Collaborative Research: A Test of the Out-of-Sequence Model for the Main Central Thrust, Western Nepal

最近对喜马拉雅造山带的研究中出现的最有趣的数据集之一是Harrison等人（1997年）和Catlos等人（2001年a，2001年b）报告的来自尼泊尔中部和东部与主中央冲断层（MCT）有关的变质岩中石榴石晶体中独居石包裹体的U-Th-Pb年龄。 一些独居石包裹体结晶，并纳入配子在晚中新世-上新世的时间。 地质温度和地质压力资料表明，变质温度为500 ~ 500 ℃，压力为8-12 kbar。 由于泥质沉积物中的独居石在埋藏过程中被破坏到矿物组合记录的深度，独居石年龄很可能记录了喜马拉雅造山作用期间石榴石生长的时间（Harrison等人，1998年）。 因此，独居石年龄包含着对喜马拉雅逆冲系统，特别是MCT及其近端下盘岩石的运动学重建至关重要的信息，对这些研究中提供的独居石年龄的解释表明，MCT在晚中新世被重新激活，MCT下盘的岩石逐渐被并入上盘并上升到地表。 许多独立的证据表明，这一假设可能是正确的，包括“o Ar/“Ar冷却年龄（Copeland等人，1991; Macfarlane等人，1992; Copeland等人，2001年）;（2）水准测量和GPS研究（杰克逊和Bilham，1994年; Bilham等人，1997; Larsen等人，1998年）;和（3）尼泊尔中部MCT区的新构造和地貌研究（例如，Bilham等人，1997年）。 虽然合理，MCT再激活假说包含了一些令人惊讶的运动学过程。 其中最重要的是，在晚中新世-上新世期间，MCT上发生了约40 km的滑动，以便将石榴石及其独居石包裹体输送到表面。 如果MCT确实被重新激活，它将是（到目前为止）有史以来记录的逆冲断层上最大的无序事件。 尽管在逆冲带模型中，序列外逆冲作用已被广泛接受，但它通常仅限于相对较小的位移（几公里）。 假设的规模的再活化事件将显着改变喜马拉雅褶皱冲断带如何运作，以及褶皱冲断带一般如何运作的现有概念。 可以想象的是，沿着尼泊尔的MCT的极端侵蚀速率沿着已经将褶皱冲断带转变为亚临界状态的近终端状态，停止其向前传播并完全重新组织主要冲断的轨迹。 因此，无序MCT假说是值得仔细和严格的审查。 了解年轻独居石年龄的关键在于获得最年轻独居石年龄的MCT以下岩石的结构。 不幸的是，在尼泊尔中部（独居石研究已经在那里进行）MCT下面的岩石的地层学和结构没有很好的记录。 MCT在油田中的确切位置仍然存在激烈的争论，因此样品的构造地层背景仍然存在疑问。 替代的顺序重新激活的MCT可以同样很好地解释年轻的独居石年龄。 在这项工作中，PI将在尼泊尔西部实施一项对顺序失调假设的关键测试。 他们将采集样品，用于含配子岩石的U-Th-Pb独居石测年和云母岩性的“Ar/“Ar测年，这些样品将沿沿着南北走向从南部的主边界逆冲断层到北部的藏南拆离体。 在过去的六年里，他们已经建立了尼泊尔西部MCT南部小喜马拉雅带的区域地层学、结构、地质年代学和Nd同位素地球化学（DeCelles等人，1998 a，1998 b，2000，2001;罗宾逊等人，2001年，2002年）。 他们建议从跨越MCT带的岩石中收集的石榴石中的独居石包裹体中获得U-Th-Pb年龄。 他们还将详细绘制该区域的地图，并收集样品进行U-Pb锆石和Nd同位素分析，以便在该区域准确定位MCT。 “Ar/“Ar冷却年龄应有助于记录逆冲岩席就位的区域历史，这将需要支持任何解释发生了什么沿着MCT。 建议的工作应有助于解决是否MCT经历了重大（几十公里）的滑动在晚中新世上新世的时间。 MCT问题的结果将对造山楔的一般模型产生影响，特别是快速侵蚀是否可以在MCT大规模复活所需的规模上重新定位主要逆冲作用的轨迹。 此外，拟议的“Ar/“Ar定年法应能为喜马拉雅地区逆冲岩席就位的时间确定提供前所未有的详细程度和精确度。 由于喜马拉雅山与青藏高原的生长和全球海洋化学的变化密切相关，PI的结果应该具有超越喜马拉雅山构造的应用。

One of the most interesting data sets to emerge from recent studies of the Himalayan orogenic belt consists of U-Th-Pb ages reported by Harrison et al. (1997) and Catlos et al. (2001a, 2001b) from monazite inclusions within garnet crystals in the metamorphic rocks associated with the Main Central thrust (MCT) in central and eastern Nepal. Some of the monazite inclusions crystallized and were incorporated into the gamets during late Miocenc-Pliocene time. Geothen-nometry and geobarometry data indicate that metamorphic temperatures ranged from 500'-SOO'C and pressures ranged from 8-12 kbar. Because detn'tal monazite in pelitic sediments is destroyed during burial to the depths recorded by the mineral assemblages, the monazite ages most likely record the timing of garnet growth during Himalayan orogenesis (Harrison et al., 1998). Thus, the monazite ages contain infon-nation that is vital for kinematic reconstructions of Himalayan thrust systems, particularly the MCT and its proximal footwall rocks.The interpretation of the monazite ages offered in these previous studies suggests that the MCT was reactivated during late Miocene time, and that rocks in the footwall of the MCT were progressively incorporated into the hanging wall and raised to the surface. A number of independent lines of evidence suggest that this hypothesis may be correct, including 'o Ar/ " Ar cooling ages (Copeland et al., 1991; Macfarlane et al., 1992; Copeland et al., 2001); (2) levelling and GPS studies (Jackson and Bilham, 1994; Bilham et al., 1997; Larsen et al., 1998); and (3) neotectonic and geomorphic studies of the MCT zone in central Nepal (e.g., Bilham et al., 1997). Although