The uppermost 2 km of the oceanic crust created at the fast spreading (135 mm yr À1 , full rate) equatorial East Pacific Rise (EPR) is exposed for tens of kilometers along escarpments bounding the Hess Deep Rift. Mosaics of large-scale digital images from the remotely operated vehicle (ROV) Argo II and direct observations from the submersible Alvin document a degree of geological complexity and variability that is not evident from most studies of ophiolites or prevailing models of seafloor spreading. Dramatic variations in the thickness and internal structure are documented in both the basaltic volcanic and sheeted dike rock units. These rock units are characterized by extensive faulting, fine-scale fracturing, and rotations of coherent crustal blocks meters to tens of meters across. The uppermost basaltic lavas are essentially undeformed and have overall gently inclined flow surfaces. Through most of the basaltic lava unit, however, lava flow contacts dip (208-708W) toward the EPR and generally increase in dip downward in the section. Dikes cutting the lavas and in the underlying sheeted dike unit generally dip (908 -408E) away from the EPR. Deeper level gabbroic rocks show little evidence of the intense fracturing typical of the overlying units. We interpret this upper crustal structure as the result of subaxial subsidence within 1-2 km of the EPR that accommodated the thickening of the basaltic lava unit to $500 m. Variations in the thickness of lava and dike units and spatially related structures along the rift escarpments suggest temporal fluctuations in magma supply. These results indicate that substantial brittle deformation accompanied waxing and waning volcanism during the accretion of the crustal section exposed at the Hess Deep Rift. If this type of structure is typical of uppermost oceanic crust generated at the EPR, these processes may be common along fast spreading mid-ocean ridges.

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[1] We performed analyses of topographic variation (surface roughness) using a new 2-D mapping method which shows that understanding the relationship between data resolution, Hurst exponent, y intercept, RMS deviation, and cell size is important for assessing surface processes. We use this new method to assess flows at six field sites in Kilauea caldera, Hawaii, using three data sets at different resolutions, TOPSAR (10 m/pixel), airborne lidar (1 m/pixel), and tripod-mounted lidar (0.02-0.03 m/pixel). The flows studied include ponded pahoehoe flows, compound pahoehoe flows, and jumbled, slabby pahoehoe. The 2-D quantification of surface roughness for the Kilauea lava flows indicates that features formed during emplacement and modification of the flows exhibit statistically distinct roughness signatures. The 2-D method provides a tool for unit mapping based on surface roughness. Key findings indicate that the new 2-D method provides more robust results than 1-D methods for surface roughness because of larger 2-D sample sizes and the removal of 1-D directional bias leading to a reduction in error. Furthermore, data set resolution relative to the scale of the features under study is important to consider when designing a 2-D surface roughness study. Future applications to topographic data sets from Mars will provide information on flow emplacement conditions and spatial and temporal evolution of volcanic provinces on Mars.

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Integrated Ocean Drilling Program (IODP) Hess Deep Expedition 345 was designed to sample lower crustal primitive gabbroic rock that formed at the fast-spreading East Pacific Rise (EPR) in order to test competing models of magmatic accretion and the intensity of hydrothermal cooling at depth. The Hess Deep Rift was selected to exploit tectonic exposures of young EPR plutonic crust, building upon results from Ocean Drilling Program Leg 147 as well as more recent submersible, remotely operated vehicle, and nearbottom surveys. The primary goal was to acquire the observations required to test end-member crustal accretion models that were in large part based on relationships from ophiolites, in combination with mid-ocean ridge geophysical studies. This goal was achieved with the recovery of primitive layered olivine gabbro and troctolite with many unexpected mineralogical and textural relationships, such as the abundance of orthopyroxene and the preservation of delicate skeletal olivine textures. Site U1415 is located within the Hess Deep Rift along the southern slope of the intrarift ridge between 4675 and 4850 m water depths. Specific hole locations were selected in the general area of the proposed drill sites (HD-01B-HD-03B) using a combination of geomorphology, seafloor observations, and shallow acoustic subbottom profiling data. A total of 16 holes were drilled. The primary science results were obtained from coring of two ~110 m deep reentry holes (U1415J and U1415P) and five single-bit holes (U1415E and U1415G-U1415I). Despite deep water depths and challenging drilling conditions, reasonable recovery for hard rock expeditions (15%-30%) was achieved at three 35-110 m deep holes (U1415I, U1415J, and U1415P). The other holes occupied during this expedition included three failed attempts to establish reentry capability (Holes U1415K, U1415M, and U1415P) and six jet-in tests to assess sediment thickness (Holes U1415A-U1415D, U1415F, and U1415L). Olivine gabbro and troctolite are the dominant plutonic rock types recovered at Site U1415, with minor gabbro, clinopyroxene oikocryst-bearing gabbroic lithologies, and gabbronorite. These rocks exhibit cumulate textures similar to those found in layered mafic intrusions and some ophiolite complexes. All lithologies are primitive, with Mg# between 76 and 89, falling within the global range of primitive oceanic gabbros. Spectacular modal and grain size layering was prevalent in >50% of the recovered core, display