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

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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.