Magnetic susceptibility also allows precise comparison between zones of deformation and occurrences of oxide gabbro. Evaluation of this for the entire core would provide substance for an entire paper on its own. At this time, I consider only a small 19.5-m segment of core, which provides some guidance on this score (Fig. F28). Core between 944.5 and 964 mbsf comprises the longest single interval obtained during Leg 176 in which rock is continuously deformed. The great majority of the segment has at least gneissic texture (crystal/plastic deformation grade 2). Much of it is porphyroclastic (grade 3), and there are seven intervals in which the rock is mylonitic or ultramylonitic (grades 4 and 5). Dips are consistent and relatively steep, averaging ~40° in the core reference frame.
The segment spans nine lithologic intervals, the thickest of which are gabbros and olivine gabbros. There are three narrow intervals of oxide gabbro plus an olivine microgabbro and a troctolite at the base of the sequence. Contacts between intervals dip conformably to the deformation. There are three felsic veins. This short segment thus spans almost the entire lithologic variability of rock from Hole 735B in the complicated, alternating pattern that is characteristic of the entire core.
The magnetic susceptibility log is given on the right side of Figure F28 and the deformation log on the left. The fluctuations in magnetic susceptibility reveal far more complexity to the 19.5 m of rock than is represented by the lithologic intervals. More of the rock is oxide rich and a great deal more of it is at least oxide bearing, than is suggested by the presence of only three narrow intervals of oxide gabbros (709, 711, and 715). Indeed, there are 27 peak regions in magnetic susceptibility rising above background levels in this single short sequence of rocks, 16 of which have magnetic susceptibilities exceeding 2000 x 10-6 MU. Most of these, obviously, went undetected by visual inspection of the core. Probably this has to do with the difficulty of recognizing fine-grained and dispersed oxide minerals in hand specimens of deformed rock unless their abundance is very high. In strongly foliated gabbros, ilmenite and magnetite tend to be dispersed around elongate silicates and are aligned parallel to the fabric (Fig. F29A). In oxide gabbros, they are concentrated in more visible bands where the oxides form shadow zones between aligned pyroxene porphyroclasts (Fig. F29B). The actual proportion of "background" olivine gabbro and troctolite, given by the gray portions of the susceptibility curve in Figure F28, is only 35.3%, whereas peak regions comprise 64.7% of the deformed segment.
All rocks with at least porphyroclastic texture coincide with either peak regions or narrow spikes in magnetic susceptibility. One ultramylonite and six mylonites are present at contacts between oxide gabbros (blue susceptibility peaks) and more primitive olivine gabbro or troctolite (gray background). In each case, the more deformed rock has higher magnetic susceptibility. The upper and lower contacts of the 19.5-m segment are both deformed and oxide rich. The lower contact is the mylonitic oxide-rich seam at ~77 cm from the top of the section shown in Figure F20. It was not accorded the status of a lithologic interval. The upper contact is shown in Figure F30. Interval 710, olivine gabbro, has gneissic fabric, and the pegmatitic oxide gabbro of lithologic interval 709 above it is described as porphyroclastic, although its crystal/plastic deformation grade is 0. The unrolled image and the core photograph both show that there is a 2- to 3-cm seam of dark oxide gabbro, corresponding to the lower peak in the strong doublet in magnetic susceptibility plotted on the right. On the barrel sheet describing the core, this seam was included in interval 710, olivine gabbro, rather than in interval 709, the pegmatitic oxide gabbro. Interval 708 is an unusual fine-grained lithology, termed leucocratic disseminated-oxide troctolite, and it produced the upper peak of the high-susceptibility spike. Its contact with the olivine gabbro above it is inclined and parallel to several other contacts and deformation fabric just beneath, as indicated by the arrows next to the core photo, and farther downsection. The unrolled image suggests that pegmatitic material in interval 709 is actually distributed unevenly in fine-grained rock resembling that of interval 708, most of which wound up in the archive half of the core, whereas the working half was used for the core description.
The relationship between the distribution of oxide gabbros and deformation in this short segment of core epitomizes the principal structural problem of Hole 735B. The zones of greatest deformation coincide strongly with oxide gabbros where they are juxtaposed with more primitive olivine gabbros and troctolites. This can be asserted more confidently in light of correlations with magnetic susceptibility than it can just using visual descriptions of the core alone. Those provide only nine lithologic intervals without much clue about their relationships. However, this 19.5-m segment is a coherent structural entity in the core, bounded by a particular highly deformed oxide-rich lithology at both the top and bottom and with the same lithology internally punctuating the sequence at equivalently strongly deformed intervals at numerous points within. The intimate relationship between zones of deformation and oxide gabbros in this segment cannot be denied. The deformed oxide gabbros at the upper and lower contacts suggest that strongly differentiated, iron-rich magmas played a role in the structural emplacement of the entire segment, perhaps in the manner of lubricants, greatly reducing effective stress at the upper and lower shear boundaries.
The structural problem comes down to the question of whether deformation followed crystallization of strongly differentiated, iron-rich magmas that intruded more primitive rock or whether it facilitated migration of those differentiated magmas along dilatant zones of shear, the magmas then crystallizing while deformation was occurring (e.g., Natland et al., 1991; Natland and Dick, 2001). The choice depends in part on how closely zones of deformation correspond to occurrences of oxide gabbro. Obviously, if deformed rocks are not discerned visually as carrying oxide minerals, then the correspondence between deformation and oxide gabbros will be considered either weak or nonexistent. Magnetic susceptibility provides a great deal of additional information, more firmly demonstrating a connection between zones of deformation and the distribution of oxide gabbros.
As to the general case, Figure F31 is a histogram for the number of magnetic susceptibility measurements (N = 1342) of different intensities that correspond to crystal/plastic deformation grades 2-5 (gneissic to ultramylonitic fabrics). Fully 65.9% of all such whole-round core recovered during Leg 176 occurs in peaks or peak regions, leaving 34.1% in background olivine gabbros and troctolites. Seams of oxide gabbro with magnetic susceptibility >2000 x 10-6 MU constitute 32.4% of the measurements. Given the different proportions of rock in the core, oxide gabbros are approximately six times more likely to have gneissic or greater deformation fabric than olivine gabbro and troctolite. Leaving out gneissic rocks, the association of deformation and oxide gabbros is even more pronounced. Some 78% of porphyroclastic, mylonitic, and ultramylonitic rocks (deformation grades 3-5) are present at peaks and peak regions. Much of the remaining 22% is present in immediately adjacent olivine gabbros and troctolites, a consequence of the shear couple across contacts with strongly deformed oxide gabbros. An oxide gabbro (>2000 x 10-6 MU) is almost exactly 15 times more likely to have these strongly deformed fabrics than olivine gabbro or troctolite.