Reviews of Deep Drilling into Oceanic Crust
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Deep drill holes bored into in situ oceanic crust were reviewed. Since the beginning of the Deep Sea Drilling Project, 45 drill holes have penetrated over ca. 50 m into normal oceanic crust, however, most are concentrated in the north Atlantic and east Pacific Oceans. Basement ages of most holes are younger than 20 Ma, and are rather biased toward younger crust compared to the average age of the oceanic crust (61 Ma). Only five deep drill holes have penetrated over 500 m into the basement, three of which drilled into slow spread crust formed at < 4 cm/yr, with only one hole is fast spread crust formed at >8 cm/yr and one in intermediately spread crust.Three deep holes (332B, 395A, 418A) drilled into slow spread crust formed at Mid-Atlantic Ridge gave the first evidence of magnetic reversals through the vertical oceanic crust, and showed that the slow spread upper oceanic crust away from hot spots is dominantly composed of pillow lavas with a normal MORB-like affinity. Younger 332B and 395A holes (3.5 and 7.3 Ma) gave poor core recovery (18-21%), while the oldest Hole 418A (110 Ma) yielded a fairly high recovery of 72%.Hole 504B is the only hole to penetrate the extrusive rocks and most of the way through the sheeted dike complex (1836.5 m sub-basement). Average core recovery dropped from 29.8-25.3% in the lava and transition zone down to 14.3% in the sheeted dike complex. Unfortunately, the 504B lava is the depleted extremity of MORBs from intermediate-fast spread ridges. One of the most important findings of Hole 504B is a discrepancy between the seismic velocity structure and the downhole lithology in that the Layer 2/3 boundary resides in the middle of the sheeted dikes, as interpreted by the difference in porosity and bulk density.Hole 1256D is dedicated to coring typical oceanic crust and ultimately penetrates the entire crust into the upper mantle. The site is located on the 15-Ma Cocos plate generated at a superfast rate (22 cm/yr). 502-m-long cores of basement (48% recovery) are lavas showing moderately evolved MORB-like compositions similar to those from the present fast spread ridges. The hole has been cleaned and left ready for future drilling, possibly into Layer 3.The above examples of deep drill holes show that the major obstacles to ultradeep drilling are hole collapse and poor core recovery. Riser drilling is expected to overcome these obstacles for “the 21-century Mohole”.Keywords:
Pillow lava
Dike
Basement
Seamount
Adakite
Dike
Seafloor Spreading
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Three seamounts close to the south end of the Pratt-Welker Seamount Chain, Gulf of Alaska, have been sampled to test whether or not mantle plume-related volcanism extends south of Bowie Seamount. Lavas recovered from Oshawa, Drifters, and Graham seamounts are weathered, Mn-encrusted pillow lavas and sheet-flow fragments, commonly with glassy rims. The glasses and holocrystalline rocks are tholeiitic basalts, with light rare earth element depleted to flat primitive mantle normalized incompatible element patterns and radiogenic isotope compositions within the ranges of mid-ocean ridge and near-ridge seamount basalts from the Explorer and northern Juan de Fuca ridges. Chemically, the seamount lavas strongly resemble older, "shield-phase" tholeiitic rocks dredged from the flanks of southern Pratt-Welker seamounts, but are distinct from the younger alkaline intraplate lavas that cap Pratt-Welker edifices. The weathered, encrusted basalts were most likely erupted in a near-ridge environment, adjacent to Explorer Ridge, between 11 and 14 Ma. No evidence of plume-related activity is found in this area. Compared with northeast Pacific mid-ocean ridge and alkaline intraplate basalts, Graham seamount lavas have anomalously high 206 Pb/ 204 Pb, which does not appear to be a function of sea-floor alteration, magma contamination, or mixing between previously identified mantle components. All near-ridge seamounts in the northeast Pacific exhibit isotopic heterogeneity that does not correlate with major or trace element composition, suggesting that the mantle sources of all near-ridge seamounts have been variably depleted by prior, but recent melting events.
