Wide-Angle Seismic Imaging of Two Modes of Crustal Accretion in Mature Atlantic Ocean Crust
Wide-Angle Seismic Imaging of Two Modes of Crustal Accretion in Mature Atlantic Ocean Crust
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大西洋成熟地壳中两种地壳增生模式的广角地震成像
DOI：
10.1029/2019jb019100
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发表时间：
2020-06-01
影响因子：
3.9
通讯作者：
Henstock, T. J.
中科院分区：
文献类型：
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作者：
Davy, R. G.;Collier, J. S.;Henstock, T. J.
我们提出了一个高分辨率的2-D P波速度模型，从225公里长的活动地震剖面，收集类似于60-75马大西洋中部地壳。该轮廓穿过由一个变换偏移和三个非变换偏移分隔的五个脊段。所有脊不连续面都具有相似的主要特征，与偏移无关。我们确定了两种类型的地壳段。第一个显示了经典的两层速度结构，其中高梯度层2（类似于0.9 s（-1））位于低梯度层3（0.2 s（-1））之上。在这里，PmP与7.5 km s（-1）等值线重合，并且在1 km以下速度增加到>7.8 km s（-1）。我们解释这些部分岩浆的强大，PmP代表地壳和地幔之间的岩石学边界。第二个具有降低的速度梯度之间的对比度的上地壳和下地壳和PmP浅于7.5 km s（-1）的等值线。我们解释这些部分为构造为主，PmP代表一个蛇纹石化（蚀变）前。虽然速度-深度分布适合在以前的信封缓慢蔓延的地壳，我们的研究结果表明，这样的概括给人一种误导的印象均匀。我们估计，这两种地壳样式在大西洋海底的比例相等。在两个构造主导的部分，我们作出了第一个广角地震识别埋藏在成熟（>20马）大西洋地壳的洋核杂岩。它们具有类似于20公里宽的“穹隆”形态，基底浅，上地壳速度增加。我们解释他们的中地壳地震速度反演的蚀变和岩石类型组合的对比，跨地壳规模的拆离断层。
We present a high-resolution 2-D P-wave velocity model from a 225-km-long active seismic profile, collected over similar to 60-75 Ma central Atlantic crust. The profile crosses five ridge segments separated by a transform and three nontransform offsets. All ridge discontinuities share similar primary characteristics, independent of the offset. We identify two types of crustal segment. The first displays a classic two-layer velocity structure with a high gradient Layer 2 (similar to 0.9 s(-1)) above a lower gradient Layer 3 (0.2 s(-1)). Here, PmP coincides with the 7.5 km s(-1) contour, and velocity increases to >7.8 km s(-1) within 1 km below. We interpret these segments as magmatically robust, with PmP representing a petrological boundary between crust and mantle. The second has a reduced contrast in velocity gradient between the upper and lower crust and PmP shallower than the 7.5 km s(-1) contour. We interpret these segments as tectonically dominated, with PmP representing a serpentinized (alteration) front. While velocity-depth profiles fit within previous envelopes for slow-spreading crust, our results suggest that such generalizations give a misleading impression of uniformity. We estimate that the two crustal styles are present in equal proportions on the floor of the Atlantic. Within two tectonically dominated segments, we make the first wide-angle seismic identifications of buried oceanic core complexes in mature (>20 Ma) Atlantic Ocean crust. They have a similar to 20-km-wide "domal" morphology with shallow basement and increased upper crustal velocities. We interpret their midcrustal seismic velocity inversions as alteration and rock-type assemblage contrasts across crustal-scale detachment faults.