Fast instantaneous oceanic plate velocities recorded by the Cretaceous Laytonville limestone: Paleomagnetic analysis and kinematic implications
Fast instantaneous oceanic plate velocities recorded by the Cretaceous Laytonville limestone: Paleomagnetic analysis and kinematic implications
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白垩纪莱顿维尔石灰岩记录的快速瞬时海洋板块速度：古地磁分析和运动学意义
DOI：
10.1029/jb095ib10p15503
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发表时间：
1990
影响因子：
--
通讯作者：
N. Sleep
中科院分区：
文献类型：
--
作者：
J. Tarduno;Michael McWillams;N. Sleep
加州北方方济各中央带混杂岩带101-89 Ma莱顿维尔石灰岩的新古地磁结果证实了Alvarez等人（1980）首次提出的南半球起源。新的莱顿维尔石灰岩露头的发现和研究扩展了古地磁记录，并提供了确定的年龄，地层极性和沉积半球。古生物学测定表明，五个部分目前被推翻，四个是目前正面朝上，正如预期的随机分布的九块混杂岩。确切的吸积时间是未知的，但地质合理的估计范围在70和50 Ma之间，导致“平均”估计的向极速度在30和14厘米/年之间。“平均”值是通过将总的古纬度移动除以磁化和估计的吸积年龄之间的差异获得的;它们反映了在很长一段时间内的净向极迁移，并且是最小估计值，因为它们只记录了向极运动的分量。通过160个古地磁样品在1200万年内随年龄的减小而系统地倾斜减小，记录了一个“瞬时”极向速度。科.保守估计，这一最低速度为15厘米/年。通过平均速度计算预测的值与在露头内测量的值之间的一致性作为对磁化的主要性质的重要检查。这个最小速度比现今任何板块的速度都要高得多。在一个相对较小的板块边界的老洋壳的俯冲被假设为一个驱动机制来解释所测量的速度。再加上最小的瞬时极向速度，这一假设表明，俯冲板块的速度可能变化高达5倍。类似的板块几何形状和由此产生的速度可能在过去的其他时间存在，但它们的发现可能受到低保存潜力的阻碍。
New paleomagnetic results from 101–89 Ma Laytonville Limestone of the Franciscan Central Belt melange of northern California confirm a southern hemisphere origin as first proposed by Alvarez et al. (1980). Discovery and study of new Laytonville Limestone outcrops extend the paleomagnetic record and provide a definitive determination of age, stratigraphic polarity, and hemisphere of deposition. Paleontological determinations show that five sections are presently overturned and four are presently right side up, as expected from a random distribution of nine blocks in a melange. The exact accretion time is unknown, but geologically reasonable estimates range between 70 and 50 Ma, leading to “averaged” estimates of poleward velocity between 30 and 14 cm/yr. The “averaged” values are obtained by dividing the total paleolatitude shift by the difference between magnetization and estimated accretion ages; they reflect net poleward transport over a long time interval and are minimum estimates because they record only the poleward component of motion. An “instantaneous” poleward velocity is recorded by the systematic inclination decrease of 160 paleomagnetic samples with respect to decreasing age over the 12 m.y. section. Conservative estimates place this minimum velocity at 15 cm/yr. The consistency between the values predicted by the average velocity calculations and those measured within the outcrops serves as an important check on the primary nature of the magnetization. This minimum velocity is much higher than any present-day plate velocity. The subduction of old oceanic crust at the boundary of a relatively small plate is hypothesised as a driving mechanism to explain the measured velocities. Coupled with the minimum instantaneous poleward velocities, this hypothesis suggests that the velocities of subducting plates may vary up to a factor of 5. Similar plate geometries and resultant velocities have probably existed at other times in the past, but their discovery may have been hindered by low preservation potential.