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）和Catlos等人（2001a, 2001b）从尼泊尔中部和东部与主中央逆冲（MCT）相关的变质岩中石榴石晶体中的独居石包裹体中报告的U-Th-Pb年龄。部分独居石包裹体在晚中新世-上新世期间结晶并合并到配子中。地温测量和地球气压测量数据表明，变质温度范围为500′- 50′c，压力范围为8-12 kbar。由于泥质沉积物中的齿状独居石在埋藏到矿物组合记录的深度时被破坏，所以独居石年龄很可能记录了喜马拉雅造山运动期间石榴石生长的时间（Harrison et al., 1998）。因此，独居石年龄包含了对喜马拉雅逆冲系统，特别是MCT及其近下盘岩石的运动学重建至关重要的信息。以往的研究对独居石年龄的解释表明，中新世晚期MCT被重新激活，MCT下盘的岩石逐渐被纳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）；(2)水准和GPS研究（Jackson and Bilham, 1994; Bilham et al., 1997; Larsen et al., 1998）；(3)尼泊尔中部MCT带的新构造和地貌研究（如Bilham et al., 1997）。虽然合理，但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，在中新世晚期至上新世期间，MCT上发生了大约40公里的滑动，以便将石榴石及其独居石包裹体运送到地表。如果MCT真的被重新激活，这将是（迄今为止）有记载的逆冲断层上最大的乱序事件。虽然逆冲带模型广泛接受了逆冲逆冲，但它通常局限于相对较小的位移（几公里）。假设震级的再激活事件将显著改变目前关于喜马拉雅褶皱-冲断带如何运作以及褶皱-冲断带一般如何运作的概念。可以想象，沿尼泊尔MCT的极端侵蚀速度已经将褶皱冲断带转移到亚临界的接近终点状态，使其向前传播停滞，并完全重组了主要冲断的位置。因此，无序MCT假说值得仔细和严格的检验。了解年轻独居石年龄的关键在于MCT以下岩石的结构，从中可以获得最年轻的独居石年龄。不幸的是，尼泊尔中部MCT（在那里进行了monazite研究）下面的岩石的地层和结构没有很好的记录。MCT在野外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。MCT无序再激活的替代方法同样可以很好地解释年轻独居石的年龄。在这项工作中，PI将在尼泊尔西部实施对无序假设的关键测试。他们将沿南至主边界冲断带至北至藏南分离带的南北构造采集含子岩U-Th-Pb独居石定年和云母岩“Ar/”Ar定年样品。在过去的六年里，他们已经建立了尼泊尔西部MCT以南小喜马拉雅带的区域地层、构造、地质年代学和Nd同位素地球化学（DeCelles et al., 1998a, 1998b, 2000,2001; Robinson et al., 2001,2002）。他们建议从从横跨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地区的地图，并收集样品进行U-Pb锆石和nd同位素分析，以便在野外准确定位MCT。“Ar/”Ar冷却年龄应该有助于记录逆冲板就位的区域历史，这将需要支持对MCT沿线发生的任何解释。这项工作将有助于解决中新世晚期-上新世期间MCT是否经历了大的（几十公里）滑动。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大规模重新激活所需的规模上重新定位主要逆冲的位置。此外，提出的“Ar/”Ar定年法应该为喜马拉雅地区逆冲板就位的时间提供前所未有的细节和精度。由于喜马拉雅与青藏高原的生长和全球海洋化学的变化密切相关，因此PI的结果应该可以应用于喜马拉雅构造以外的领域。

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.