Extension and Volcanism During Propagation of the Northwest Basin and Range into Southeastern Oregon

壮观的波纹地貌被称为北美西部的盆地和山脉省，主要是北美板块在科罗拉多高原和内华达山脉之间延伸的产物。正断层使山谷底部（“盆地”）相对于相邻的山峰（“山脉”）落下。该提案的一个主要目标是解决这个问题：这个壮观的地区是什么时候成为北美景观的一部分的？在北纬39度左右的内华达州，该省的宽度在三次主要的增加中向西增加了大约100公里，分别是在2600万到2200万年前，大约1500万到1400万年前，以及1000万到300万年前。沃克巷是沿盆地和山脉省西部边界的一个广泛的走滑和斜滑断层带，在2600万年前开始形成，并在900万年前向北扩展，接近北纬41度的喀斯喀特山脉南部。虽然盆地和山脉在东西和南北方向上的生长在内华达州相对广为人知，但很少有数据约束盆地和山脉类型的断层何时“到达”俄勒冈州。Grunder和Meigs正在对两个关键问题进行为期3年的调查：盆地和山脉是何时在俄勒冈州东南部形成的？是什么因素限制了它在俄勒冈州的发展？他们能够回答这些问题，因为盆地和山脉的发展都是在火山活动之前和伴随着火山活动。火山岩是解决这些问题的核心，因为用传统的同位素测年技术很容易确定它们的年代。本研究的主要观测内容是盆岭正断层与火山岩的关系。例如，可以根据断层与某一特定火山单元的就位时间是更早、更年轻还是同时期来确定某一特定地点开始伸展的时间。有了这些限制条件，研究人员就能够确定盆地和山脉何时扩展到俄勒冈州南部，扩张的速度是多少，单个断层的生长速度是多少，以及正常断层是由火山活动模式控制的，还是相反，断层控制了火山活动发生的位置。这些数据有助于目前关于火山作用在世界各地盆地和山脉型省的发展中的作用的争论。此外，它们提供了断层率的新视角，这有助于限制正常断层对俄勒冈州东南部人口中心（包括克拉马斯瀑布、湖景和本德）造成的地震危险。其他的社会效益包括培训俄勒冈州立大学的研究生和本科生，扩大未被充分代表的群体在科学领域的参与，以及将研究成果与正在进行的盆地-山脉地球物理研究（包括EarthScope）相结合。

The spectacular corrugated landscape known as the Basin and Range Province in western North America is the product of extension of the North American Plate largely between the Colorado Plateau and the Sierra Nevada. Normal faults drop valley bottoms (the 'basins') relative to adjacent mountain peaks (the 'ranges'). A major goal of this proposal addresses the question: when did this spectacular region become a part of the North American landscape? In Nevada at approximately 39 degrees North, the province grew about 100 kilometers incrementally westward in width in three main increments, at 26 to 22 million years ago, approximately 15 to 14 million years ago, and 10 to 3 million years ago. The Walker Lane, a broad zone of strike-slip and oblique-slip faulting that lies along the western boundary of the Basin and Range Province, initiated after 26 million years ago and apparently propagated northward approaching the southern Cascade Range at 41 degrees North after 9 million years ago. Whereas the growth of the Basin and Range in east-west and north-south directions is relatively well known in Nevada, few data constrain when Basin and Range-style faulting "arrived" in Oregon. Grunder and Meigs are conducting a 3-year investigation into two key questions--When did the Basin and Range form in southeastern Oregon? And, what factors limited how far into Oregon it grew? They are able to answer these questions because the development of the basins and ranges was both preceded and accompanied by volcanic activity. Volcanic rocks are central to resolving the questions because they are easily dated using conventional isotopic dating techniques. The essential observation of this study is the relationship between Basin and Range normal faults and volcanic rocks. Timing of the initiation of extension at a given place can be identified, for example, depending on whether faults can be shown to be older, younger, or coeval with emplacement of a given volcanic unit. With these constraints, the investigators are able to determine when the Basin and Range propagated into southern Oregon, at what rate that expansion occurred, at what rate individual faults have grown, and whether the normal faulting was controlled by the pattern of volcanism or, conversely, whether the faults controlled where volcanism occurred. These data contribute to the ongoing debate about the role of volcanism in the development of Basin and Range-type provinces around the world. Moreover, they provide a new perspective on the rate of faulting, which helps constrain the seismic hazard posed by normal faults to the population centers of southeastern Oregon (including Klamath Falls, Lakeview, and Bend). Additional societal benefits include the training of graduate and undergraduate students at Oregon State University, broadening of participation of underrepresented groups in science, and integration of research results with ongoing geophysical studies in the Basin-Range, including EarthScope.