Oblique Non-Ridge Parallel Magmatic Layering and Foliations in Lower Crustal Gabbros from Hess Deep Rift
Abstract
The lower oceanic crust beneath fast-spreading ridges is thought to form by magmatic intrusion and subsequent flow. However, the accretionary mechanisms responsible for the formation of the lower crust are debated. To constrain accretionary models, we present an integrated rock fabric study of gabbroic rocks recovered at IODP Expedition 345 Site 1415 from Hess Deep, a tectonic rift at the western end of the Cocos-Nazca spreading center. Core sections from U1415 Holes J and P, 4850 meters below sea-level, revealed primitive gabbroic rocks from a dismembered lower crustal section that initially formed at the East Pacific Rise (EPR). Core from Hole J shows distinct cm-scale magmatic modal layering, whereas core from Hole P displays local irregular banding and a much weaker foliation. The synthesis of magnetic and structural measurements from the two most coherent sections of core from U1415 Holes J and P, based on consistent paleomagnetic remanence directions from adjacent discrete samples, provides some constraints on the initial orientation of layering and foliation. Solid angles between remanence directions and poles to layering/foliation or anisotropy of magnetic susceptibility (AMS) minimum axes can be used to constrain the range of possible initial orientations of foliations at the EPR. Samples from the upper portion of Hole J Unit II yield angles between the remanence directions and the poles to magmatic layering of 20°-40°, implying that layering strikes were initially oriented at high-angles (>45°) to the EPR-axis, with dips >45°. Magmatic fabrics in Hole P are less common and so we use AMS as a proxy for bulk silicate fabrics. The minimum principal axes of magnetic fabrics (i.e. Kmin) generally coincides with poles to magmatic foliations and b-axis maxima from plagioclase crystallographic preferred orientation, suggesting that AMS is a reasonable proxy for bulk silicate fabrics. Samples from Hole P Unit II yield a wide range of angles from 25°-65°, implying foliation strikes were initially oriented at >20° to the EPR. Similar, non-ridge parallel foliations have been documented in 30% of uppermost gabbro outcrops in the Oman ophiolite, but have not yet been observed from modern fast-spread crust. Although the origin is unclear, oblique gabbro fabrics must constrain accretionary models of lower crust formation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFMOS31D2069H
- Keywords:
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- 0450 Hydrothermal systems;
- BIOGEOSCIENCESDE: 1021 Composition of the oceanic crust;
- GEOCHEMISTRYDE: 3618 Magma chamber processes;
- MINERALOGY AND PETROLOGYDE: 3621 Mantle processes;
- MINERALOGY AND PETROLOGY