PALEOMAGNETIC RECORD OF PLATE-MARGIN TECTONIC PROCESSES ALONG THE WESTERN EDGE OF NORTH-AMERICA
PALEOMAGNETIC RECORD OF PLATE-MARGIN TECTONIC PROCESSES ALONG THE WESTERN EDGE OF NORTH-AMERICA
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DOI：
10.1029/jb085ib12p07115
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发表时间：
1980-01-01
影响因子：
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通讯作者：
BECK, ME
中科院分区：
文献类型：
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作者：
BECK, ME
北美西部边缘的古地磁记录显示出一致的、系统的不一致性。事实上，对位于大陆边缘数百公里范围内的上新世前岩石单元进行的大约36项高质量古地磁研究中，没有一项位于适当的参考极附近（根据克拉通的高质量古地磁数据构建）。这些不一致的古地磁研究也没有表现出简单的随机分散，就像预期的那些磁化后历史包括再加热、复杂的构造变形或化学变化的岩体一样。相反，它们的古地磁极明显地系统地偏离了参考曲线，进入了大西洋的一般区域。要实现这一点，构造上需要磁化岩体相对于北美向北移动，顺时针旋转，或两者兼而有之。古地磁记录的一致性表明，北美西部边缘的大部分地区都经历了这种块体运动，因此是异地的，其规模至少与典型的古地磁研究区域一样大。它进一步认为，向北运输和顺时针旋转已在塑造科迪勒拉的主要因素。科迪勒拉山脉不同部分的古地磁记录之间的差异可能反映了特定板块构造历史的差异。例如，尽管在门多西诺角以南和温哥华岛南部以北的区域（太平洋西北部），外来地块的北向搬运是重要的，但只发现顺时针旋转。这与第三纪大部分时间法拉隆板块对太平洋西北部海岸线的稳定逆冲作用相一致，这与南北方向的稳定或间歇性转换活动相反。岩石带，包括南加州的半岛山脉岩基和不列颠哥伦比亚省的海岸深成杂岩，似乎也参与了一般的向北迁移和顺时针旋转，尽管未被发现的构造倾斜的可能性掩盖了记录。然而，考虑到大型倾斜深成岩体的热行为，磁化后的倾斜可能很小。如果是这样的话，那么大部分西部的科迪勒拉白垩纪岩基（不包括内华达州山脉）也是外来的。向北迁移和顺时针旋转可以通过各种机制来实现，包括公认的板块构造过程。其中一些机制进行了讨论。然而，北美和西部板块之间的右旋剪切是每种机制的核心。看来，剪切也有可能破坏和内部修改旧的加积岩，产生内部块体旋转。这些较老的、被破坏的异地沉积物似乎被拉到了大陆西部边缘的高度衰减的构造纹层中。
The paleomagnetic record for the western edge of North America shows consistent, systematic discordance. Virtually none of roughly three dozen high‐quality paleomagnetic studies of pre‐Pliocene rock units located within several hundred kilometers of the continental margin are lying anywhere near their appropriate reference poles (constructed from high‐quality paleomagnetic data from the craton). Nor do these discordant paleomagnetic studies show simple random scatter, as might be expected for rock bodies whose postmagnetization histories include reheating, complex tectonic deformation, or chemical change. Instead, their paleomagnetic poles are clearly displaced systematically away from the reference curve into the general area of the Atlantic Ocean. To accomplish this tectonically requires that the magnetized rock body move northward, rotate clockwise, or both, in relation to North America. The consistency of paleomagnetic record argues that most of the western edge of North America has undergone such block movement and therefore is allochthonous, at a scale at least as large as the area of a typical paleomagnetic study. It further argues that northward transport and clockwise rotation have been prime elements in shaping the Cordillera. Differences between paleomagnetic records for separate parts of the Cordillera probably reflect differences in specific platetectonic histories. For instance, although northward transport of allochthonous blocks is important south of Cape Mendocino and north of southern Vancouver Island, in the area between (the Pacific Northwest), only clockwise rotations are found. This is consistent with steady under‐thrusting of the Pacific northwest coastline by the Farallon plate for most of the Tertiary, which is in contrast to the steady or intermittent transform activity to the north and south. Batholith belts, including the Peninsular Range batholith of Southern California and the Coast Plutonic Complex of British Columbia, also seem to have been involved in the general northward transport and clockwise rotation, although the possibility of undetected tectonic tilts clouds the record. However, consideration of the thermal behavior of large, tilted plutonic blocks suggests that post‐magnetization tilts probably are small. If so, then most western Cordilleran Cretaceous batholiths (not including the Sierra Nevada) also are allochthonous. Northward transport and clockwise rotation could be accomplished by a variety of mechanisms, involving recognized plate tectonic processes. Some of these mechanisms are discussed. However, dextral shear between North America and plates to the west is central to each mechanism. It seems likely that shear also has disrupted and internally modified older accreted terranes, producing internal block rotations. Such older, disrupted allochthonous terranes appear to be drawn out into highly attenuated tectonic laminae plastered on the western edge of the continent.