Passive seismic study of a magma-dominated rift: the Salton Trough

非技术解释一个被广泛接受的范式从地壳厚度和热流的角度解释了构造主导的大陆裂谷(即大陆上正在伸展或已经伸展的地区，通过地震断层的伸展)的结构。没有等同的范式来解释魔力主导的裂谷(伸展的省份，火山及其潜在的火成岩侵入迫使伸展程度)的演变，主要是因为我们缺乏测试数值模型的观测数据集。了解这些裂谷盆地的形成和演化具有重要的社会意义，因为它们蕴藏着地球上大部分的S油气储量。美国国家科学基金会S地球物理计划通过这个项目与美国国家科学基金会S边缘和地球望远镜-科学计划合作，为一个裂谷提供一套详细的此类观测，在这个裂谷中，地壳和上地幔的关键特征被埋在一个深盆地之下，即加利福尼亚州最南端的索尔顿海槽。萨尔顿海槽位于圣安德烈亚斯转换系统的南端，有很高的破坏性地震发生的风险，本项目中获得的所有新信息将成为社区南加州速度模型的一部分，该模型是整个地区评估强地面运动和地震危险性的基础技术描述萨尔顿海槽是连接加利福尼亚州海湾和圣安德烈亚斯断层系统的岩浆控制的裂谷。由于裂谷埋藏在一堆厚厚的科罗拉多河沉积物之下，令人惊讶的是，目前人们对地壳的侵入总量以及裂谷边缘内外的岩浆分布知之甚少。我们正在用岩石圈下部的速度测量来确定这些分布和体积。波速在很大程度上受地壳内成分的控制，而在最上层地幔内则受温度(和流体含量)的控制。确定下地壳岩性的最佳方法是将可控震源折射剖面获得的最可靠的P波速度与S波速度结合起来，后者通常更好地受到我们正在进行的联合面波/接收函数反演的约束。我们正在索尔顿海槽部署40个被动地震仪，标称间距为5公里，与NSF(边际-地球范围)-美国地质勘探局可控震源剖面共线。NSF-USGS的折射/反射剖面提供了结构约束，包括莫霍面旅行时，以及一个大大改进的P波速度模型。以前使用加利福尼亚州最南端的永久宽带站进行的面波研究将提供联合反演的相速度，并根据这里获得的数据计算出新的接收器函数。附加的构造约束(基于纵波反射和折射剖面)可以施加在联合反演上，以获得跨越岩浆裂谷的最佳确定的纵波-横波-岩性剖面之一。密集的接收函数和确定的速度剖面相结合，可以对板块边界(圣安德烈亚斯)走滑断层是抵消莫霍面还是消失在韧性中地壳进行新的测试。索尔顿海槽是加利福尼亚州海湾伸展省的最北端，它裂解了北美大陆，并将下加利福尼亚和半岛山脉转移到太平洋板块。尽管沿该省的总伸展相似，但产生了非常不同的伸展结构，从南部海湾的海底扩张到北部海湾和索尔顿海槽14-20公里厚的大陆或过渡地壳。横跨索尔顿海槽的新剖面提供了一个与最近确定的横跨墨西哥湾的剖面相比较的剖面，以加强我们对早期海洋裂谷及其向大陆传播的认识，并了解岩浆活动沿走向变化的原因。

Non-technical explanationA well-accepted paradigm explains the architecture of structurally dominated continental rifts (that is, regions of the continents that are stretching or have been stretched, by extension on earthquake-faults) in terms of crustal thickness and heat flow. No equivalent paradigm explains the evolution of magmatically dominated rifts (stretched provinces in which volcanoes and their underlying igneous intrusions force the amount of extension), largely because we lack the observational data-sets to test numerical models. Understanding the formation and evolution of such rift basins is societally important because they contain much of Earth?s hydrocarbon reserve. NSF?s Geophysics program through this project is teaming with NSF?s MARGINS and Earthscope-Science programs to provide a detailed set of such observations for a rift in which the key features in the crust and upper mantle are buried beneath a deep basin, the Salton Trough in southernmost California. The Salton Trough is located across the southern end of the San Andreas transform system that has high risk of producing a devastating earthquake, and all new information obtained in this project will become a part of the community Southern California velocity model that is the basis for assessing strong ground motion and earthquake hazard throughout that areaTechnical descriptionThe Salton Trough is a magma-dominated rift linking the Gulf of California to the San Andreas fault system. Because the rift is buried beneath a thick pile of Colorado River sediments, surprisingly little is currently known about the total volume of intrusion into the crust and the magma distribution within and beyond the rift margins. We are determining these distributions and volumes with velocity measurements in the lower lithosphere. Wavespeeds are largely controlled within the crust by composition, and within the uppermost mantle by temperature (and fluid content). Lithology of the lower crust is best determined by combining P-wave velocities that are most reliably obtained from controlled-source refraction profiles with S-wave velocities that are typically better constrained by the joint surface-wave/receiver-function inversions that we are carrying out. We are deploying 40 passive seismometers across the Salton Trough, at nominal 5-km spacing, collinear with the NSF(MARGINS-Earthscope)-USGS controlled-source seismic profile. The NSF-USGS refraction/reflection profile is providing structural constraints, including Moho travel-time, and a vastly improved P-wave velocity model. Previous surface-wave studies carried out using permanent broadband stations across southernmost California will provide the phase-velocities for joint inversions with new receiver functions calculated from the data acquired here. Additional structural constraints (based on the P-wave reflection and refraction profiling) can be imposed on the joint inversions to obtain one of the best-determined Vp-Vs-lithologic profiles across a magmatic rift. The combination of dense receiver functions and well-determined velocity profiles allow a new test of whether the plate-bounding (San Andreas) strike-slip faults offset the Moho or disappear into a ductile middle crust.The Salton Trough is the northernmost part of the Gulf of California extensional province that rifted the North American continent and transferred Baja California and the Peninsular Ranges to the Pacific plate. Despite similar total extension along the province, very different extensional structures have been produced, from seafloor spreading in the southern Gulf to 14-20 km thick continental or transitional crust in the northern Gulf and Salton Trough. The new profile across the Salton Trough provide a comparative section to those recently determined across the Gulf, to enhance our knowledge of early ocean rifting and its propagation into a continent, and to understand the causes of along-strike variation in magmatic activity.