Revealing the Nature of Contemporary Uplift and Collapse in the Sierra Nevada - Great Basin System

内华达山脉今天的构造活动有多活跃？大盆地的扩张在多大程度上是由引力坍缩驱动的？内华达山脉和大盆地是如何相互作用的？内华达山脉在控制大盆地的崩塌和伸展中起什么作用？这些问题是EarthScope项目的核心任务，因为它们代表了太平洋-北美板块边界如何演变的谜团的关键部分，以及是什么过程造成的。然而，尽管大地测量学最近取得了进展，我们仍然无法回答任何学童可能会问的简单问题：“内华达山脉今天上升的速度有多快？”“大盆地是在上升还是下降？”为了解决这些问题，这个为期3年的项目将利用来自EarthScope板块边界观测站的高精度GPS数据来测量地壳的垂直运动，准确地描述内华达山脉-大盆地地区的隆起和崩塌。这些测量将为潜在过程提供关键的一阶证据。测量垂直运动很重要，因为它表明势能通量，这是构造/地幔动力学过程的基本特征，包括均衡、造山运动（造山运动）、重力塌陷和地幔上涌。为了使垂直GPS数据具有物理意义，人们正在实施一些方法，将地表高度的变化精确地参考到地球中心（地球质心）。为了将垂直速率的误差降低到一个有用的水平（每年亚毫米），研究人员正在利用GPS可观测建模的几个最新进展，包括全球范围的模糊分辨率、绝对天线校准、卫星发射机相位中心变化、GPS卫星上的非重力力建模和大气折射。一旦得到每个GPS站点垂直位置的精确时间序列，垂直变化的时空格局就可以根据感兴趣的构造过程以及非构造效应（如大气压力载荷和水文效应）进行解释。该项目的主要数据集是由专门设计的密集分布的GPS站点“PBO样带”获取的，该站点横跨东西向的整个大盆地和内华达山脉，纬度39º-40º，以及覆盖太平洋-北美板块边界及其以外的更广泛间隔的PBO网络。此外，内华达大学里诺分校自2004年以来建立并运行了约300个站点的“MAGNET”半连续GPS网络，为该项目提供数据。尽管来自MAGNET的测量并非一直连续，但初步研究表明，它们可以准确地跟踪GPS站高的季节和长期变化；因此，通过为垂直运动的复杂模式提供更多的空间细节，MAGNET补充了连续的PBO测量。到本项目结束时（2012年中期），试点研究预测，大多数GPS站的垂直速度精度为0.5毫米/年，这一水平有助于科学解释，并有助于回答项目的研究问题。

How tectonically active is the Sierra Nevada today? To what extent is Great Basin extension driven internally by gravitational collapse? How do the Sierra Nevada and Great Basin interact? What is the role of the Sierra Nevada in controlling collapse and extension of the Great Basin? These questions are central to the mission of the EarthScope Program, as they represent key parts to the puzzle of how the Pacific-North America plate boundary is evolving, and what processes are responsible. Yet, despite recent advances in geodesy, we still do not have answers to simple questions that any schoolchild might ask: "How fast is the Sierra Nevada going up today?"; "Is the Great Basin going up or down?"To address these questions, this 3-year project will accurately characterize uplift and collapse in the Sierra Nevada - Great Basin region by exploiting high precision GPS data from EarthScope's Plate Boundary Observatory to measure vertical motion of the Earth's crust. These measurements will provide crucial first-order evidence on the underlying processes. Measuring vertical motion is important because it indicates the flux of potential energy, a fundamental characteristic of tectonic/mantle dynamic processes, including isostasy, orogeny (mountain building), gravitational collapse, and mantle upwelling. To make the vertical GPS data physically meaningful, methods are being implemented that will accurately reference the changes in surface height to the center of the Earth (the Earth center of mass). In order to reduce the errors in vertical rates to a useful level (sub-millimeter per year), the investigators are making use of several recent advances in GPS observable modeling, including global-scale ambiguity resolution, absolute antenna calibrations, satellite transmitter phase center variations, the modeling of non-gravitational forces on the GPS satellites, and atmospheric refraction. Once an accurate time series of vertical position is derived for each GPS station, the spatial and temporal patterns of vertical variations are then interpreted in terms of both the tectonic processes of interest, as well as non-tectonic effects (such as atmospheric pressure loading and hydrological effects).The primary data set for this project is being acquired by the specially designed "PBO Transect" of densely-spaced GPS stations, spanning east-west the entire Great Basin and Sierra Nevada range at latitude 39º-40º, together with the more broadly spaced PBO network covering the Pacific-North American plate boundary and beyond. In addition, the University of Nevada, Reno, has since 2004 built and operated the ~300-station "MAGNET" semi-continuous GPS network, which provides data for this project. Although the measurements from MAGNET are not continuous all of the time, pilot studies show that they can accurately track seasonal and secular change in GPS station height; thus MAGNET complements the continuous PBO measurements by providing much more spatial detail to the complex patterns of vertical motions. By the end of this project (mid-2012), pilot studies predict that vertical velocities for most GPS stations have an accuracy of 0.5 mm/yr, at a level that is useful for scientific interpretation, and toward answering the project's research questions.