HANGING WALL SITES

Taken together, the structural and preliminary paleomagnetic observations in the upper 400 m of the footwall of Atlantis massif suggest that models for core complex exhumation that require large viscous strains and footwall rotations need to be reevaluated. Structures indicative of high displacements by either ductile or brittle processes were not observed, and paleomagnetic data document a lack of significant tectonic rotation in the upper 120 m of the massif. Site U1310 was drilled to assess any petrogenetic relationship between volcanic rocks in the hanging wall and potential source rocks recovered in the footwall, to document faulting and any rotation of the hanging wall, and to compare the alteration and deformation styles between the footwall and hanging wall cores. The site was located where the hypothesized detachment that forms the corrugated central dome might be intersected once drilling reached the base of the hanging wall. The position of a seismic reflector (Canales et al., 2004) beneath Site U1310 may indicate that the detachment occurs at a depth of 250–350 mbsf. Recovery of rock from within the detachment zone was a high priority, inferred to be more likely where hanging wall volcanic rocks may armor the fault than at Site U1309, where seafloor exposure of the detachment is likely to have degraded (or even removed) the material due to weathering and/or erosion. In addition, the difficulty associated with the bare-rock spud makes recovery of a thin detachment zone unlikely at the footwall site.

Because of drilling difficulties at Site U1310, we developed an alternate strategy for delivering some of the objectives outlined for the hanging wall site. Site U1311 is on the lower eastern slope of the central dome, along seismic Line Meg-10 (Canales et al, 2004), where the D-reflector is interpreted to surface before diving east beneath the hanging wall. The site can be interpreted as a possible klippe of hanging wall rock, stranded above the detachment fault. We note that if this target is not a klippe, but instead is either the detachment surface itself or post-exposure volcanic deposits, objectives related to penetration into D-reflector and into the footwall would still be addressed at this site.

Site U1310 is located ~9.3 km west of the center of the rift valley. TOBI and DSL120 side-scan sonar data and bathymetry indicate that the site is on a small fault-bound horst, ~600 m east of the break in slope inferred to mark the termination of the detachment fault exposed on the central dome. The site coincides with the eastern end of Alvin Dive 3643, Cruise AT3-60, in an area with numerous small northeast-striking scarps (both southeast and northwest facing) (Fig. F20). Based on nearby Alvin observations and sampling, these scarps expose pillow basalt.

A push test at the end of the camera survey indicated there was as much as 6 m of unlithified microfossil ooze in Hole U1301A (30°11.4850'N, 42°03.9256'W, 2583 mbsl). No coring was attempted at this site, but ~1 kg of broken basaltic material was recovered from the hammer drill casing when it returned to the rig floor. The dominant rock type is sparsely plagioclase-phyric fine-grained pillow basalt. A single ~10 cm pillow fragment has a palagonitized glass rim grading through a spherulitic zone to a microcrystalline interior. The remaining material ranges in size from ~5 cm to dust. It includes palagonitized glass fragments, some of which are encrusted by calcareous sediment and/or iron-manganese oxides, angular and drill-rounded fine-grained basalt fragments, and minor calcareous sediment and iron manganese oxides. The finer material includes separate fragments and coatings of ferrous and siliceous welding slag derived from assembly and disassembly of the bottom-hole assembly (BHA).

The first core recovered from Hole U1310B (30°11.4842'N, 42°03.9197'W; 2583.5 mbsl) contained 1.3 m of fist-sized and smaller pieces of basalt from the 13.5 m interval below a 5 m thick sediment cover. Although we cored as deep as 23 m, the second core barrel was still in the lower BHA when it severed, so no rock was recovered from below 18.5 mbsf. Most of the basalt fragments recovered from Core 304-U1310B-1R were broken, alteration along these fracture surfaces being minimal and the interior >1% altered, suggesting they are derived from in situ pillows. The piece interiors are also almost unaltered. Vesicles compose 3%–5% of the pieces analyzed. Vesicles close to fracture surfaces are internally discolored brown but not filled; spherulitic zones appear light brown in places, and within these zones, plagioclase appears white, rather than transparent. Some fragments have a glassy rim 1–3 mm thick, with relatively fresh glass. Thin zones of palagonite are mostly confined to the outer pillow surface. Glass samples were taken for onshore analysis prior to the postcruise sampling party.

