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

技术说明：该项目利用全大陆范围的高地震速度下地壳层（根据P波速度大于7.0公里/秒，称为“7.x层”）绘图，以回答有关大陆地壳生长的基本问题：是否如先前假设的那样，基性底侵作用在元古代比稳定的太古代地壳更常见？2.它是否与裂谷和地幔柱轨迹有关？3.地壳内岩浆分异作用是否产生镁铁质下地壳？4.大陆碰撞时会发生底侵作用吗？综合利用可移动阵列上的“双莫霍面”的接收函数映射、接收函数和环境噪声面波的联合反演、先前收集的折射数据、下地壳捕虏体，7.x层范围与构造省的空间和时间关系用于绘制横跨美国大陆的7.x层，并将其存在与大陆组装的时间序列联系起来，非技术性解释/更广泛的意义：北美大陆地壳的组装和生长是大陆块体之间碰撞、地壳增厚和伸展以及广泛熔融的一系列事件。大陆地壳通常由具有慢得多的地震波速度的材料组成（即，地震波穿过物质的速度）比下面的地幔更快。然而，几个大陆建设过程导致形成一个较低的地壳层，具有较高的地震速度，位于典型的地幔和地壳值之间。该项目利用来自全大陆EarthScope移动式阵列的地震数据绘制这一高速下地壳层的地理范围。该层相对于地表上绘制的地质区和已知大陆碰撞、断裂和地壳熔融区的范围，提供了关于该层就位时间和机制的信息。更多的信息来自火山爆发时被运送到地表的下地壳岩石和过去记录的地震爆炸数据。EarthScope提供了第一次整理整个大陆宽度的所有这些数据的机会，以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.