Collaborative Research: A four-dimensional view of deformation in the Eastern Alaska Range - where did the slip on the Denali fault go?

2002年，阿拉斯加中南部的德纳里断层在7.9级地震中破裂，这是有记录以来最大的大陆走滑地震（岩石块之间的水平运动）之一。这一事件引起了人们对这条很少被研究但很重要的断层的关注，这条断层穿越了阿拉斯加石油管道和阿拉斯加的两条主要高速公路。科学家预测迪纳利断层地震只有走滑运动，但相反，2002年的地震开始于一次逆冲地震（把一块岩石放在另一块岩石上），并沿着一条以前未知的逆冲断层抬升了阿拉斯加山脉的岩石。这一意想不到的地震隆升模式为科学界提供了逆冲断层可以吸收迪纳里走滑断层的横向运动的额外证据。如果这些类型的断层相互作用持续了数百万年，那么确定迪纳利断层旁边的逆冲断层的数量将有助于解决一个长期存在的争议，即整个迪纳利断层总共发生了多少位移，并解释为什么迪纳利断层四面被大山包围（形成阿拉斯加山脉）。揭开迪纳里断层附近逆冲断层和隆起的历史，不仅有助于解决这些有趣的科学问题，而且还有助于了解这些断层在过去有多重要，以及今天阿拉斯加山脉最大的地震危险在哪里。为了进一步了解该地区的断层如何将德纳里断层的横向滑动转化为阿拉斯加山脉的隆起，项目研究人员将在该地区安装7个临时地震仪来记录地震和地面运动。通过解释这些地震信号，这些数据可以产生断层在地下和山脉根部的三维ct扫描图像。项目调查人员还将进行地质测绘研究，以了解断层在地表的位置。地质图，结合岩石的三维图像，将显示出主要断层存在的深度，以及它们是否切割了整个地壳。该项目将涉及几种类型的同位素测年方法，以确定这些断层的运动时间和速度。年龄信息将为研究提供第四个维度，即时间，并将显示哪些断层在遥远的过去和最近的过去都是最活跃的。这项研究的最终目标是记录具有逆冲和走滑运动的断层是如何连接到德纳里主断层的，以及它们在抬升山脉中起什么作用。石油管道在走滑地震中没有断裂，但不能承受大的逆冲地震，研究结果对更新石油管道的地震危险性具有重要意义。该项目将为学生提供实地方法和实验室应用方面的培训，总体成果将通过外展讲座、口语表演以及高速公路路边解释标志的创作和安装向公众发布。迪纳里断裂带水平偏移史的现代和深部时间限制表明，在200公里的距离上，东向西变化，在新生代有多达360公里的滑动“缺失”。缺少的滑动已经通过地壳缩短和/或断层主链的走滑断裂被弥补的可能性尚未得到彻底的研究。与邻近的地壳块体相比，阿拉斯加山脉东部的古缝合带地壳明显较厚。初步结果表明，渐新世以来，高角和低角逆冲断层的组合作用是此次地壳增厚的重要组成部分。本研究将验证地壳缩短和增厚可能在减小迪纳里断裂走滑分量中起主要作用的假设。另一种假设是，缝合带内的走滑断裂是滑块流出的重要机制。这两种假设都没有被该地区以前的研究人员充分研究过。为了解决这一研究问题，项目研究人员将收集并结合地质图数据、热年代学和地震成像，以确定地壳缩短和增厚的机制、时间和程度，并将其与阿拉斯加山脉南部缝合带先前未被识别的走滑断层进行比较。7个宽带地震仪将安装在一个80乘30公里（平均间距15-20公里）的区域内，将提供比地球范围可移动阵列提供的更详细的地壳厚度和地壳内部不连续性视图。低温度时计将包括同位素系统，记录从400摄氏度到65摄氏度的冷却，当结合沉积物跟踪（碎屑锆石）和结构研究时，将提供该地区地壳结构的4-d分析。这些新结果与现有的热年代学、地质年代学和地震数据相结合，将对长期逆走滑断层的滑动消散方式产生重要的地质约束。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The Denali fault in south-central Alaska ruptured in a 7.9 magnitude earthquake in 2002, one of the largest continental strike-slip (horizontal motion of blocks of rocks past each other) earthquakes ever recorded. The event brought attention to this little-studied but major fault, which crosses the trans-Alaska oil pipeline as well as the two main highways in Alaska. Scientists predicted Denali fault earthquakes to have only strike-slip motion, but instead, the 2002 quake started as a thrust (putting one block of rock over another) earthquake and uplifted rocks in the Alaska Range along a previously unknown thrust fault. The unexpected earthquake uplift pattern provided the scientific community additional evidence that thrust faults can siphon lateral motion from the strike-slip Denali fault. If these types of fault interactions persist for millions of years, then determining the amount of thrust faulting next to the Denali fault could help solve a long-lived controversy of how much total displacement has taken place across the Denali fault and explain why the Denali fault is surrounded by large mountains on all sides (forming the Alaska Range). Unraveling the history of thrust faulting and uplift next to the Denali fault will not only help chip away at these intriguing scientific questions but will also inform how significant these faults have been in the past and where the greatest seismic hazards in the Alaska Range are today.To further understand how faults in the region may be transforming lateral slip from the Denali fault into uplift of the Alaska Range, project researchers will install 7 temporary seismometers in this area to record earthquakes and ground motion. By interpreting these seismic signals, the data can produce 3-dimensional CAT-scan type image of where the faults are in the subsurface and the