Constraining the Architecture of Active Thrust Systems of the Central Nepalese Himalaya

尽管我们对喜马拉雅构造演化的认识最近取得了许多进展，但仍然存在一些重要的争议。其中一个中心是将喜马拉雅山峰从山脚下分离出来的戏剧性地形转变与其背后的主要变形结构之间的关系。这场争论的部分原因是主要中央冲断系统(MCT)的痕迹与地形前锋在地理上几乎重合。尽管MCT长期以来一直被大多数喜马拉雅地质学家认为是休眠的，但一些观察结果表明，MCT对更高的喜马拉雅山脉具有持续的差异抬升和折返作用，并向一些人暗示了MCT的积极作用：(1)至少17 Ma以来，MCT上盘中的结晶岩石一直是向孟加拉扇输送沉积物的主要来源；(2)微震活动呈线性趋势聚集，与MCT的地形转变和绘制的轨迹相吻合；(3)最近的大地测量研究表明，更高喜马拉雅山脉快速差异抬升；(4)晚中新世-上新世40Ar/39Ar冷却年龄和Th-Pb独居石年龄分别反映了MCT带现代深部折返和同运动变质作用。已经提出了三种具有根本不同构造含义的替代模式来解释地形转变：(1)MCT上中新世活动后的侵蚀撤退；(2)喜马拉雅唯一逆冲带地壳规模坡度以上的断层-弯曲褶皱变形；(3)MCT上的新近(上新世)或活动逆冲。如果我们要了解喜马拉雅造山楔是如何在中新世-最近的时间间隔内演化的，那么确定山脚-喜马拉雅-更高喜马拉雅过渡带是瞬时侵蚀前锋、地下斜坡的地形特征还是活动断裂系统的踪迹是至关重要的。每个模型都预测了独特的抬升和掘出的空间模式和时间，可以用来评估它们的相对优点。本研究项目通过多学科研究验证这些模型，结合构造制图、构造地貌学和专题40Ar/39Ar和(U-Th)/He年代学，以探索尼泊尔中部Burhi Gandaki和Trisuli流域山麓-高喜马拉雅过渡带的结构和热演化。这种地貌和地质年代学方法的结合旨在提供晚中新世到最近隆升和折返模式的几种独立测量--独立于目前用于约束地壳结构的现有大地测量和微地震数据--从而有助于为喜马拉雅造山楔的下一代演化模型提供信息。

Whipple0087508Despite many recent advances in our understanding of the tectonic evolution of the Himalaya, perhaps the quintessential collisional orogen, some important controversies remain. One of these centers on the relationship between the dramatic topographic transition separating the high Himalayan peaks from their foothills and the major deformational structures that underlie it. This controversy is fueled in part by the geographic near-coincidence of the trace of the Main Central Thrust (MCT) system with the topographic front. Although the MCT has long been considered dormant by most Himalayan geologists, a number of observations require continued differential uplift and exhumation of the Higher Himalaya and suggest to some an active role of the MCT: (1) crystalline rocks in the hanging wall of the MCT have been the dominant source of sediment delivered to Bengal fan since at least 17Ma; (2) microseismicity clusters on a linear trend that coincides with both the topographic transition and the mapped trace of the MCT; (3) recent geodetic studies indicate rapid differential uplift of the Higher Himalaya; and (4) late Miocene-Pliocene 40Ar/39Ar cooling ages and Th-Pb monazite ages indicate recent deep exhumation and synkinematic metamorphism in MCT zone, respectively. Three alternative models with fundamentally different tectonic implications have been advanced to explain the topographic transition: (1) erosional retreat following Miocene activity on the MCT; (2) fault-bend fold deformation above a crustal-scale ramp in the Himalayan Sole Thrust; and (3) recent (Pliocene) or active thrusting on the MCT. Determining whether the foothills-Higher Himalaya transition is a transient erosional front, the topographic signature of a subsurface ramp, or the trace of an active fault system is critical if we are to understand how the Himalayan orogenic wedge has evolved over the Miocene-Recent interval. Each model predicts distinctive spatial patterns and timing of uplift and exhumation that can be exploited to evaluate their relative merits. This research project is testing these models through a multi-disciplinary study, combining structural mapping, tectonic geomorphology, and topical 40Ar/39Ar and (U-Th)/He geochronology to explore the structural and thermal evolution of the foothills-Higher Himalaya transition in the Burhi Gandaki and Trisuli drainages of central Nepal. This combination of geomorphic and geochronologic approaches is designed to provide several independent measures of late Miocene to recent uplift and exhumation patterns -- independent of available geodetic and microseismic data currently used to constrain crustal architecture - and thus to help inform the next generation of evolutionary models for the Himalayan orogenic wedge.