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

岩浆主导的裂谷的被动地震研究：索尔顿海槽

• 批准号: 0911743
• 负责人: Simon Klemperer
• 金额: $ 25.24万
• 依托单位: Stanford University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2009
• 资助国家: 美国
• 起止时间: 2009-09-15 至 2015-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0911743&HistoricalAwards=false
• 关键词: Passive; seismic; study; magma; dominated

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.

非技术解释一种广为接受的范式从地壳厚度和热流角度解释了结构主导的大陆裂谷（即大陆正在拉伸或已经拉伸的区域，通过地震断层的延伸）的结构。没有等效的范式可以解释岩浆主导的裂谷（火山及其潜在的火成岩侵入物迫使其延伸的延伸省份）的演化，很大程度上是因为我们缺乏观测数据集来测试数值模型。 了解此类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的大量碳氢化合物储量。 通过该项目，美国国家科学基金会的地球物理学计划与美国国家科学基金会的 MARGINS 和 Earthscope-Science 计划合作，为裂谷提供一系列详细的观测结果，其中地壳和上地幔的关键特征埋藏在深盆地（加利福尼亚州最南端的索尔顿海槽）之下。索尔顿海槽位于圣安德烈亚斯变换系统的南端，具有产生毁灭性地震的高风险，该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，该模型是评估整个地区强烈地面运动和地震危险的基础技术描述索尔顿海槽是连接加利福尼亚湾和圣安德烈亚斯断层系统的岩浆主导的裂谷。 由于裂谷被埋在厚厚的科罗拉多河沉积物下面，令人惊讶的是，目前人们对侵入地壳的总体积以及裂谷边缘内外的岩浆分布知之甚少。 我们正在通过岩石圈下部的速度测量来确定这些分布和体积。地壳内的波速主要由成分控制，而最上地幔内的波速则主要由温度（和流体含量）控制。下地壳的岩性最好通过将从受控源折射剖面中最可靠地获得的纵波速度与横波速度相结合来确定，横波速度通常更好地受到我们正在进行的联合表面波/接收器函数反演的约束。 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（MARGINS-Earthscope）-USGS 受控源地震剖面共线。 NSF-USGS 折射/反射剖面提供了结构约束，包括莫霍面传播时间和大幅改进的 P 波速度模型。先前使用加利福尼亚州最南端的永久宽带站进行的表面波研究将提供用于联合反演的相速度，以及根据此处获取的数据计算出的新接收器函数。 可以对联合反演施加额外的结构约束（基于 P 波反射和折射剖面），以获得跨岩浆裂谷的最佳确定的 Vp-Vs 岩性剖面之一。 密集的接收器函数和明确的速度剖面相结合，可以对板块边界（圣安德烈亚斯）走滑断层是否抵消莫霍面或消失在韧性中地壳中进行新的测试。索尔顿海槽是加利福尼亚湾伸展省的最北端，该省使北美大陆裂谷并将下加利福尼亚州和半岛山脉转移到太平洋板块。尽管沿该省的总伸展相似，但产生了截然不同的伸展结构，从南部海湾的海底扩张到北部海湾和索尔顿海槽的 14-20 公里厚的大陆或过渡地壳。 索尔顿海槽的新剖面图为最近确定的整个海湾剖面图提供了一个比较剖面，以增强我们对早期海洋裂谷及其传播到大陆的了解，并了解岩浆活动沿走向变化的原因。