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Mantle plume
Transform fault
Hotspot (geology)
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Pillow lava
Seabed
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Pillow lava
Seafloor Spreading
Rift zone
Lava field
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Seamount
Mid-Atlantic Ridge
Isotopes of strontium
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Rare, fault‐bounded escarpments expose natural cross sections of ocean crust in several areas and provide an unparalleled opportunity to study the end products of tectonic and magmatic processes that operated at depth beneath oceanic spreading centers. We mapped the geologic structure of ocean crust produced at the East Pacific Rise (EPR) and now exposed along steep cliffs of the Pito Deep Rift near the northern edge of the Easter microplate. The upper oceanic crust in this area is typified by basaltic lavas underlain by a sheeted dike complex comprising northeast striking, moderately to steeply southeast dipping dikes. Paleomagnetic remanence of oriented blocks of dikes collected with both Alvin and Jason II indicate clockwise rotation of ∼61° related to rotation of the microplate indicating structural coupling between the microplate and crust of the Nazca Plate to the north. The consistent southeast dip of dikes formed as the result of tilting at the EPR shortly after their injection. Anisotropy of magnetic susceptibility of dikes provides well‐defined magmatic flow directions that are dominantly dike‐parallel and shallowly plunging. Corrected to their original EPR orientation, magma flow is interpreted as near‐horizontal and parallel to the ridge axis. These data provide the first direct evidence from sheeted dikes in ocean crust for along‐axis magma transport. These results also suggest that lateral transport in dikes is important even at fast spreading ridges where a laterally continuous subaxial magma chamber is present.
Dike
Seafloor Spreading
Magma chamber
Transform fault
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Pillow lava
Seafloor Spreading
Rift zone
Echelon formation
Lava field
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The Masirah Ophiolite is a good example of thin oceanic crust. Below pillow lavas and a sheeted dike complex with a relatively normal thickness of 1–1.5 km, the gabbroic lower crust barely exceeds 500 m in thickness. In spite of this reduced thickness, the oceanic crust preserves all members of a model ophiolite in a coherent lithostratigraphic sequence. The crust was formed during the uppermost Jurassic (circa 150 Ma) when the Indian‐Madagascar plate separated from the African‐Arabian plate and is therefore related to the opening of the coeval Somali basin. Geological relationships indicate that this portion of oceanic crust was formed at a ridge‐transform intersect. The peculiarly reduced thickness of the gabbro layer is interpreted as the result of a weak magma supply at the edge of a ridge segment, rather than the consequence of a tectonic thinning. The cooling effect due to the vicinity of two large continental lithospheric blocks (Indian‐Madagascar and African‐Arabian plates) during this initial stage of the oceanization might have been an additional factor contributing to the reduction of the crustal thickness.
Pillow lava
Seafloor Spreading
Dike
Oceanic basin
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Pillow lava
Seafloor Spreading
Rift zone
Lava field
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The structures of North Arm Mountain ophiolitic basalt and diabase indicate that these formed as a frozen lid to an underlying magma chamber in a system analogous to present-day mid-ocean spreading systems. Volcanics consist of pillow lavas, flows, and breccias. Volcanics grade downward into sheeted dikes by increase in dike frequency across a transition commonly less than 50 m thick. Sheeted dikes, subparallel throughout, dip away from the paleospreading axis, located to the west of present ophiolite position as indicated by chilled margin bias, implying rotation of the lid down and away from the axis. One-way chilling statistics, dike thickness, and dike intrusion sequence analyses all point toward a relatively narrow but complex zone of dike intrusion. The mean measured dike thickness is 0.90 m. Diabase contains zones of fracturing subparallel to dike trends, interpreted to reflect oceanic fissuring and faulting. These commonly terminate downward in foliated gabbro and (or) lineated amphibolite. Diabase grades into gabbros across a complex transition wherein (1) dikes grade into gabbro by gradual decrease in dike frequency, (2) dikes grade abruptly into gabbro, and (3) gabbro intrusions truncate and stope overlying diabase.
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Pillow lava
Massif
Sill
Breccia
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