The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite
The Role of Pre-existing Mechanical Anisotropy on Shear Zone Development within Oceanic Mantle Lithosphere: an Example from the Oman Ophiolite
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DOI：
10.1093/petrology/egg099
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发表时间：
2004-02
影响因子：
3.9
通讯作者：
K. Michibayashi;D. Mainprice
中科院分区：
文献类型：
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作者：
K. Michibayashi;D. Mainprice
对阿曼蛇绿岩Hilti地幔剖面的构造和组构分析表明，剪切带的发育可能是由大洋板块碎裂引起的，受古山脊原有地幔组构的影响。在地幔剖面上详细的构造映射显示出一个东—西流动方向的平缓起伏构造。一条NW—SE走滑剪切带横贯该水平构造。橄榄石在叶理中的晶体优先取向（CPO）以（010）轴向模式为主，而不是更常见的（010）[100]模式，这表明接近莫霍的水平流动涉及非同轴流动。在低温下形成的剪切带内的橄榄石CPO具有（001）[100]模式和左旋剪切感。橄榄石的CPO随着糜棱化的进展和晶粒尺寸的减小而减弱，最终发展为具有随机CPO模式的超糜棱岩。橄榄石[010]轴始终是次垂直的，即使在剪切带内水平叶理已旋转到次垂直方向。这些观察结果表明，主要的力学各向异性（地幔结构）已经很容易转变为次要的构造（剪切带）。这是由于海洋拆离过程中橄榄石滑移体系的变化和冷却过程中相关构造的变化。我们认为原生橄榄石CPO组构可能在地幔的后续构造发育中发挥重要作用。因此，洋幔岩石圈在俯冲和逆冲过程中的构造行为可能受到洋扩张中心发育的初始力学各向异性的强烈影响。
Structural and fabric analysis of the well-exposed Hilti mantle section, Oman ophiolite, suggests that shear zone development, which may have resulted from oceanic plate fragmentation, was influenced by pre-existing mantle fabric present at the paleoridge. Detailed structural mapping in the mantle section revealed a gently undulating structure with an east---west flow direction. A NW---SE strike-slip shear zone cuts across this horizontal structure. The crystal preferred orientation (CPO) of olivine within the foliation is dominated by (010) axial patterns rather than more commonly observed (010)[100] patterns, suggesting that the horizontal flow close to the Moho involved non-coaxial flow. Olivine CPO within the shear zone formed at low temperature is characterized by (001)[100] patterns and a sinistral sense of shear. The olivine CPO becomes weaker with progressive mylonitization and accompanying grain size reduction, and ultimately develops into an ultra-mylonite with a random CPO pattern. The olivine [010]-axis is consistently sub-vertical, even where the horizontal foliation has been rotated to a sub-vertical orientation within the shear zone. These observations suggest that the primary mechanical anisotropy (mantle fabric) has been readily transformed into a secondary structure (shear zone) with minimum modification. This occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling. We propose that primary olivine CPO fabrics may play a significant role in the subsequent structural development of the mantle. Thus, the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center.