Mantle melting, lithospheric strength and transform fault stability: Insights from the North Atlantic
Mantle melting, lithospheric strength and transform fault stability: Insights from the North Atlantic
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地幔熔化、岩石圈强度和转换断层稳定性：来自北大西洋的见解
DOI：
10.1016/j.epsl.2021.117351
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发表时间：
2022
影响因子：
5.3
通讯作者：
Hey, Richard
中科院分区：
文献类型：
--
作者：
Martinez, Fernando;Hey, Richard
洋盆的错断脊转换构造是板块构造最突出的表现之一。然而，为什么这种构型比连续的发散边界更受青睐，这一点仍然没有得到解决。我们研究这个问题，使用地幔布格异常（MBA）在北大西洋对比脊系统。Reykjanes海岭北部的湾转换断层没有转换偏移，其特点是快速传播的熔融中心沿着线性轴和连续的MBA低。在南部，毗邻的大西洋中脊有典型的脊段，每个脊段下面都有一个离散的MBA“靶心”低，并被转换和非转换不连续性所抵消。我们假设，地幔熔融的模式，反映在MBA，创造化学和流变学的变化，有利于或阻碍转换断层形成的残留地幔。在脊段内，地幔熔融有效地提取了水，产生干燥而坚固的残余地幔。在岩段的末端，低程度的熔融和低效率的熔体提取保持潮湿和软弱的地幔。在线性Reykjanes海岭连续熔融和快速传播的熔融中心创建连续强大的地幔可能弱流变变化在高角度的开口方向，不利于转换断层。与此相反，稳定的分段地幔熔融大西洋中脊形成带的强和弱的残留地幔的扩张方向对齐，后者创造有利的剪切变形和转换断层的位置。我们的假设也解释了地球端元扩张速率缺乏转换断层。在超慢脊，整体熔化是有限的和不规则的，熔体提取是低效的。在超快速海脊，地幔熔融是普遍的，熔体提取是有效的。在这两种情况下，有组织的蔓延平行的成分流变变化不形成和转换断层是不利的。我们的模型表明，除了随着年龄的冷却和加强，地幔熔融的模式塑造了海洋岩石圈的流变结构和板块构造的几何形状。
The offset ridge-transform structure of ocean basins is one of the most prominent expressions of plate tectonics. Yet why this configuration is favored over a continuous divergent boundary has remained unresolved. We examine this issue using mantle Bouguer anomalies (MBAs) from contrasting ridge systems in the North Atlantic. The Reykjanes Ridge north of the Bight transform fault has no transform offsets and is characterized by rapidly propagating melting centers along a linear axis and a continuous MBA low. To the south, the adjoining Mid-Atlantic Ridge has typical ridge segments each underlain by a discrete MBA “bulls-eye” low and offset by transform and non-transform discontinuities. We hypothesize that the pattern of mantle melting, as reflected in the MBAs, creates chemical and rheologic variations in the residual mantle that either favor or hinder transform fault formation. Within ridge segments, mantle melting efficiently extracts water producing dry and strong residual mantle. At segment ends, low extents of melting and inefficient melt extraction preserve damp and weak mantle. On the linear Reykjanes Ridge continuous melting and rapidly propagating melting centers create continuous strong mantle with possibly weak rheologic variations at high angles to the opening direction, not favoring transform faults. In contrast, stable segmented mantle melting on the Mid-Atlantic Ridge forms bands of strong and weak residual mantle aligned in the spreading direction, the latter creating favorable locations for shear deformation and transform faults. Our hypothesis also explains the lack of transform faults at Earth's endmember spreading rates. At ultra-slow ridges, overall melting is limited and irregular and melt extraction is inefficient. At ultra-fast ridges, mantle melting is pervasive and melt extraction is efficient. In both cases, organized spreading-parallel compositional rheological variations do not form and transform faults are not favored. Our model implies that beyond cooling and strengthening with age, the pattern of mantle melting shapes the rheological structure of oceanic lithosphere and the geometry of plate tectonics.
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