Is continental collision thick- or thin-skinned? Combining local seismicity with receiver functions in the Zagros Fold-and-Thrust Belt

伊朗扎格罗斯地区是一个早期的大陆-大陆碰撞带，阿拉伯板块向北-东北方向进入欧亚板块。一个山脉，高扎格罗斯山脉，已经建成；在其阿拉伯板块一侧，扎格罗斯褶皱-冲断带显示出厚厚的沉积盖层的强烈变形，这些盖层占据了大部分的缩短。大陆碰撞模式的末端成员有薄皮情况（没有基底参与，如美国西部的塞维尔造山运动）和厚皮情况（基底参与，如美国西部的拉拉米德造山运动）。扎格罗斯褶皱冲断带可以说是当今地球上年轻大陆碰撞带的最好例子，我们试图确定扎格罗斯在碰撞模型的光谱上的位置，以及扎格罗斯的不同部分是否可能以不同的方式变形。争论的焦点在于沉积物-基底界面的深度与地震活动的深度之间的关系。扎格罗斯地区是进行此类研究的一个很好的实验室，因为该地区仪器相对较好，并且经常经历中等震级的地震活动。我们将采用先进的重新定位方法来精确确定地震深度，并利用高频接收函数分析来绘制地震震源区的沉积物-基底界面。我们还将使用S-P延迟时间空间成像，以避免由于对地壳速度结构的不完全了解而导致估计深度的偏差。在构造活跃的地球上，板块之间是相对运动的，有些板块的碰撞持续了数千万年。在这样的碰撞中，大洋板块的密度足以沉入地幔；它们形成了一个狭窄的、明确的俯冲带，就像最近在日本、智利和印度尼西亚发生的大地震一样。大陆板块的浮力更大，当两个大陆板块相遇时，一个板块不会简单地下沉到另一个板块的下面；两侧有浮力的地壳块相互碰撞并堆积起来，形成山脉和高原，并造成大范围的地震带。喜马拉雅是一个长期的大陆碰撞的例子，它消耗了印度大陆的大部分地区，并使欧亚地壳从其板块边界变形了数千公里。伊朗的扎格罗斯是早期的大陆碰撞，阿拉伯向东北移动进入亚洲，并在此过程中经历了缩短、增厚和折叠。我们项目的目标是确定地壳的哪一层吸收了变形；研究结果将帮助科学家了解大陆碰撞从开始到以喜马拉雅山为代表的后期阶段是如何发展的。该项目也与地震灾害的研究直接相关。该项目涉及两名美国地震学家、一名伊朗地震学家和一名法国地质学家之间的国际合作。因此，这项研究是由nsf -地球物理和国际科学与工程办公室（OISE）共同资助的。国际合作使我们能够将新开发的分析技术应用于难以获取的地震数据，并在一个30多年来一直缺乏科学合作的地区促进科学合作。该项目包括美国和伊朗合作者之间的访问，以及由美国pi教授的伊朗学生地震学短期课程。

The Zagros region of Iran is an early-stage continent-continent collision zone, with the Arabian plate moving north-northeastwards into the Eurasian plate. A mountain range, the High Zagros, has been built as a result; on its Arabian plate side, the Zagros Fold-and-Thrust belt shows strong deformation of the thick sedimentary cover that takes up much of the shortening. Models of continental collision have as end-members the thin-skinned case (no basement involvement; e.g., the Sevier orogeny in the Western U.S.) and the thick-skinned case (basement involvement; e.g., the Laramide orogeny in the Western U.S.). The Zagros Fold-and-Thrust Belt is arguably the best example of a young continental collision zone on Earth today, and we seek to determine where on the spectrum of collisional models the Zagros lies, and whether different parts of the Zagros may be deforming in different styles. Much of the debate hinges upon the depth of the sediment-basement interface relative to the depth of seismicity. The Zagros region is an excellent laboratory for such a study because it is relatively well instrumented and experiences frequent seismicity at moderate magnitudes. We will apply advanced relocation methods to accurately determine earthquake depths and high-frequency receiver function analysis to map the sediment-basement interface in the source regions of the earthquake. We will also use imaging in S-P delay time space to avoid biases in estimated depths due to incomplete knowledge of the velocity structure of the crust. On the tectonically active Earth, plates are in movement relative to each other, with some plates colliding for tens of millions of years. Oceanic plates are dense enough to sink into the mantle during such a collision; they form a narrow, well-defined subduction zone, such as the ones that gave rise to recent megaquakes in Japan, Chile, and Indonesia. Continental plates are more buoyant, and when two continental plates meet, one does not simply sink underneath the other; the buoyant crustal blocks on each side collide and stack up, building mountains, high plateaus, and causing a wide zone of earthquakes. The Himalaya is an example of a long-lived continental collision that has consumed much of the Indian continent and deformed the Eurasian crust for thousands of kilometers from its plate boundary. The Zagros in Iran is an early-stage continental collision, with Arabia moving northeastwards into Asia and experiencing shortening, thickening, and folding in the process. Our project's aim is to determine which layers of the crust are absorbing the deformation; the results will help scientists understand how continental collisions develop between their onset and a late stage such as represented by the Himalaya. The project has direct relevance to research in earthquake hazards as well. This project involves an international collaboration between two U.S. seismologists, an Iranian seismologist, and a French geologist. Consequently, this research is funded jointly by NSF-Geophysics and the Office of International Science and Engineering (OISE). The international collaboration allows us to apply newly developed analysis techniques to difficult-to-access seismic data and to promote scientific cooperation in a region where it has been lacking for over 30 years. The project includes visits between the U.S. and Iranian collaborators and a short course in seismology for Iranian students, taught by the U.S. PIs.