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

2002年，阿拉斯加中南部的迪纳利断层在一次7.9级地震中破裂，这是有史以来最大的大陆走滑（岩石块相互通过的水平运动）地震之一。这一事件引起了人们对这一几乎没有研究但却很重要的断层的关注，该断层横跨阿拉斯加输油管道和阿拉斯加的两条主要公路。科学家们预测德纳里断层地震只有走滑运动，但相反，2002年的地震是以逆冲（将一块岩石放在另一块岩石上）地震开始的，并沿着一个以前未知的逆冲断层沿着抬升了阿拉斯加山脉的岩石。意外的地震隆起模式为科学界提供了额外的证据，证明逆冲断层可以从走滑的德纳里断层虹吸横向运动。如果这些类型的断层相互作用持续了数百万年，那么确定德纳里断层附近的逆冲断层的数量可以帮助解决德纳里断层发生了多少总位移的长期争议，并解释为什么德纳里断层周围都是大山（形成阿拉斯加山脉）。揭开德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，而且还将告知这些断层在过去有多重要，以及阿拉斯加山脉今天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侧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，记录地震和地面运动。通过解释这些地震信号，数据可以产生三维CAT扫描类型的图像，显示断层在地下和山脉根部的位置。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，结合深度岩石的3D图像，将显示主要断层在深度上存在的位置，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，以确定这些断层的运动时间和速度。年龄信息将为研究提供第四维，时间，并将显示在遥远的过去和最近的过去哪些断层最活跃。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里断层，以及它们在抬升山脉方面发挥了什么作用。这些结果对于更新石油管道的地震危险潜力非常重要，石油管道在走滑地震中幸存下来而没有断裂，但没有设计成能够承受大推力地震。该项目将为学生提供现场方法和实验室应用方面的培训，并将通过外联讲座、口头表演以及创建和安装公路路边解释标志向公众发布总体结果。德纳里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 km的距离内从东到西变化，在新生代有多达360公里的滑动“缺失”。通过地壳缩短和/或断层主链的走滑断层作用，缺失的滑动已经被吸收的可能性还没有得到彻底的研究。与邻近的地壳块体相比，阿拉斯加山脉东部的古缝合带区域的地壳明显更厚。初步结果表明，这一地壳增厚的一个重要组成部分发生在渐新世以来，通过高角度和低角度的逆冲断层的组合。这项研究将测试的假设，地壳缩短和增厚可以发挥重要作用，减少走滑分量的德纳里断层。另一种假设是，缝合带内的走滑断层作用是滑脱流失的重要机制。这两种假设都没有被该地区以前的研究人员充分研究过。为了解决这个研究问题，项目研究人员将收集和联合收割机地质图数据，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，并将其与以前未被认识到的阿拉斯加山脉南部缝合带的走滑断层进行比较。7个宽频带地震仪将安装在80公里乘30公里的区域内（平均间距15-20公里），它们将提供比EarthScope移动式阵列更详细的地壳厚度和地壳内不连续性情况。低温热年代学将包括同位素系统，记录从400到65摄氏度的冷却，并与沉积物跟踪（碎屑锆石）和结构研究相结合，将提供该地区地壳结构的四维分析。这些新的成果，结合现有的热年代学，地质年代学和地震数据，将产生重要的地质约束滑动如何消散沿着长寿transpressive走滑future.This奖项反映了NSF的法定使命，并已被认为是值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估的支持。

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.