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利断层在7.9级地震中破裂，这是有记录以来最大的大陆走滑(岩石块相互水平运动)地震之一。这一事件引起了人们对这条鲜为人知但却很大的断层的关注，该断层横跨跨阿拉斯加的石油管道以及阿拉斯加的两条主要高速公路。科学家预测德纳利断层地震只有走滑运动，但2002年的地震始于一次逆冲(将一块岩石覆盖另一块岩石)地震，阿拉斯加山脉的岩石沿着以前未知的逆冲断层隆起。意想不到的地震抬升模式为科学界提供了更多证据，证明逆冲断层可以虹吸走滑的德纳利断层的侧向运动。如果这种类型的断层相互作用持续数百万年，那么确定德纳利断层旁边的逆冲断层的数量可能有助于解决德纳利断层发生了多少总位移的长期争议，并解释为什么德纳利断层被四面都是大山包围(形成了阿拉斯加山脉)。揭开德纳利断层附近的逆冲断层和隆起的历史，不仅有助于破解这些有趣的科学问题，还将了解这些断层过去的重要性，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利断层的侧滑转变为阿拉斯加山脉的隆起，项目研究人员将在该地区安装7台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，这些数据可以生成断层位于地下和山根的三维CAT扫描图像。项目调查人员还将进行地质填图研究，提供断层在地表的位置。地质图与岩石深处的3D图像相结合，将显示出主要断层在深处存在的位置，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，以确定这些断层上的时间和运动速度。年龄信息将为这项研究提供第四个维度--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如何连接到主要的德纳利断层，以及它们在抬升山脉中所起的作用。这些结果对于更新这条输油管道的地震危险性具有重要意义。这条输油管道在走滑地震中幸存下来，没有破裂，但没有设计成能够承受大推力地震。该项目将为学生提供野外方法和实验室应用方面的培训，总体结果将通过外展讲座、口头表演以及创建和安装公路路旁解说标志向公众传播。德纳利断层水平偏移历史的现代和深层时间限制已被证明在约200公里的距离内从东向西变化，在新生代有多达360公里的滑移“失踪”。缺失的滑动可能是通过地壳缩短和/或走滑断层离开断层主线而被吸收的，这一可能性尚未得到彻底调查。与邻近的地壳区块相比，阿拉斯加东部山脉在一个古老的缝合带地区有着明显更厚的地壳。初步结果表明，自渐新世以来，通过高角度和低角度逆冲断层的组合，地壳增厚的一个重要组成部分发生了。这项研究将检验这一假设，即地壳缩短和增厚可能在减少德纳利断层的走滑分量方面发挥主要作用。另一种假设是，缝合带内的走滑断层是滑脱出血的重要机制。这两个假说都没有被这一领域的前辈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来确定地壳缩短和增厚的机制、时间和程度，并将其与阿拉斯加南部山脉缝合带以前未发现的走滑断层作用进行比较。7台宽带地震仪将安装在80×30公里(平均间距15-20公里)的区域内，将比地球望远镜可移动阵列提供更详细的地壳厚度和地壳内不连续的情况。Low-T热计时将包括记录400至65摄氏度冷却的同位素系统，当结合沉积物追踪(碎屑锆石)和结构研究时，将提供该区域地壳结构的四维分析。这些新的结果与现有的热年代学、地质年代学和地震数据相结合，将对滑动如何沿着长寿的走滑断层消散产生重要的地质约束。这一裁决反映了NSF的法定使命，并通过使用基金会的智力优势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.