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由哈里森等人报告的U-Th-Pb年龄组成。(1997)和Catlos等人。(2001a，2001b)与尼泊尔中部和东部的主要中央逆冲(MCT)有关的变质岩中的Gamet晶体中的独居石包裹体。部分独居石包裹体在晚中新世-上新世结晶并混入石榴石中。大地测量学和地球重力测量数据表明，变质温度为500‘-800’℃，压力为8-12kbar。由于泥质沉积物中的独立独居石在埋藏过程中被破坏，达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的生长时间(Harison等人，1998)。因此，独居石年龄包含了对喜马拉雅冲断系统，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。这些研究提供的独居石年龄解释表明，MCT在中新世晚期重新激活，MCT下盘的岩石逐渐并入上盘并上升到地表。一些独立的证据表明，这一假设可能是正确的，其中包括：(1)40Ar/39Ar冷却年龄(Copeland等人，1991年；Macfarlane等人，1992年；Copeland等人，2001年)；(2)水准和GPS研究(Jackson和Bilham等人，1994；Bilham等人，1997；Larsen等人，1998)；以及(3)尼泊尔中部MCT带的新构造和地貌研究(如Bilham等人，1997)。尽管MCT再激活假说是合理的，但它包含了一些令人惊讶的运动学过程。其中最重要的是，为了将配子及其独居石包裹体输送到地表，MCT在晚中新世-上新世期间发生了大约40公里的滑动。如果MCT真的被重新激活，这将是(到目前为止)有记录的逆冲断层上最大的无序事件。尽管无序推覆作用现在在逆冲带模型中被广泛接受，但它通常仅限于相对较小的位移(几公里)。假想规模的重新激活事件将显著改变喜马拉雅褶皱冲断带如何运行的现有概念，以及褶皱冲断带的总体运行方式。可以想象，尼泊尔MCT沿线的极端侵蚀速度使褶皱冲断带进入了近十年的亚临界状态，停止了向前传播，并完全重组了主要冲断的轨迹。因此，失序的MCT假说值得仔细和批判性地检验。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，尼泊尔中部MCT(进行独居石研究的地方)下方岩石的地层和结构没有得到很好的记录。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无序重新激活的替代方法同样可以很好地解释年轻的独居石年龄。在这项工作中，派将在尼泊尔西部对无序假说进行关键测试。他们将采集含伽玛特岩石的独居石U-Th-Pb定年和从南部主边界逆冲到北部藏南滑脱的南北交汇处云母岩的Ar/Ar定年。在过去的六年里，他们已经建立了尼泊尔西部MCT以南的小喜马拉雅带的区域地层、构造、年代学和Nd同位素地球化学(DeCelle等人，1998a，1998b，2000,2001；Robinson等人，2001,2002)。他们建议从横跨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。他们还将详细绘制区域地图，并收集样品用于U-Pb锆石和ND同位素分析，以便在野外准确定位MCT。40Ar/3‘Ar冷却年龄应该有助于记录逆冲片侵位的区域历史，这将需要它来支持对MCT沿线发生的任何解释。拟议的工作应有助于解决MCT在晚中新世-上新世期间是否经历了重大(数十公里)滑动。MCT问题的结果将对造山楔的一般模型产生影响，特别是侵蚀是否能以MCT大规模快速恢复活动所需的规模重新定位主要冲断的轨迹。此外，所提出的“Ar/3‘Ar测年”将为喜马拉雅地区冲断板侵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，PI的结果应该适用于喜马拉雅构造以外的地区。

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 gamet 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 garnets during late Miocene-Pliocene time. Geothen-nometry and geobarometry data indicate that metamorphic temperatures ranged from 500'-800'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 information 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 40 Ar/ 39 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 gamets 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 ten-ninal 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 micaccous 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 40 Ar/ 3' 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 'd erosion can relocate the locus of major thrusting on a scale required by large-scale rapi I reactivation of the MCT. In addition, the proposed " Ar/ 3' 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.