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Earth's magnetic field is recorded as oceanic crust cools, generating lineated magnetic anomalies that provide the pattern of polarity reversals for the past 160 million years.sup.1. In the lower (gabbroic) crust, polarity interval boundaries are proxies for isotherms that constrain cooling and hence crustal accretion. Seismic observations.sup.2-4, geospeedometry.sup.5-7 and thermal modelling.sup.8-10 of fast-spread crust yield conflicting interpretations of where and how heat is lost near the ridge, a sensitive indicator of processes of melt transport and crystallization within the crust. Here we show that the magnetic structure of magmatically robust fast-spread crust requires that crustal temperatures near the dike-gabbro transition remain at approximately 500 degrees Celsius for 0.1 million years. Near-bottom magnetization solutions over two areas, separated by approximately 8 kilometres, highlight subhorizontal polarity boundaries within 200 metres of the dike-gabbro transition that extend 7-8 kilometres off-axis. Oriented samples with multiple polarity components provide direct confirmation of a corresponding horizontal polarity boundary across an area approximately one kilometre wide, and indicate slow cooling over three polarity intervals. Our results are incompatible with deep hydrothermal cooling within a few kilometres of the axis.sup.2,7 and instead suggest a broad, hot axial zone that extends roughly 8 kilometres off-axis in magmatically robust fast-spread ocean crust. A record of Earth's magnetic field constructed from near-bottom magnetization observations and oriented samples provides three-dimensional imaging of magnetic stripes in fast-spread crust, and suggests slow cooling off-axis, as opposed to deep hydrothermal cooling close to the spreading ridge., Author(s): Sarah M. Maher [sup.1] , Jeffrey S. Gee [sup.1] , Michael J. Cheadle [sup.2] , Barbara E. John [sup.2] Author Affiliations: (1) Scripps Institution of Oceanography, University of California, [...]

Three-quarters of the oceanic crust formed at fast-spreading ridges is composed of plutonic rocks whose mineral assemblages, textures and compositions record the history of melt transport and crystallization between the [...]

Atlantis Bank is an anomalously uplifted oceanic core complex adjacent to the Atlantis II transform, on the southwest Indian Ridge, that rises >3 km above normal seafloor of the same age. Models of flexural uplift due to detachment faulting can account for ~1 km of this uplift. Postdetachment normal faults have been observed during submersible dives and on swath bathymetry. Two transform-parallel, large-offset (hundreds of meters) normal faults are identified on the eastern flank of Atlantis Bank, with numerous smaller faults (tens of meters) on the western flank. Flexural uplift associated with this transform-parallel normal faulting is consistent with gravity data and can account for the remaining anomalous uplift of Atlantis Bank. Extension normal to the Atlantis II transform may have occurred during a 12 m.y. period of transtension initiated by a 10[degrees] change in spreading direction ca. 19.5 Ma. This extension may have produced the 120-km-long transverse ridge of which Atlantis Bank is a part, and is consistent with stress reorientation about a weak transform fault. Keywords: oceanic core complex, southwest Indian Ridge, transtension, Atlantis II.

Three-dimensional numerical simulations have been constructed to investigate the distributions of apparent dihedral angles expected in unequilibrated rock textures for comparison with those expected in equilibrated rock textures. For unequilibrated, fully crystallized monomineralic rocks, the simulation shows that the distribution of apparent dihedral angles is nonrandom and similar to that expected from an equilibrated rock, and is largely independent of crystal shape and crystal nuclei distribution. This new distribution, together with the one previously derived for an equilibrated texture, defines a pair of apparent dihedral angle curves which encompass curves corresponding to any degree of textural equilibrium within a monomineralic rock. The location of a measured curve within this range allows a quantitative assessment of the degree of textural equilibrium within that rock. The simulations also show that unequilibrated, partially crystallized rocks generate apparent dihedral angle distributions for closed triangular pores that, depending on crystal shape, may mimic equilibrated dihedral angle distributions. They also indicate that unless a rock is in complete equilibrium, different dihedral angle distributions are produced by measuring the apparent dihedral angles of closed triangular or open nontriangular pores. An important conclusion of this work is that apparent dihedral angle curves for unequilibrated textures are similar to those for equilibrated textures. Therefore care must be taken when interpreting textural equilibrium from apparent dihedral angle curves.

Cluster analysis has the potential to quantify the size and characteristics of groups or clusters of grains and crystals within a rock texture. We present two techniques: (1) the complete linkage hierarchical cluster analysis (CLHCA) technique that we use to analyze grain-center distributions, and (2) the density linkage cluster analysis (DLCA) technique that we use to analyze grain boundary distributions. CLHCA applies a cluster algorithm to the distance matrix calculated from grain-center coordinates. This produces a hierarchical cluster distribution that is characteristic of the spatial pattern of the texture. This cluster distribution is normalized to that of a random distribution of points to identify clustered and ordered patterns in the data. The technique successfully identified 3-D cluster sizes from 2-D sections of a modal data set. In the application to real rock textures, the CLHCA of a komatiite cumulate identified a glomerocryst population with cluster sizes in the range of 0.3-2.5 mm, and also a variation of cluster patterns up through the cumulate pile. The CLHCA of a polymodal texture from a pyroxene-scapolite-sphene granulite indicated a clustered distribution for the sphene, which was restricted in growth pattern by the pyroxene and scapolite in the texture. DLCA computes a single solution of clusters that conforms to the search criteria input into the algorithm. DLCA performed poorly when used to identify distributions of large, randomly packed clusters of small spheres, mainly identifying smaller cluster patterns within each large cluster. Application of DLCA to the komatiite cumulate example indicated cluster lengths of 0.72-0.80 mm similar to the CLHCA results, and an average aspect ratio of 1.52.

The article presents the highlights of the three-leg Océano Profundo 2015 expedition that was conducted by the U.S. National Oceanic and Atmospheric Administration (NOAA) Office of Ocean Exploration and Research (OER) to study and characterize the deep waters around Puerto Rico and the U.S. Virgin Islands (USVI). Among the activities is the collection of high-resolution multibeam bathymetry, backscatter, and subbottom data of the seafloor.

Examines the distributions of dihedral angles in unequilibrated rock textures for comparison with equilibrated rock textures. Nonrandom distribution of apparent dihedral angles in nonequilibrated rocks; Similarity in dihedral angle curves in equilibrated and unequilibrated textures.