Collaborative Research: Study of the Peruvian flat slab and its effects on the continental lithosphere

合作研究：秘鲁平板及其对大陆岩石圈影响的研究

• 批准号: 0943991
• 负责人: Susan Beck
• 金额: $ 28.8万
• 依托单位: University of Arizona
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-08-01 至 2015-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0943991&HistoricalAwards=false
• 关键词: Collaborative; Research; Study; Peruvian; flat

This project will install 40 seismometers in southern Peru in order to study the causes and consequences of flat-slab subduction. Unlike most subduction zones where one plate descends beneath another plate at a relatively constant dip angle, flat-slab subduction zones are characterized by a descending plate that reaches some depth (in this case ~100 km) and then flattens, traveling horizontally for hundreds of kilometers before resuming its descent into the mantle. It is this type of unusual subduction geometry that may have caused the formation of large mountains far from tectonic margins such as the Rocky Mountains in the western United States or the Sierras Pampeanas in central Argentina.However, very little is known about how this type of subduction zone forms, or what processes link this geometry to the formation of large inland mountain ranges. Existing theories generally involve subducting oceanic plateaus whose thick crust makes them neutrally buoyant at some depth until the crustal material undergoes a phase change that increases its density. While horizontal, the flat-lying slab releases its water, but does not form volcanism as is usually observed in subduction zones because the temperatures are too low. The released water would instead accumulate between the two plates until flat-slab subduction ended, at which time the water would interact with inflowing hot mantle material to create a large flare up of volcanism at the surface. Today there are only two flat-slab subduction zones in the world. One is in central Chile and Argentina, and has been associated with the Sierras Pampeanas. Several seismic studies have now been performed in this area, and have found surprising results. There does not appear to be any evidence for water above the horizontal plate in Chile and Argentina, but there is evidence that silica has been added. This may help to explain the formation of early continents which required significant silica enrichment to maintain their buoyancy.The flat slab in Chile/Argentina is, however, much narrower than the one that has been postulated as the cause of the Rocky Mountains in the western United States. The only other flat slab in existence today, beneath southern Peru, is much broader than it?s counterpart in Chile, and is therefore a better analogue. We propose to investigate the structures in the mantle along the southern half of this broad flat-slab subduction region to see if we find similar structures to those found in Chile and Argentina, and to better understand whether such a subduction geometry could be responsible for the formation of the Rocky Mountains. This project will use broadband seismology to image the crust, mantle lithosphere, downgoing plate, and sub-slab mantle beneath south-central Peru in order to improve our understanding of flat slab subduction. Flat slab subduction has become a popular concept used to explain a wide host of geological observations including the cessation of arc volcanism, thick-skinned deformation far removed from tectonic plate margins, and the formation of high plateaus. Its usefulness as an explanation, however, stems in part from the paucity of details available on both the requirements for its genesis and the consequences of its existence. Perhaps the best-known invocation of a paleo-flat-slab is that of the Farallon plate during the Laramide, which was purportedly responsible for the formation of the Rocky Mountains and associated ignimbrite flare-up. Some have also attributed the formation of the Bolivian Altiplano to flat slab subduction, citing its width and volcanic history. However, to make these theories truly testable hypotheses, better constraints on two key questions are required: 1) How do flat slabs form? and 2) What effects do they have on the continental lithosphere? Today, both of the truly "flat" slabs lie along the South American margin: one below central Chile and western Argentina at ~30° S, and one beneath most of Peru between ~3° S and 15° S. These slabs vary greatly in size; however, neither is believed to be as wide (along strike) or as broad (perpendicular to strike) as the suggested Farallon flat slab must have been in order to have caused all of the associated Laramide-age tectonic features. It is therefore vital to understand what factors contribute to the formation of a flat slab and how flat slabs are dynamically supported if we are to comprehend whether these factors can reasonably be scaled upwards and be applied to the Laramide flat slab. In order to understand the effect of the flat slab on continental lithosphere, we need to understand the nature of the "filling" between the horizontal portion of the downgoing slab and the base of the overriding crust. Tight constraints on the nature of this material (including its composition, stress state, and evolution over time) are key to understanding any coupling between flat slab subduction and inland crustal deformation. Results from the CHARGE deployment, which studied the central Chilean flat slab, contradicted some previously held assumptions and raised many additional questions about the nature of flat slab subduction. Peru represents the widest flat slab currently in existence, and as such could be argued to be the best location to study how a possible Laramide age flat slab could have formed, and what kinds of geologic observables we might expect to be able to find today as a result. In order to provide answers to these fundamental questions about the structure and dynamics of flat slabs, we propose to deploy 40 broadband seismometers above the Peruvian flat slab in three roughly linear transects. The instruments will be deployed for approximately two years and the data set thus obtained will provide an unprecedented look into the workings of a large, broad flat slab segment. We propose to carry out a variety of analyses on the data, including body wave and surface wave tomography, receiver function analysis, shear wave splitting analysis, and a variety of other tools. These analyses are tied tightly to the investigation of the two fundamental questions outlined above and will provide tight constraints on the isotropic and anisotropic structure of the crust, mantle lithosphere, slab, and sub-slab mantle. In turn, these structural constraints will provide information about mantle dynamics, the transmission of stress through the crust and mantle lithosphere, and the processes which have modified the continental lithosphere.This project is supported by the Geophysics Program and the Americas Program of the Office of International Science and Engineering

