The breakup of supercontinent Pangea occurred ∼200 Ma forming the Eastern North American Margin (ENAM). Yet, the precise timing and mechanics of breakup and onset of seafloor spreading remain poorly constrained. We investigate the relict lithosphere offshore eastern North America using ambient-noise Rayleigh-wave phase velocity (12–32 s) and azimuthal anisotropy (17–32 s) at the ENAM Community Seismic Experiment (CSE). Incorporating previous constraints on crustal structure, we construct a shear velocity model for the crust and upper ∼60 km of the mantle beneath the ENAM-CSE. A low-velocity lid (V_S of 4.4–4.55 km/s) is revealed in the upper 15–20 km of the mantle that extends ∼200 km from the margin, terminating at the Blake Spur Magnetic Anomaly (BSMA). East of the BSMA, velocities are fast (>4.6 km/s) and characteristic of typical oceanic mantle lithosphere. We interpret the low-velocity lid as stretched continental mantle lithosphere embedded with up to ∼15% retained gabbro. This implies that the BSMA marks successful breakup and onset of seafloor spreading ∼170 Ma, consistent with ENAM-CSE active-source studies that argue for breakup ∼25 Myr later than previously thought. We observe margin-parallel Rayleigh-wave azimuthal anisotropy (2%–4% peak-to-peak) in the lithosphere that approximately correlates with absolute plate motion (APM) at the time of spreading. We hypothesize that lithosphere formed during ultra-slow seafloor spreading records APM-modified olivine fabric rather than spreading-parallel fabric typical of higher spreading rates. This work highlights the importance of present-day passive margins for improving understanding of the fundamental rift-to-drift transition.

中文翻译：

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北美东部近海岩石圈结构和地震各向异性：对大陆分裂和超慢传播动力学的影响

超大陆 Pangea 发生了约 200 Ma 的分裂，形成了北美东部边缘（ENAM）。然而，破裂和海底扩张开始的精确时间和机制仍然缺乏约束。我们在 ENAM 社区地震实验 (CSE) 中使用环境噪声瑞利波相速度 (12-32 秒) 和方位各向异性 (17-32 秒) 来研究北美东部近海的残余岩石圈。结合先前对地壳结构的限制，我们为 ENAM-CSE 下方的地壳和上约 60 公里的地幔构建了一个剪切速度模型。低速盖 ( V _S4.4-4.55 公里/秒）出现在从边缘延伸约 200 公里的地幔上部 15-20 公里处，终止于布雷克 Spur 磁异常（BSMA）。在 BSMA 以东，速度很快（>4.6 km/s），具有典型的大洋地幔岩石圈特征。我们将低速盖解释为拉伸的大陆地幔岩石圈，其中嵌入了高达 15% 的保留辉长岩。这意味着 BSMA 标志着海底扩张的成功破裂和开始 ∼170 Ma，与 ENAM-CSE 活性源研究一致，该研究认为破裂比以前认为的晚约 25 Myr。我们观察到岩石圈中边缘平行的瑞利波方位各向异性（2%–4% 峰峰值），这与扩张时的绝对板块运动 (APM) 大致相关。我们假设在超慢海底扩张过程中形成的岩石圈记录了 APM 修饰的橄榄石结构，而不是具有较高扩散率的典型扩散平行结构。这项工作强调了当今被动利润率对于提高对基本裂谷到漂移过渡的理解的重要性。