Constraints on crustal structure of adjacent OCCs and segment boundaries at 13°N on the Mid-Atlantic Ridge
Constraints on crustal structure of adjacent OCCs and segment boundaries at 13°N on the Mid-Atlantic Ridge
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大西洋中脊邻近OCC和北纬13°段边界的地壳结构约束
DOI：
10.1093/gji/ggz074
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发表时间：
2019
影响因子：
2.8
通讯作者：
Peirce C
中科院分区：
文献类型：
--
作者：
Peirce C
大西洋中脊的13°N部分是一个在形态学上得到充分研究的缓慢扩张脊段，其中充满了海洋核心复合体（OCC）。在本文中，我们提出了一个200公里长的2-D地震和重力断面通过这一段，边界断裂带的南部和脊不连续的北部的结果。我们使用这个样带来考虑OCC演化的两个端元模型，其中一个称为分段尺度模型，意味着它们是相互联系的，它们的determination是单个段长特征的一部分，另一个称为Localizedmodel，每个OCC在结构上是孤立的。地壳一直是相对薄的平行于洋脊的，平均厚度为1.5km，在OCC之下，莫霍面标志着速度梯度过渡到地幔的顶部，而不是明显的速度不连续性。虽然每个OCC不是在相同的结构位置上穿越，但它们显示出不同的地壳速度-密度结构随深度的变化，这种结构中的沿着轴变化反映了它们之间的测深深度。较旧的OCC与当前活跃的13°20 'N OCC具有形成鲜明对比的速度深度特征。13°20′N OCC的独特之处在于它不像它的邻居那样在地壳较浅的深度显示出较高的相对速度，而13°30′N OCC的地壳明显较薄。我们的P波地震走时层析成像和重力正演模拟表明，13°N段的OCC在深度上并不相互连接。在13°30′N OCC以北，我们的模型还表明地壳正在被岩浆刷新，或海岭轴线目前正在经历岩浆增生，并伴随着海岭尖端的扩展，该扩展发生在海岭的不连续面上，该不连续面标志着海岭的北方边缘。该剖面还穿过标志着13°N段和南海岭南部界限的马拉松和墨丘利断裂带-变换交点外角。沿着剖面，马拉松断裂带比墨丘利断裂带（1.8亿年对1.11亿年）更年轻（1.11亿年对1.88亿年）的洋壳。当综合考虑时，地震和重力模型都表明，马拉松断裂带测深谷附近的地壳变薄，加上一个低密度区域，很可能反映了上地幔的蛇纹化。此外，断裂带之间捕获的地壳似乎绕东西向轴相对旋转，并向北抬升，向上运动位于测深凹陷的北方侧边缘，而不是其中心。无论是外隅角还是以断裂带为边界的地壳，其速度-深度特征都与13°N段的OCC相似，而不是正常增生的洋壳，特别是在中上地壳。总体而言，我们的研究结果支持OCC演化的局部化模型，并表明断裂带并不像现有模型所要求的那样在变形-断裂转变时立即锁定。
The 13°N segment of the Mid-Atlantic Ridge is an example of a morphologically well-studied slow spreading ridge segment populated with oceanic core complexes (OCCs). In this paper we present the results of an ∼200-km-long 2-D seismic and gravity transect through this segment, the bounding fracture zones to the south and the ridge discontinuity to the north. We use this transect to consider the two end-member models of OCC evolution in which one, referred to as theSegment-scalemodel, implies they are interconnected with their detachments being part of a single segment-long feature, and the other, theLocalizedmodel, that each OCC is structurally isolated.We show, using the 7.5 km s−1velocity contour as the base of crust marker, that the crust is consistently relatively thin ridge-parallel, at ∼5 km thick on average, and that, beneath the OCCs, the Moho marks the top of a velocity gradient transition into the mantle, rather than a distinct velocity discontinuity. Although each OCC is not traversed in an identical structural location, they show a different crustal velocity–density structure with depth, with along axis variations in this structure mirrored by the bathymetric deeps between them. Older OCCs have a contrasting velocity–depth signature to the currently active 13°20′N OCC. The 13°20′N OCC is distinct in that it does not show higher relative velocity at shallower crustal depth like its neighbours, while the 13°30′N OCC has an apparently thinner crust. Our combinedP-wave seismic traveltime tomography and gravity forward modelling suggests that the OCCs of the 13°N segment are not interconnected at depth. To the north of the 13°30′N OCC, our modelling also suggests that the crust is being magmatically refreshed, or that the ridge axis is currently undergoing magmatic accretion with an associated ridge tip propagation occurring across the ridge discontinuity that marks its northern edge.The profile also crosses the Marathon and Mercurius fracture zones that mark the southern limit of the 13°N segment and the southern ridge-transform intersection outside corner. Along profile, Marathon fracture zone offsets younger (∼1 Myr versus ∼8 Myr) oceanic crust than Mercurius fracture zone (∼8 Myr versus ∼11 Myr). When considered in combination, both seismic and gravity modelling suggest crustal thinning in the direct vicinity of the bathymetric valley of Marathon fracture zone, coupled with a region of low density that, most likely, reflects serpentinization of the uppermost mantle. In addition, the crust captured between fracture zones appears relatively rotated about an E–W axis and uplifted to the north, with the upwards motion accommodated on the northern lateral edge of the bathymetric depression rather than in its centre. Both the outside corner and the crust bounded by fracture zones have velocity–depth characteristics similar to that of the 13°N segment OCCs rather than normally accreted oceanic crust, particularly in the upper-to-middle crust.Overall, our results support theLocalizedmodel of OCC evolution and suggest that fracture zones do not become locked immediately on transform-to-fracture transition as current models dictate.
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