该项目将在秘鲁南部安装40个地震米，以研究平板俯冲的原因和后果。与大多数俯冲带以相对恒定的倾角下降到另一盘板下方的大多数俯冲带不同，平板俯冲带的特征是降落的板块达到一定深度（在这种情况下为〜100 km），然后平坦，然后水平行驶数百公里，然后恢复其下降到地幔中。 It is this type of unusual subduction geometry that may have caused the formation of large mountains far from tectonic margins such as the Rocky Mountains in the western United States or the Sierras Pampianas in central Argentina.However, very little is known about how this type of subduction zone forms, or what processes link this geometry to the formation of large inland mountain ranges.现有的理论通常涉及俯冲海洋高原，其厚的外壳使它们在一定深度的中性浮力，直到地壳材料经历了增加其密度的相变。虽然水平，但扁平的平板释放了水，但由于温度太低，因此不会像俯冲区中通常观察到的火山一样。释放的水将在两个板之间积聚，直到平面俯冲结束为止，此时，水会与流动的热地幔材料相互作用，以在地面形成大型火山爆发。今天，世界上只有两个平板俯冲带。一个人在智利中部和阿根廷，与塞拉斯汤板植物有关。现在已经在该领域进行了几项地震研究，并发现了惊喜结果。似乎没有任何证据表明智利和阿根廷水平板上的水有任何证据，但是有证据表明已经添加了二氧化硅。这可能有助于解释早期持续的形成，这需要大量的二氧化硅富集来维持其浮力。但是，智利/阿根廷的平板比被释放为美国西部落基山的原因的扁平板要窄得多。当今唯一存在的其他平板板在秘鲁南部的下方，比智利的同行宽大得多，因此是一个更好的类似物。我们建议研究这个宽阔的平板俯冲区域沿着地幔中的结构，以查看我们是否发现与智利和阿根廷中发现的结构相似，并更好地了解这种俯冲几何形状是否可能导致落基山的形成。该项目将使用宽带地震学来对秘鲁中南部下区的地壳，地幔岩石圈，下板和次板地幔进行成像，以提高我们对平板俯冲的理解。平板俯冲已成为一种流行的概念，用于解释广泛的地质观测值，包括停止弧火山，皮肤厚的变形远距离从构造板缘去除，并形成高质量的。然而，它作为解释的有用性，部分原因是缺乏有关其起源要求及其存在后果的细节的匮乏。也许最著名的古平板锯齿是在拉amide期间的法拉隆板块的调用，据称是落基山脉和相关的Ignimbrite耀斑的形成的原因。有些人还归因于玻利维亚山利波拉的形成，理由是其宽度和火山历史。但是，要使这些理论真正可检验的假设，需要对两个关键问题的更好限制：1）平板如何形成？ 2）它们对连续岩石圈有什么影响？如今，两个真正的“平坦”平板位于南美边缘：智利中部和阿根廷西部的一个下方的板，在〜30°S下方，秘鲁大部分位于〜3°S和15°S下方。这些板的大小差异很大；但是，据信，这两种情况都不是宽（沿着罢工）或宽阔（垂直于罢工），而建议的Farallon扁平板一定是为了造成所有相关的Laramide-Age Age构造特征。因此，重要的是要了解哪些因素有助于形成平板的形成，以及如果我们要理解这些因素是否可以合理地向上缩放并应用于Laramide Flat Slab，则如何动态支撑平板。为了理解平板对连续岩石圈的影响，我们需要了解下板的水平部分之间“填充”的性质和覆盖层的底部。对该材料的性质的严格约束（包括其组成，应力状态和随着时间的流逝）是理解平板俯冲和内陆地壳变形之间的任何耦合的关键。研究了智利中部平板的指控部署的结果与一些先前的假设相矛盾，并提出了有关平板俯冲性质的许多其他问题。秘鲁代表了目前存在的最宽的平板，因此可以认为是研究可能形成的Laramide Age Age Flat Slab的最佳地点，以及我们可能希望今天能够找到哪种地质观察。为了提供有关扁平平板的结构和动力学的这些基本问题的答案，我们建议在三个大致线性样带中部署秘鲁平板上方的40个宽带地震仪。这些仪器将部署大约两年，因此获得的数据集将提供前所未有的查看，以了解一个大型宽平板细分市场的运作方式。我们建议对数据进行各种分析，包括身体波和表面波断层扫描，接收器功能分析，剪切波拆分分析以及各种其他工具。这些分析与对上面概述的两个基本问题的研究紧密相关，并将对地壳，地幔岩石圈，平板和sup-Slab地幔的各向同性和各向异性结构产生严格的约束。反过来，这些结构性约束将提供有关地幔动态的信息，压力通过地壳和地幔岩石圈的传播以及修改了连续岩石圈的过程。该项目得到了地球物理计划和国际科学和工程办公室的支持

项目成果

Foreland uplift during flat subduction: Insights from the Peruvian Andes and Fitzcarrald Arch

平坦俯冲期间的前陆隆起：来自秘鲁安第斯山脉和菲茨卡拉德拱门的见解

• DOI: 10.1016/j.tecto.2018.03.005
• 发表时间: 2018
• 期刊: Tectonophysics
• 影响因子: 2.9
• 作者: Bishop, Brandon T.;Beck, Susan L.;Zandt, George;Wagner, Lara S.;Long, Maureen D.;Tavera, Hernando
• 通讯作者: Tavera, Hernando