root of the mountains. Project investigators will also conduct geologic mapping studies, which provide a view of where the faults are at the surface. The geologic maps, combined with 3D images of the rocks at depth, will show where major faults exist at depth and if they cut the earth's entire crust. The project will involve several types of isotopic dating methods to determine the timing and rate of motion on these faults. The age information will provide the fourth dimension to the study, time, and will show which faults have been most active in both the distant and recent past. The ultimate goal of the study is to document how faults that have thrust and strike-slip motion connect into the main Denali fault, and what role they play in uplifting the mountain range. The results are important for updating the seismic hazard potential for the oil pipeline, which survived the strike-slip earthquake without breaking but is not engineered to withstand a large thrust earthquake. This project will provide training for students in field methods and laboratory applications and the overall results will be distributed to the public through outreach talks, spoken word performances, and the creation and installation of a highway wayside interpretative sign.The modern and deep time constraints on the horizontal offset history of the Denali Fault have been shown to vary east to west over a distance of ~ 200 km, with as much as 360 km of slip "missing" in the Cenozoic. The potential that the missing slip has been taken up through crustal shortening and/or strike-slip faulting off the main strand of the fault has not been thoroughly investigated. The eastern Alaska Range has significantly thicker crust in an ancient suture zone region compared to neighboring crustal blocks. Preliminary results indicate that a significant component of this crustal thickening occurred since the Oligocene through a combination of high-angle and low-angle thrust faults. This study will test the hypothesis that crustal shortening and thickening could play a major role in reducing the strike-slip component of the Denali fault. An alternate hypothesis is that strike-slip faulting within the suture zone is the important mechanism for bleeding off the slip. Neither of these hypotheses have been fully investigated by previous researchers in this region. To address this research question, project researchers will collect and combine geologic map data, thermochronology, and seismic imaging to determine the mechanisms, timing, and extent of crustal shortening and thickening, and compare that to previously unrecognized strike-slip faulting in the southern Alaska Range suture zone. Seven broadband seismometers, which will be installed over a region of 80 by 30 km (average spacing 15-20 km), will provide a more detailed view of the crustal thickness and intra-crustal discontinuities than the EarthScope Transportable Array can provide. Low-T thermochronometry will include isotopic systems that record cooling from 400 to 65 degrees C, and when combined with sediment tracking (detrital zircons) and structural studies will provide a 4-d analysis of the region's crustal structure. These new results, integrated with existing thermochronology, geochronology, and seismic data, will yield important geologic constraints on how slip dissipates along long-lived transpressive strike-slip faults.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.