History of rift propagation and magnetization intensity for the Cocos-Nazca sspreading Center
History of rift propagation and magnetization intensity for the Cocos-Nazca sspreading Center
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科科斯-纳斯卡扩张中心裂谷传播和磁化强度的历史
DOI：
10.1029/95jb00762
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发表时间：
1995
影响因子：
--
通讯作者：
R. Hey
中科院分区：
文献类型：
--
作者：
Douglas S. Wilson;R. Hey
对收集的几乎平行于构造流线的磁异常剖面进行分析，可以详细解释科科斯-纳斯卡扩张中心的复杂构造历史。考虑了扩展速率变化、脊跃变、不对称扩展、磁化强度变化和水深测量的磁异常正演模型与观测异常有很好的一致性。通过具有三个速率变化的异常4A，可以将扩展速率限制为共同的有限旋转历史。约1.5 Ma和4.1 Ma的速率变化对应于速率梯度的变化，并发生在反转时间尺度的校准部分，因此它们无疑可以被识别为板块运动的真实变化。∼15%的速率在5.2 Ma左右的下降可以被解释为板块运动的变化，或者是对时间标度的较老部分校准不佳的伪影。4.1 Ma的变化特别重要，因为许多时间尺度是基于该板块对0-6 Ma的恒定扩张率的假设。裂谷扩展在山脊轴几何结构的持续重组中起主导作用。传播主要是远离热点，跳跃主要是向南。繁殖速度从30毫米/年到120毫米/年不等，通常接近70毫米/年。大多数传播序列的起源很难解释，但许多似乎涉及到离散的向南脊跃，在热点附近形成了一个新的部分。磁异常幅度是熔岩中铁含量的可靠示踪剂。对于8 Ma以来磁化强度的沿轴变化，可以得出几个概括：仅在至少150公里长的区段的远端(相对于加拉帕戈斯热点)观察到高磁化；在高磁化区段末端的偏移量至少为15公里；高磁化度区段与热点重建位置之间没有大于30-45公里的偏移量。我们对这些模式的解释表明，在岩浆供应的梯度导致演化岩浆沿轴向流动的地方，分馏熔岩喷发。供给的梯度是由热区衍生的软流层在狭窄管道中的亚轴向流动造成的。这种流动只被20-30公里的偏移量部分阻挡，但被50公里的偏移量完全阻挡。
Analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the Cocos-Nazca spreading center. Forward models of the magnetic anomalies accounting for spreading rate variations, ridge jumps, asymmetric spreading, magnetization intensity variations, and bathymetry show excellent agreement with observed anomalies. Spreading rates can be constrained to a common finite rotation history through anomaly 4A with three changes in rates. Rate changes at about 1.5 Ma and 4.1 Ma correspond to changes in rate gradients and occur during the well-calibrated part of the reversal timescale, so they can unquestionably be identified as true changes in plate motion. A ∼15% rate decrease at about 5.2 Ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale. The change at 4.1 Ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0–6 Ma. Rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis. Propagation has been predominantly away from the hotspot, with jumps predominantly south-ward. Propagation rates have ranged from 30 to 120 mm/yr, commonly near 70 mm/yr. Origin of most propagation sequences is difficult to interpret, but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot. Magnetic anomaly amplitude appears to be a reliable tracer of Fe content of lavas. Several generalizations can be drawn about along-axis variations in magnetization intensities since 8 Ma: high magnetizations are only observed at the far ends (relative to the Galapagos hotspot) of segments at least 150 km long; offset at the end of a high-magnetization segment is at least 15 km; and there are no offsets larger than 30–45 km between high-magnetization segments and the reconstructed position of the hotspot. We interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along-axis flow of evolved magma. The gradients in supply result from subaxial flow of hotspot-derived asthenosphere in a narrow conduit. This flow is only partly obstructed by an offset of 20–30 km but entirely blocked by an offset of 50 km.