alijos Rocks west of Southern Baja California (Fig. F7). The suggested advantage of the Alijos site is higher paleolatitude, allowing determination of magnetic polarity with azimuthally unoriented cores. The other significant difference recognized before survey work is lower spreading rate, ~120–130 mm/yr instead of 200–220 mm/yr, with a presumably greater depth to the uppermost gabbros.

The site survey work focused on seismic reflection and refraction. MCS reflection and refraction experiments to ocean-bottom hydrophones (OBHs) were conducted separately because of differences in desired shot intervals. MCS work used a tuned array of 10 air guns shooting to a new 480-channel, 6-km streamer, with a nominal shot interval of 37.5 m (15–18 s). With a hydrophone spacing of 12.5 m, this geometry gives 80-fold coverage with 6.25-m midpoint spacing. Refraction shooting to grids of 10–11 OBHs using 20 air guns was at a shot interval of 90 s (130–180 m) for most of the grid and 150 s (300 m) for the outermost shots. The grid geometry was designed for well-constrained measurements of velocities in both across-strike and along-strike directions to depths of 1.5–2.0 km and to cover to Mohorovicic Discontinuity (Moho) depths in the across-strike direction only (Figs. F10, F12, F13). Because of time constraints and delays from several causes, refraction surveying was only conducted at two of the three Guatemala Basin sites.

The Guatemala Basin sites have 200–300 m of sediment cover resulting from their formation near the paleoequator. Referring to the sites in the order MCS was collected, grid 1 was chosen to include ODP Site 844 at a line crossing and to be centered near the magnetic Anomaly 5D(y) boundary, and grids 2 and 3 were centered on Anomalies 5C(y) and 5B(o) along a flow line perpendicular to anomaly strike (Fig. F9). Grids 1 and 3 have refraction data. Grid 1 is quite shallow for its ~17-Ma age at 3400–3500 m, and basement at Site 844 is at 3705 m (Fig. F12). Relief on basement as seen in MCS is extremely low, with the largest scarps having ~30-m amplitude. Subtle horizontal reflections ~1.6–1.7 s below basement are interpreted to correspond with the Moho. A cluster of seamounts with minimum depth of 2790 m is present near the southern tip of the grid.

Grid 3 is deeper than grid 1 at 3600–3700 m, and basement at ~3900 m is near normal depth for the ~15-Ma age (Fig. F10). In the southwest half of the grid, abyssal hill fabric is visible through the sediment cover and larger scarps approach 100-m amplitude. The northeast half of the grid has low relief, comparable to grid 1. Reflection data here commonly show complex reflectors at 1.3–1.8 s below basement, indicating dipping interfaces in the lower crust or upper mantle, probably including some Moho reflections. Upper crustal reflectors at ~0.4–0.8 s into basement are often bright and tend to have gentle (~20°) apparent dips in the isochron direction, with more horizontal apparent dips in the spreading direction (Figs. F14, F15). Analysis of refraction data in this grid shows crustal structure that is fairly typical for off-axis Pacific seafloor. Upper Layer 2 velocities are 4.5–5 km/s, a gradual transition between Layers 2 and 3 is at ~1.5 km below basement, and total crustal thickness is ~5–5.5 km (Fig. F16). Velocities of the uppermost crust are slowest in the southwestern part of the grid where the abyssal hill relief is greatest.

In contrast to the Cocos plate sites, grid 4 near Alijos Rocks has thin (50–100 m) sediment, slightly deep water (3800–4300 m) for the ~16.5-Ma age, and extremely high relief for the fast spreading rate (Fig. F13). Individual scarps are commonly 150 and up to 400 m. MCS data show few coherent reflections below Layer 2 in preliminary stacks. Receiver gathers for refraction data are broadly similar to the Cocos plate sites, perhaps suggestive of slightly slower velocities at ~1 km below basement.

Magnetic data at the Cocos plate sites show trends parallel to the previously mapped regional trend, with no evidence for isochron offsets at ~1-km detection limit within the grids (Fig. F11) and perhaps 3- to 5-km detection limit outside the grids. The Alijos grid is located within an area where magnetic and topographic features are linear for 30–40 km, but right-stepping offsets of a few kilometers leave the local trend a few degrees counterclockwise of the regional trend.

Of the three survey grids with refraction data, grid 3 (the southwesternmost and youngest of the Cocos plate grids) was chosen as the primary target for Leg 206 because its depth and basement relief are closest to normal. Within this grid, several factors affected the final site selection. The slower seismic velocities in southwestern part of the grid indicate more porous and possibly more rubbly material that may lead to poorer drilling conditions. OBH failure on line 23 along the southeastern part of the grid led to limited constraints on velocity determinations there.

A very bright upper crustal reflector is observed on much of line 21 in the northeastern part of the grid and for short distances on lines 27 and 28 where they cross line 21 (see Fig. F15). The reflector dips northwest and projects updip to a ~50-m-high hill that strikes northeast, perpendicular to the normal abyssal hill trend. The character of the reflection and its relation to the nearly vertical velocity gradient determined by refraction analysis are both more consistent with a narrow low-velocity zone rather than a simple interface between materials of different velocity. All of these relations suggest that the reflector might be a thrust fault, possibly driven by thermal contraction of the lower lithosphere. Because such a fault might lead to very poor drilling conditions at about the depths gabbro might be encountered on a return leg, this area was dismissed as a potential site for deep drilling. The remaining area near the northern corner of the grid appears very suitable for deep drilling, and the intersection of lines 22 and 27 (proposed Site GUATB-03C), where the velocity control is best, was the primary drilling location for Leg 206, Site 1256 (Figs. F10, F14, F15).

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