Moho and lower crustal reflectivity beneath a young rift basin: results from a two-ship, wide-aperture seismic-reflection experiment in the Valencia Trough (western Mediterranean)
Moho and lower crustal reflectivity beneath a young rift basin: results from a two-ship, wide-aperture seismic-reflection experiment in the Valencia Trough (western Mediterranean)
复制标题
年轻裂谷盆地下方的莫霍面和下地壳反射率：巴伦西亚海槽（地中海西部）两船大孔径地震反射实验的结果
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发表时间：
1994
期刊：
影响因子：
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通讯作者：
A. Watts
中科院分区：
文献类型：
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作者：
J. Collier;P. Buhl;M. Torné;A. Watts
摘要
我们展示了巴伦西亚海槽下的下地壳和莫霍面的新图像，巴伦西亚海槽是地中海西部的一个年轻的裂谷盆地。这些图像是从两艘船的大口径反射实验中获得的，并显示了一些在以前可用的常规单船反射剖面上无法区分的特征。
以前只能断断续续地看到的莫霍面，现在可以自信地在整个盆地中追踪到。我们建立了今天的深度-莫霍面图，并估计了整个盆地的地壳减薄程度。地壳减薄以盆地中部最大，β值为3.15±0.25.在盆地边缘，β值下降到1.5±0.1.
盆地不同部位下地壳和莫霍面的反射特征不同。我们已经能够将这些差异与伸展的量联系起来。因此，我们解释说，观测到的地壳低反射率的变化是由最近的(新近纪)拉张事件造成的，该事件打开了巴伦西亚海槽。沿伊比利亚边缘有发育良好的低地壳反射率，由1-4公里长的1-2s双向时间(TWT)组成，近水平的反射体被解释为反射莫霍面的更连续的、尽管不是很强的反射体。在近海，这种下地壳反射单位迅速变薄，以至于在距离海岸线40-50公里处看不到它，那里的地壳被拉伸了1.7±0.1倍。随着较低的地壳反射率变得不可检测，反射莫霍面成为一个强劲的、连续的事件。然而，当β超过2.4±0.2时，莫霍面是一个弱事件，难以追踪。我们推测，伸展作用本身或与之相关的熔融作用显着削弱甚至破坏了盆地中央的低地壳反射率，并增强了中等伸展的莫霍面。
巴利阿里边缘有些反常，因为在巴伦西亚海槽伸展的同时，由于逆冲和褶皱作用，似乎存在地壳的弯曲载荷。下地壳显示出弱的但局部可变的下地壳反射率的证据。可能仅仅是因为莫霍面向下弯曲，从而限制了上地幔的减压，从而限制了上地幔的熔融，才保留了下地壳的反射率。这表明，熔融本身而不是伸展作用是下地壳改造的主要机制。
SUMMARY
We present new images of the lower crust and Moho beneath the Valencia Trough—a young rift basin in the western Mediterranean. These images were obtained from a two-ship, wide-aperture reflection experiment and show several features not distinguishable on previously available conventional single-ship reflection profiles.
The Moho, which was previously only seen intermittently, can now be confidently traced throughout the basin. We have constructed a present-day depth-to-Moho map and estimated the degree of crustal thinning for the whole basin. Crustal thinning is at a maximum in the centre of the basin, where β values reach 3.15 ± 0.25. At the margins of the basin the β value decreases to 1.5 ± 0.1.
The reflective character of the lower crust and Moho is different beneath different parts of the basin. We have been able to correlate these differences with the amount of stretching. We therefore interpret the variations of the observed lower crustal reflectivity as having been caused by the most recent (Neogene) stretching event that opened the Valencia Trough. Along the Iberian margin there is well-developed lower crustal reflectivity consisting of 1-2s two-way time (TWT) of 1-4 km long, near-horizontal reflectors underlain by a more continuous, although not significantly stronger, reflector interpreted to be the reflection Moho. Offshore, this lower crustal reflective unit thins rapidly, such that it is undetectable 40-50 km from the coastline where the crust has been stretched by a factor of 1.7 ± 0.1. As the lower crustal reflectivity becomes undetectable the reflection Moho becomes a robust, continuous event. Where β exceeds 2.4 ± 0.2, however, the Moho is a weak event and difficult to trace. We infer that either the extension itself or associated melting significantly weakened or even destroyed the lower crustal reflectivity in the centre of the basin and enhanced the Moho where extension was moderate.
The Balearic margin is somewhat anomalous in that there appears to have been flexural loading of the crust due to thrusting and folding that occurred at the same time as extension in the Valencia Trough. The lower crust shows evidence of weak, but locally variable lower crustal reflectivity. It is possible that the lower crustal reflectivity was preserved simply because the Moho was flexed downward and so decompression, and hence melting, of the upper mantle was restricted. This suggests that the melting itself rather than the extension is the primary mechanism of lower crustal modification.