Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones
Magnetic anisotropy of serpentinized peridotites from the MARK area: Implications for the orientation of mesoscopic structures and major fault zones
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MARK 地区蛇纹石化橄榄岩的磁各向异性：对细观结构和主要断层带方向的影响
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发表时间：
2002
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影响因子：
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通讯作者：
J. Karson
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作者：
R. M. Lawrence;J. Gee;J. Karson
[1]幔源蛇纹化橄榄岩既可沿断裂带沿着出露，也可在构造伸展极缓至中等扩张脊的区域出露，在这些环境中可能构成浅层地壳的重要部分。在这里，我们研究的潜力，剩磁数据和结构特征的蛇纹橄榄岩从大洋钻探计划（ODP）网站920（大西洋中脊南部凯恩，马克）提供见解的构造过程负责这些深埋岩石在海底的曝光。来自920号站点的214个样本的古地磁数据记录了一个非常一致的倾角（36.1° +0.8°/-1.4°），比预期的地心轴向偶极倾角（40.7°）或该站点的现今倾角（41.9°）都要浅。我们表明，在这些样品中的近univectorial剩磁很可能是一个部分热剩磁，可能增加了在中等温度下的粘性过程。这些特性是在从蛇纹石化发生的相对高温（>350°C）冷却期间获得的。因此，剩磁方向提供了一些信息的蛇纹岩隆升的最新阶段。然而，解释这一构造历史是复杂的存在一个明显的磁组构，这可能导致偏转的剩磁。我们使用磁化率各向异性和剩磁各向异性之间的关系来估计这种偏转的幅度和方向。校正后的反射倾斜（平均39.5°）更接近于该地点的时间平均倾斜，表明在蛇纹石化和冷却至350°C后，反射层几乎没有或没有可分辨的倾斜。考虑到剩磁的各向异性相关偏转，也使我们能够更准确地将岩芯内的各种结构特征恢复到其地理方向。在这种重新定位后，在这些岩石中占主导地位的中观叶理，定义的斜方辉石和次平行的蛇纹岩脉的首选方向，具有平均取向，密切平行的区域规模的断层带的西部中央谷壁。
[1] Mantle-derived serpentinized peridotites are exposed both along fracture zones and in areas of extreme tectonic extension at slow to intermediate spreading ridges and may constitute a significant volume of the shallow crust in these environments. Here we examine the potential of magnetic remanence data and structural features in serpentinized peridotites from Ocean Drilling Program (ODP) Site 920 (Mid-Atlantic Ridge south of Kane, MARK) to provide insights into the tectonic processes responsible for the exposure of these deep-seated rocks at the seafloor. Paleomagnetic data from 214 samples from Site 920 document a remarkably consistent inclination (36.1° +0.8°/-1.4°) that is shallower than either the expected geocentric axial dipole inclination (40.7°) or present-day inclination (41.9°) at the site. We show that the nearly univectorial remanence in these samples is likely to be a partial thermoremanence, possibly augmented by viscous processes at moderate temperatures. These properties were acquired during cooling from the relatively high temperatures (>350°C) at which serpentinization occurred. The remanence directions therefore provide some information on the latest stages of uplift of the serpentinite massif. However, interpretation of this tectonic history is complicated by the presence of a pronounced magnetic fabric, which presumably resulted in a deflection of the remanence. We estimate the magnitude and direction of this deflection using a relationship between the anisotropy of magnetic susceptibility and remanence anisotropy. The corrected remanent inclinations (mean 39.5°) more closely approximates the time-averaged inclination at the site, indicating that the massif experienced little or no resolvable tilt after serpentinization and cooling to 350°C. Accounting for the anisotropy-related deflection of the remanence also allows us to more accurately restore various structural features within the core to their geographic orientation. After this reorientation the dominant mesoscopic foliation in these rocks, defined by the preferred orientation of orthopyroxene and subparallel serpentine veins, has an average orientation that closely parallels the regional-scale fault zones on the western median valley wall.