Lowrider: The Where, When, and How of Continental Crustal Underplating

技术说明：本项目采用高地震速度（称为“7”）的地壳下层全大陆测绘。x层（基于P速度大于7.0 km/s）来回答有关大陆地壳生长的基本问题：基性底沉积在元古代是否比稳定的太古宙地壳更普遍？2. 它是否与裂谷和地幔柱轨迹有关？3. 地壳内部的岩浆分异是否产生了基性下地壳？4. 在大陆碰撞时是否会发生海底沉积？结合可移动阵列上的“双莫霍”接收函数映射、接收函数和环境噪声表面波的联合反演、先前收集的折射数据、下地壳捕虏体以及7的时空关系。X层扩展到构造省，用于绘制7。并将其存在与大陆组合的时间序列和随后的构造过程联系起来。非技术解释/更广泛的意义：北美大陆地壳的组合和生长是一系列大陆块体之间碰撞、地壳增厚和伸展以及广泛融化的事件。大陆地壳通常是由地震波速（即地震波穿过物质的速度）比下面的地幔慢得多的物质组成的。然而，一些大陆构造过程导致了位于典型地幔和地壳值之间具有较高地震速度的下地壳层的形成。该项目利用全大陆EarthScope可移动阵列的地震数据绘制了这一高速下地壳层的地理范围。相对于地表绘制的地质区域和已知的大陆碰撞、分裂和地壳融化区域，该层的范围提供了有关该层就位时间和机制的信息。其他信息来自火山爆发期间被带到地表的地壳下部岩石，以及过去记录的地震爆炸数据。EarthScope提供了第一个机会来整理整个大陆宽度范围内的所有这些数据，以便回答有关大陆地壳如何形成的基本问题。

Technical description:This project uses continent-wide mapping of a lower crustal layer with high seismic velocity (termed "7.x layer" based on P velocities greater than 7.0 km/s) to answer fundamental questions about the growth of continental crust: 1. Is mafic underplating more common in Proterozoic than stable Archean crust, as previously postulated? 2. Is it associated with rifting and plume tracks? 3. Does magmatic differentiation within the crust generate a mafic lower crust? 4. Does underplating occur during continental collision? The combination of receiver function mapping of a 'double Moho' across the Transportable Array, joint inversions of receiver functions and ambient noise surface waves, previously collected refraction data, lower crustal xenoliths, and spatial and temporal relationships of 7.x layer extent to tectonic provinces is used to map the 7.x layer across the continental U.S. and relate its presence to the time sequence of continental assembly and subsequent tectonic processes.Nontechnical explanation / broader significance:The assembly and growth of the North American continental crust is a sequence of episodes of collisions between continental blocks, crustal thickening and extension, and widespread melting. Continental crust is usually composed of material that has much slower seismic wavespeeds (i.e., the velocity with which earthquake waves travel through the material) than the underlying mantle. However, several of the continent-building processes lead to the formation of a lower crustal layer with higher seismic velocities that lie in between typical mantle and crustal values. This project maps the geographic extent of this high-velocity lower crustal layer using seismic data from the continent-wide EarthScope Transportable Array. The extent of the layer relative to geological provinces mapped on the surface and known areas of continental collision, breakup, and crustal melting provides information on the timing and mechanism of emplacement of the layer. Additional information comes from lower crustal rocks that were transported to the surface during volcanic eruptions and from seismic explosion data that were recorded in the past. EarthScope offers the first opportunity to collate all these data across the entire width of a continent in order to answer fundamental questions about how continental crust is built.