reasonable, the MCT reactivation hypothesis incorporates some surprising kinematic processes. Paramount among these is the requirement that approximately 40 km of slip on the MCT occurred during late Miocene-Pliocene time in order to convey the garnets and their monazite inclusions to the surface. If the MCT was indeed reactivated, it would be (by far) the largest out-of-sequence event on a thrust fault ever documented. Whereas out-of-sequence thrusting is now widely accepted in thrust belt models, it generally is restricted to relatively minor displacements (a few km). A reactivation event of the hypothesized magnitude would significantly alter current concepts of how the Himalayan fold-thrust belt operates, and how foldthrust belts in general operate. It is conceivable that the extreme rate of erosion along the MCT in Nepal has shifted the fold-thrust belt into a near terminal state of subcriticality, stalling its forward propagation and completely reorganizing the locus of major thrusting. Thus, the out-ofsequence MCT hypothesis is worthy of careful and critical examination. The key to understanding the young monazite ages lies in the structure of the rocks below the MCT from which the youngest monazite ages were obtained. Unfortunately, the stratigraphy and structure of the rocks below the MCT in central Nepal (where the monazite studies have been executed) are not well documented. Exact placement of the MCT in the field is still hotly debated, such that the tectonostratigraphic context of the samples remains in doubt. Alternatives to out-of-sequence reactivation of the MCT can explain equally well the young monazite ages. In this work, the PI's will implement a critical test of the out-of-sequence hypothesis in western Nepal. They will collect samples for U-Th-Pb monazite dating of gamet-bearing rocks and " Ar/ " Ar dating of micaceous lithologies along north-south transacts from the Main Boundary thrust in the south to the South Tibetan detachment in the north. They have already established the regional stratigraphy, structure, geochronology, and Nd isotope geochemistry of the Lesser Himalayan zone south of the MCT in western Nepal during the past six years (DeCelles et al., 1998a, 1998b, 2000, 2001; Robinson et al., 2001, 2002). They propose to obtain U-Th-Pb ages from monazite inclusions in garnets collected from rocks that span the MCT zone. They will also map the zone in detail and collect samples for U-Pb zircon and Nd-isotopic analysis in order to locate the MCT exactly in the field. The " Ar/ " Ar cooling ages should help to document the regional history of thrust sheet emplacement, which will be needed to support any interpretation of what occurred along the MCT. The proposed work should help to resolve whether the MCT experienced major (several tens of km) slip during late Miocene-Pliocene time. The result of the MCT question will have an impact on general models for orogenic wedges, in particular whether rapid erosion can relocate the locus of major thrusting on a scale required by large-scale reactivation of the MCT. In addition, the proposed " Ar/ " Ar dating should provide an unprecedented level of detail and precision for the timing of thrust sheet emplacement in the Himalaya. Because the Himalaya is intimately related to the growth of the Tibetan Plateau and changes in global ocean chemistry, the PI's results should have applications beyond Himalayan tectonics.