In thin section, the pillow interiors appear fresh, with 5%–10% seriate plagioclase needles and radiating clusters in a glassy groundmass characterized by branching, feathery quench textures. Sparse anhedral olivine and prismatic plagioclase microphenocrysts are no more than 0.5 mm in size, with rare grains up to several millimeters in long dimension. Crystal clots composed of subophitic intergrowths of plagioclase and olivine are relatively common. Major and trace element geochemistry suggests that the basalt is a primitive tholeiite in composition. The basalt has 49.35 wt% SiO₂, 10.23 wt% MgO, 9.66 wt% Fe₂O₃, 15.21 wt% Al₂O₃, 11.94 wt% CaO, 2.02 wt% Na₂O, 0.05 wt% K₂O, and 0.95 wt% TiO₂. Site U1310 basalt has 7.8 ppm Ba, 72 ppm Sr, 23.9 ppm Y, 48.2 ppm Zr, 234 ppm V, and 34.3 ppm Sc. Sample 304-U1310B-1R-1, 16–18 cm, is characterized by high Mg and low trace element contents, consistent with the observation of olivine microphenocrysts noted in thin section.

Site U1311 is located on a small knoll near the break in slope where the corrugated dome meets the adjacent volcanic, hanging wall block. Recovery of fresh, glassy vesicular basalt in Hole U1311A does not provide a definitive test of the hypothesis that the site is located on a small klippe above the detachment fault, as outlined above. The eruptive basalt could have come from the inferred hanging wall, as hypothesized, or it could have been erupted onto or against the slope of the easternmost domal surface at anytime after exposure at the seafloor.

Hole U1311A (30°10.6091'N, 42°04.1904'W; 2552 mbsl) is located on the southern slope of the knoll. A 60 m x 60 m survey with the VIT camera documented a ~3600 m² area of mud- and rubble-covered seafloor, with a moderate slope to the south-southeast. Along the northeastern corner of the survey area, a moderately to steeply southeast-dipping, >20 m high scarp oriented east-northeast (~75°) crops out and is characterized by rounded pillow structures.

One attempt to drill at this site penetrated 12 mbsf, including 3.5 m of unconsolidated mud. Recovery from the hole produced 1.5 m (13%) of fresh, vesicular, moderately plagioclase-olivine phyric basalt pillows, with sparse glass preserved. Almost all pieces are angular and have broken along fracture surfaces within pillows—few, if any, were cut by the drill. The fracture surfaces are discolored dark brown and minimally altered. These observations suggest that the core is derived from in situ pillows. Some piece interiors include multiple gray Liesegang bands, indicating pervasive minor or incipient alteration. Vesicles close to fracture surfaces are internally discolored brown but not filled.

The basalt is dark gray to black in color and characterized by abundant (5%–10%) seriate plagioclase that locally occurs in radiating clumps. Sparse euhedral plagioclase phenocrysts range upward in size to ~2 mm. In thin sections from pillow interiors, the basalt appears fresh, with minimal darkening of the matrix and local occurrences of orange to green smectites either in vesicles or in the vicinity of olivine (Fig. F21). Randomly oriented, acicular to prismatic plagioclase, ranging in size from <0.1 to 0.5 mm, makes up ~40% of the sample. Many of the plagioclase crystals have hollow or swallowtail quench morphologies. Olivine microphenocrysts (~5%) appear either in subophitic crystal clots with plagioclase or as euhedral microphenocrysts ranging in size from 0.1 to 0.3 mm and in shape from larger prisms to smaller diamonds and more complex quenched forms. The remainder of the rock is the devitrified glass matrix, dominated by plumose, quenched clinopyroxene with anhedral interstitial plagioclase. Oxides are abundant, up to 2%, in the matrix and mostly occur as complex quench morphologies, most likely ilmenite. Vesicles occupy 3%–5% of total volume. They occur in two forms: round (~0.1 mm) and elongate, irregular, and locally interconnected. Most are unfilled, but a few are completely or partially filled by devitrified glass and a few, especially near fracture surfaces, are filled by secondary green or orange smectite. They generally range in size up to 0.5 mm, but in places are as large as 2 mm. Where present, glass is 1–3 mm thick with 50%–100% palagonitization close to outer surfaces.

The one sample of pillow basalt (Sample 304-U1311A-1R-1, 34–36 cm) analyzed for major and trace element geochemistry suggests that the basalt is a primitive tholeiite, with a Mg# of 66. The sample is characterized by loss on ignition values of –0.05 wt% and H₂O and CO₂ below detection limit. It has 48.97 wt% SiO₂, 9.63 wt% MgO, 9.82 wt% Fe₂O₃, 16.81 wt% Al₂O₃, 12.61 wt% CaO, 2.17 wt% Na₂O, 0.04 wt% K₂O, and 1.12 wt% TiO₂. The Site U1311 basalt has 8.8 ppm Ba, 83 ppm Sr, 26 ppm Y, 57 ppm Zr, and 38 ppm Sc, virtually equivalent to the basalt sampled in Hole U1310B. Basalt from both hanging wall sites reflect a depleted source.