Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhumation and three-dimensional tectonic processes
Structure of the actively deforming fold-thrust belt of the St. Elias orogen with implications for glacial exhumation and three-dimensional tectonic processes
复制标题
圣埃利亚斯造山带活跃变形褶皱冲断带的结构及其对冰川折返和三维构造过程的影响
DOI：
10.1130/ges00753.1
复制
发表时间：
2012
期刊：
影响因子：
2.5
通讯作者：
J. Spotila
中科院分区：
文献类型：
--
作者：
T. Pavlis;J. Chapman;R. Bruhn;K. Ridgway;L. Worthington;S. Gulick;J. Spotila
先前对阿拉斯加南部圣埃利亚斯造山带雅卡塔加褶皱冲断带的研究表明，高折返率与高山冰川作用有关。然而，这些研究仅对造成长期隆起变形的实际结构进行了初步处理。我们展示了横跨陆上褶皱冲断系统的两个走廊的详细地质测绘结果：Yakataga 角以西的 Duktoth 河横断面和圣埃利亚斯山地区的 Icy Bay 横断面。在德克托斯断面中，我们认识到较古老的、大致东西走向的结构，这些结构被开放的、西北走向的褶皱系统所覆盖，我们将其与东北走向、无序、可能活跃的逆冲断层系统联系起来。这些较年轻的结构叠印了褶皱逆冲叠层，其特点是结构复杂性可变，与沿含煤层位的分离折叠和始新世地层内的双工有关。在冰湾横断面中，我们认识到类似的结构风格，但不同的运动历史，受到同构造 Yakataga 地层底部角度不整合的限制。在缝合线附近的高构造层，构造显示出一致的西北走向，但在 Yakataga 组内相继较年轻的构造中，褶皱逆冲系统旋转至东西向至东北走向。我们为每个横断面提供了平衡横截面，其中我们从海上地震数据投影基底顶部，并假设具有与陆上横断面中观察到的结构相似但简化的双工系统的地下结构。这些剖面可以解释褶皱逆冲系统内 150-200 公里的缩短，我们的剖面修复也为在逆冲带中观察到的低温冷却年龄的细长靶心模式提供了一个简单的解释，这是由于生长的双相和/或反形式堆上方折返的结果。与模拟模型研究的比较表明，含煤地层中侵蚀和滑脱层发育之间的结构反馈导致了这种结构样式。尽管之前基于热年代学的研究表明逆冲带北缘存在活跃的逆冲作用，但剖面恢复表明逆冲作用是允许的，但现有数据并不要求存在逆冲作用。 Yakataga 褶皱冲断带一直被视为占主导地位的 2D 系统，但我们的工作表明 3D 过程很突出。在达克托斯断面中，我们将一组东北向的逆冲断层解释为较年轻的、无序的结构，是由于冰川侵蚀和近海沉积对造山楔的快速破坏而形成的。我们推断这些东北向的逆冲将滑移下倾转移到双系统中，形成了在横截面恢复中建模的反形式叠层，并且我们推断这些结构代表逆冲从主动逆冲锋面后退，试图重建造山楔，该楔的破坏速度与重建速度一样快，甚至更快。在冰湾横断面中，我们利用同构造 Yakataga 组下方的角度不整合提供的相对年代学来推断，早期的西北走向的褶皱-逆冲系统是沿着 Fairweather 变换形成的压压构造。持续的走滑将这些结构带入造山带费尔韦瑟段和雅卡塔加段之间的构造角，导致缩短轴逆时针旋转，直到岩石到达目前的位置。
Previous studies in the Yakataga fold-thrust belt of the St. Elias orogen in southern Alaska have demonstrated high exhumation rates associated with alpine glaciation; however, these studies were conducted with only a rudimentary treatment of the actual structures responsible for the deformation that produced long-term uplift. We present results of detailed geologic mapping in two corridors across the onshore fold-thrust system: the Duktoth River transect just west of Cape Yakataga and the Icy Bay transect in the Mount St. Elias region. In the Duktoth transect, we recognize older, approximately east-west–trending structures that are overprinted by open, northwest-trending fold systems, which we correlate to a system of northeast-trending, out-of-sequence, probably active thrusts. These younger structures overprint a fold-thrust stack that is characterized by variable structural complexity related to detachment folding along coal-bearing horizons and duplexing within Eocene strata. In the Icy Bay transect, we recognize a similar structural style, but a different kinematic history that is constrained by an angular unconformity at the base of the syntectonic Yakataga Formation. At high structural levels, near the suture, structures show a consistent northwest trend, but fold-thrust systems rotate to east-west to northeast trends in successively younger structures within the Yakataga Formation. We present balanced cross sections for each of these transects where we project the top of basement from offshore seismic data and assume a subsurface structure with duplex systems similar to, but simplified from, structures observed in the onshore transects. These sections can account for 150–200 km of shortening within the fold-thrust system, which is Our section restorations also provide a simple explanation for the observed elongate bullseye pattern of low-temperature cooling ages in the thrust belt as a consequence of exhumation above the growing duplex and/or antiformal stack. Comparison with analog model studies suggests that structural feedbacks between erosion and development of decollement horizons in coal-bearing strata led to this structural style. Although previous studies based on thermochronology suggested an active backthrust at the northern edge of the thrust belt, section restorations indicate that a backthrust is allowable but not required by available data. The Yakataga fold-thrust belt has been treated as a dominantly 2D system, yet our work indicates that 3D processes are prominent. In the Duktoth transect, we interpret a group of northeast-trending thrusts as younger, out-of-sequence structures formed in response to the rapid destruction of the orogenic wedge by glacial erosion and deposition immediately offshore. We infer that these northeast-trending thrusts transfer slip downdip into a duplex system that forms the antiformal stack modeled in cross-section restorations, and we infer that these structures represent thrusting stepping back from the active thrust front attempting to rebuild an orogenic wedge that is being destroyed as rapidly as, or more rapidly than, it is being rebuilt. In the Icy Bay transect, we use the relative chronology provided by an angular unconformity beneath the syntectonic Yakataga Formation to infer that early, northwest-trending fold-thrust systems were formed along the Fairweather transform as transpressional structures. Continued strike slip carried these structures into the tectonic corner between the Fairweather and Yakataga segments of the orogen, producing a counterclockwise rotation of the shortening axis until the rocks reached their present position.