Collaborative Research: A test of the out-of-sequence model for the Main Central Thrust, Western Nepal

合作研究：尼泊尔西部主要中央冲断层的无序模型测试

• 批准号: 0208307
• 负责人: Peter Copeland
• 金额: $ 3.96万
• 依托单位: University of Houston
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2002
• 资助国家: 美国
• 起止时间: 2002-08-01 至 2005-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0208307&HistoricalAwards=false
• 关键词: Collaborative; Research; test; out; sequence

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.

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包括 Harrison 等人报告的 U-Th-Pb 年龄。 (1997) 和 Catlos 等人。 （2001a，2001b）来自与尼泊尔中部和东部主中央逆冲断层（MCT）相关的变质岩配子晶体内的独居石包裹体。 一些独居石包裹体在中新世晚期至上新世期间结晶并并入石榴石中。 地压测量和地压测量数据表明，变质温度范围为 500'-800'C，压力范围为 8-12 kbar。 由于泥质沉积物中的沉积独居石在埋藏至矿物组合记录的深度期间被破坏，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，1998）。 因此，独居石年龄包含对喜马拉雅逆冲系统的运动学重建至关重要的信息，特别是MCT及其近端下盘岩石。这些先前研究中提供的独居石年龄解释表明，MCT在中新世晚期重新激活，并且MCT 下盘的岩石逐渐融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 40 Ar/ 39 Ar 冷却年龄（Copeland 等，1991；Macfarlane 等，1992；Copeland 等，2001）； (2) 水准测量和 GPS 研究（Jackson 和 Bilham，1994；Bilham 等，1997；Larsen 等，1998）； (3) 尼泊尔中部 MCT 地带的新构造和地貌研究（例如，Bilham 等，1997）。 MCT 重新激活假说虽然合理，但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 上发生约 40 公里的滑动，以便将配子及其独居石包裹体输送到地表。 如果 MCT 确实重新激活，这将是（迄今为止）有记录的逆冲断层上最大的无序事件。 尽管乱序冲断现在在冲断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）。 假设规模的重新激活事件将显着改变目前关于喜马拉雅褶皱逆冲带如何运作以及褶皱逆冲带一般如何运作的概念。 可以想象，尼泊尔 MCT 沿线的极端侵蚀速率已将褶皱逆冲带转变为接近十级的亚临界状态，阻止了其向前传播并完全重组了主要逆冲的轨迹。 因此，乱序 MCT 假说值得仔细和批判性的检验。 了解年轻独居石年龄的关键在于 MC​​T 以下岩石的结构，从中获得了最年轻的独居石年龄。 不幸的是，尼泊尔中部 MCT 下方岩石的地层和结构（独居石研究已在那里进行）没有很好的记录。 MCT 在野外的确切位置仍然存在激烈争论，因此样本的构造地层背景仍然存在疑问。 MCT 失序重新激活的替代方案同样可以很好地解释独居石的年轻年龄。 在这项工作中，PI 将在尼泊尔西部对无序假设进行关键测试。 他们将沿着从南部的主边界逆冲断层到北部的藏南滑脱的南北走向，收集含配子岩石的U-Th-Pb独居石测年和云母岩性的“Ar/”Ar测年样品。 他们在过去的六年里已经建立了尼泊尔西部MCT以南小喜马拉雅带的区域地层、结构、年代学和Nd同位素地球化学（DeCelles等，1998a，1998b，2000，2001；Robinson等., 2001, 2002)。 他们建议从跨越 MCT 区域的岩石中收集的石榴石中的独居石包裹体获取 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，以便在现场准确定位 MCT。 40 Ar/ 3' Ar 冷却年龄应有助于记录推力片安放的区域历史，这将需要支持对沿 MCT 发生的情况的任何解释。 拟议的工作应有助于解决 MCT 在中新世晚期至上新世时期是否经历过重大（数十公里）滑移。 MCT 问题的结果将对造山楔的一般模型产生影响，特别是侵蚀是否可以按照 MCT 大规模 rapi I 重新激活所需的规模重新定位主要逆冲的位置。 此外，所提出的“Ar/3’Ar测年应该为喜马拉雅山逆冲片的就位时间提供前所未有的细节和精度。因为喜马拉雅山与青藏高原的生长和全球海洋的变化密切相关化学方面，PI 的结果应该具有喜马拉雅构造之外的应用。