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最新研究中出现的最有趣的数据集之一是Harrison等人报道的U-Th-PB年龄。 （1997）和Catlos等。 （2001a，2001b）来自与尼泊尔中部和东部的主要中央推力（MCT）相关的变质岩石岩的独占夹杂物。 中新世晚期时期，一些单唑岩夹杂物结晶，并被掺入石榴石中。 地理数量和地理测量学数据表明，变质温度的范围为500'-800'C，压力范围为8-12 kbar。 由于在矿物质组合记录的深处埋葬期间，在毛状沉积物中的探测不可单唑被破坏，因此，独居石的年龄很可能记录了喜马拉雅造山过程中石榴石生长的时机（Harrison等，1998）。 因此，独居石的年龄包含的信息对于喜马拉雅推力系统的运动学重建至关重要，尤其是MCT及其近端脚壁岩石。对这些先前研究中的独占年龄所提供的解释表明，MCT在中期时期后期重新启动了MCT，并且在中新世晚些时候又在层面上逐渐构成了MCT的岩石，并逐渐被延伸到了悬挂的范围内，并逐渐构成了闲置的范围。 许多独立的证据表明，该假设可能是正确的，包括40 AR/ 39 AR冷却年龄（Copeland等，1991; MacFarlane等，1992; Copeland et al。，2001）; （2）水平和GPS研究（Jackson和Bilham，1994； Bilham等，1997； Larsen等，1998）； （3）尼泊尔中部MCT区的新染料和地貌研究（例如，Bilham等，1997）。 尽管合理，MCT重新激活假设包含了一些令人惊讶的运动过程。 其中最重要的是要求在中新世晚期 - 上新世时期发生大约40 km的MCT滑移，以便将配子及其独占夹杂物传达到表面。 如果确实重新激活了MCT，那将是（迄今为止）有史以来最大的推力故障。 尽管现在在推力皮带模型中被广泛接受，但通常仅限于相对较小的位移（几公里）。 假设的幅度的重新激活事件将显着改变喜马拉雅褶皱折叠皮带如何运行的当前概念，以及一般操作中的褶皱带。 可以想象，尼泊尔MCT沿着MCT侵蚀的极端速率已将折叠带的带转移到了近乎十的亚临界状态，使其正向传播停滞不前，并完全重组了主要推力的轨迹。 因此，MCT假设的序列值得进行仔细和批判性检查。 了解年轻的独居石时代的关键在于在获得最年轻的独居石时代的MCT下方的岩石结构。 不幸的是，尼泊尔中部MCT下方的岩石的地层和结构尚未得到充分证明。 MCT在该领域的确切位置仍在争论中进行了激烈的争论，因此样本的构造构造构成了疑问。 MCT的序列重新激活的替代方法可以很好地解释年轻的独居石时代。 在这项工作中，PI将对西尼泊尔西部的序列假设进行批判性检验。 他们将收集样品，以供带有配子的岩石岩石的u-th-pb ronazite约会，并从南北交易的岩石岩石岩石的“ ar/” ar art art，从南部的主要边界推力到北部的南藏族脱离。 在过去的六年中，他们已经建立了尼泊尔西部MCT以南的较小喜马拉雅地区的区域地层，结构，地质学和ND同位素地球化学（Decelles等，1998a，1998b，2000年，2000年，2001年； Robinson等，2000年； Robinson等，2001，2001，2002）。 他们建议从跨越MCT区的岩石收集的石榴石中获得U-Th-PB年龄。 他们还将详细绘制区域，并收集用于U-PB锆石和ND同位素分析的样品，以便将MCT精确定位在现场中。 40 AR/ 3'AR冷却年龄应有助于记录推力板的区域历史，这将是支持对MCT沿线发生的任何解释所需的。 拟议的工作应有助于解决中新世晚期的MCT是否经历了重大（几十公里）的滑移。 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根据MCT重新激活大规模Rapi I要求的规模重新定位主要推力的基因座。 此外，提议的“ AR/ 3'AR约会应为喜马拉雅山中推力板的时机提供前所未有的细节和精确度。由于喜马拉雅山与藏族高原的增长密切相关，并且与全球海洋化学的变化有关，因此PI的结果应超出梅拉亚扬（Himalayan）的应用。