Considerable research efforts have been directed toward revealing the structure and constitution of the lower oceanic crust as well as understanding magma chamber processes and geometries responsible for formation of the plutonic portions of the crust at spreading centers. Studies of seafloor plutonic samples have provided remarkably little direct knowledge of the structure and constitution of the plutonic foundations of the oceanic crust despite its obvious volumetric importance. Studies have likewise not clearly enabled definition of magma chamber processes that relate to production of mid-ocean-ridge basalt (MORB). The paucity of samples, the difficulties maintaining pseudostratigraphic control on dredge or dive samples in tectonically complex regions, and the altered and deformed nature of many abyssal plutonics have all in some way hampered investigations aimed at sampling a complete section of the lower plutonic crust with seafloor collections.
Drilling efforts of the Ocean Drilling Program (ODP) to investigate lower oceanic crust have been more successful than more conventional sampling techniques in obtaining in situ samples and started to yield significant results when Site 735 near the Atlantis II Fracture Zone of the Indian Ocean was drilled during Leg 118. The first significant penetration (~500 m) into a gabbroic massif was unexpectedly achieved with high recovery (~90%) toward the end of the leg (Robinson, Von Herzen, et al., 1989). In the late 1980s the Offset Drilling Working Group proposed a plan for capitalizing on sites, like Site 735, where tectonic dismemberment had created windows of opportunity to drill the lower oceanic crust and mantle. The goal of the program was to drill offset holes within dismembered sections to build a composite picture of the entire oceanic crustal section. Two legs followed during which gabbroic and ultramafic rock were drilled in the Hess Deep in the Pacific Ocean during Leg 147 (Gillis, Mevel, Allan, et al., 1993) and, subsequently, near the Kane Fracture Zone along the Mid-Atlantic Ridge (MARK area) during Leg 153 (Cannat, Karson, Miller, et al., 1995). The JOIDES Resolution revisited the Southwest Indian Ridge (SWIR) during Leg 176 to deepen the previously drilled Hole 735B and achieved penetration to 1500 m with excellent recovery. This greatly enhanced and completely transformed the way we view the nature of the plutonic section of the oceanic crust (Dick, Natland, Miller, et al., 1999; Dick et al., 2000). Finally, to partly address bare rock spudding problems during prior offset drilling legs, Leg 179 was conceived as an engineering leg to test the hammer drill-in casing system near Hole 735B and to drill a hole along the Ninetyeast Ridge for emplacement of a downhole seismometer (Pettigrew, Casey, Miller, et al., 1999). Leg 179 was unexpectedly able to add to plutonic recovery on the Atlantis Bank because of delays in a resupply ship visit that inhibited resumption of hammer tests at Site 1104. A bare rock spud using a tricone bit was drilled to 15 meters below seafloor (mbsf), followed by several days of rotary coring in Hole 1105A to a depth of 158 mbsf within the gabbroic massif of the Atlantis platform. The hole is ~1.2 km east-northeast of Hole 735B and was logged with the Formation MicroScanner (FMS) and other downhole tools (Pettigrew, Casey, Miller, et al., 1999).
Ocean Drilling Program Leg 179 set out with two primary objectives:
The second primary objective specifically included drilling a single hole as deep as possible into basement and installing a reentry cone and casing beyond basement to prepare the Ninetyeast Ridge site for installation of an ocean-bottom observatory. The observatory will be installed at a later date and will be part of the future network of seafloor observatories proposed in the International Ocean Network program. Drilling reached 493.8 mbsf, sufficient depth to create an acceptable borehole below casing for the downhole seismometer installation. Total penetration into basement was 122 m, and total penetration below casing reached 79.4 m. This significant penetration below the casing, as well as the firm attachment to basement, should isolate the geophysical instrument from noise reported from other ocean floor seismometer installations. Both these objectives were accomplished during Leg 179.
Hammer drill testing was completed at Sites 1104 and 1106 (virtually coincident locations but positioned from different beacons after a hiatus in testing while waiting for resupply). These tests provide a wealth of information regarding the viability of the hammer drill-in casing system as a new tool for investigating the solid Earth. Although the testing was not completed as planned, design changes resulting from tests that were completed were made to accommodate future legs where the hammer drill was deployed successfully. Based on the results of the first sea trial of the hammer drill-in casing system and follow-up legs, we are confident that continued development will soon allow us to sample the deep ocean crust in environments where, with rare exceptions, we have historically been unable to operate (Pettigrew, Casey, Miller, et al., 1999).
Drill string acceleration data were acquired at two sites occupied during Leg 179 (Pettigrew, Casey, Miller, et al., 1999). Myers et al. (Chap. 1, this volume) determined that drill string acceleration signals vary inversely with porosity measured from logging and core data. The signal amplitude varies with lithology, with sediment amplitude roughly one-half that of either basalt or gabbro. Significant energy radiates through the seafloor but behaves differently when passing through various formations. The authors proposed that drill string acceleration may provide a useful tool to evaluate drilling conditions and formation properties encountered at the bit.
Because of contingencies developed during Leg 179, certain ancillary objectives were not accomplished during hammer drilling, but one contingency drilling site was cored on the Atlantis Bank, Southwest Indian Ridge (Pettigrew, Casey, Miller, et al., 1999). This chapter is a synthesis of results from structural, geochemical, and downhole information now available for ODP Hole 1105A, the contingency drilling site. Drilling penetrated into a gabbroic massif on the Atlantis Bank, along the eastern transverse ridge of the Atlantis II Fracture Zone offsetting the Southwest Indian Ridge. The hole is proximal to Hole 735B, drilled into the same gabbroic massif during Legs 118 and 176. The gabbroic massif appears to have been exposed as a consequence of core complex formation at ~11.5 Ma at the northern Southwest Indian Ridge intersection with the Atlantis II Transform (Dick et al., 1991; Natland and Dick, 2002).
Located only 1.2 km from Hole 735B, Hole 1105A represents the first hole with good recovery to have been drilled in such close proximity to another high-recovery hole. The value of this lies in the attempt to correlate various aspects of each hole and the attempt to construct a two-dimensional (2-D) model of the geometry of the plutonic complex beneath the Atlantis Bank and the magmatic and structural features in both holes. This synthesis paper reports largely on the results of Leg 179 that pertain to Hole 1105A and studies of the core and logging data collected. Further details of the hammer drill sites and the NERO site where no core recovery was possible can be obtained from the Leg 179 Initial Reports volume (Pettigrew, Casey, Miller, et al., 1999). Because the leg was an engineering leg, significant results are reported in the Initial Reports volume. For developments of the of the hammer drill system since Leg 179, readers are referred to further results of tests conducted during Leg 191 (Holloway and Shipboard Scientific Party, 2001).