Extension of continental crust at the margin of the eastern Grand Banks, Newfoundland
Extension of continental crust at the margin of the eastern Grand Banks, Newfoundland
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DOI：
10.1016/j.tecto.2008.05.030
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发表时间：
2009-04
期刊：
影响因子：
2.9
通讯作者：
H. Avendonk;L. Lavier;D. Shillington;G. Manatschal
中科院分区：
文献类型：
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作者：
H. Avendonk;L. Lavier;D. Shillington;G. Manatschal
地震和重力观测从东部大浅滩，纽芬兰，断裂边缘的大陆地壳延伸的陆架边缘ODP站点1277，地幔岩石被挖出的支持一个新的模型。我们发现，最大的地壳厚度减少，从约28公里到6公里，发生在大陆坡下的大浅滩超过20公里的距离。地壳厚度的快速减少与陆架边缘下地壳异常高的地震速度（7.0-7.2 km·s-1）相吻合。该区陆-洋过渡带薄壳，基底表面光滑，不含上地壳块体和裂前沉积物。我们比较我们的地球物理结果与地球动力学模型，代表裂谷的一个相对较热的大陆岩石圈和另一个数值模型，代表裂谷的一个寒冷的岩石圈。这两种地球动力学模型都认为，大陆坡下地壳减薄是由于上地壳的伸展断层作用和中地壳的韧性剪切带造成的。地球动力学模型解释了在断裂边缘形成明显的大陆斜坡的原因：在大浅滩大陆架之下，莫霍面和坚固的下地壳向上旋转，倾斜50°，但没有明显的内部变形。在裂谷侧翼的浅层，这些坚固的下地壳岩石的存在，随后有助于将延伸局限于更远的海洋。随着持续的伸展，一些高角度的正断层可能已经旋转到接近水平的方向，这可以解释地震反射数据中缺乏可见的脆性变形。这两种地球动力学模型在远缘产生了不同程度的陆壳伸展。热裂陷模型使应变更快地局部化，留下狭窄的伸展大陆地壳区域，并在裂陷的最后阶段产生相对大量的熔体（>30%）。大陆裂解可能在热岩石圈（<500万年）中迅速发生。另一方面，冷拉张模型将大陆地壳延伸到厚度小于10公里，远缘宽度为50公里，类似于我们在大浅滩东部推断的情况。冷岩石圈模型需要大约2300万年的伸展在大陆分裂之前，它预测少得多的熔融在地幔（13%）。裂谷持续时间长、陆壳减薄带广、岩浆活动量小，冷裂谷模式最适用于纽芬兰-伊比利亚裂谷。
Seismic and gravity observations from the rifted margin of the eastern Grand Banks, Newfoundland, support a new model for extension of the continental crust from the shelf edge to ODP Site 1277, where mantle rocks are exhumed. We find that the largest decrease in crustal thickness, from about 28 km to 6 km, occurs beneath the continental slope of the Grand Banks over a distance of just 20 km. This rapid decrease in crustal thickness coincides with anomalously high seismic velocities (7.0–7.2 km·s−1) in the lower crust of the shelf edge. The thin crust of the continent–ocean transition (COT) in this area has a smooth basement surface, void of upper crustal blocks and prerift sediments. We compare our geophysical results with a geodynamical model that represents rifting of a relatively hot continental lithosphere and with another numerical model that represents rifting of a cold lithosphere. Both geodynamic models suggest that crustal thinning beneath the continental slope was achieved by extensional faulting in the upper crust and ductile shear zones in the middle crust. The geodynamic models provide an explanation for the formation of distinct continental slopes at rifted margins: Beneath the continental shelf of the Grand Banks, the Moho and the strong lower crust rotated upwards toward to a 50° dip without visible internal deformation. The presence of these strong lower crustal rocks at shallow depth in the rift flank subsequently helped to localize the extension farther seaward. With ongoing extension, some high-angle normal faults may have rotated to a sub-horizontal orientation, which would explain the lack of brittle deformation visible in the seismic reflection data. The two geodynamic models produce different amounts of extension of continental crust in the distal margins. The hot rifting model localizes strain much more rapidly, leaving narrow zones of extended continental crust, and it produces a relatively large amount of melt (>30%) in the final stages of rifting. Continental breakup may occur rapidly in hot lithosphere (<5 Myr). On the other hand, a cold extension model extends the continental crust to a thickness smaller than 10 km over a width of 50 km in the distal margin, similar to what we inferred at the eastern Grand Banks. The cold lithospheric model requires about 23 Myr of extension before continental breakup, and it predicts much less melting in the mantle (13%). The long rift duration, wide zones of thinned continental crust, and small amount of magmatism make the cold rifting model the most applicable to Newfoundland–Iberia rift.