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: 0944184
• 负责人: Drew Coleman
• 金额: $ 31.57万
• 依托单位: University of North Carolina at Chapel Hill
• 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 2016-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0944184&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个地震仪，以研究平板俯冲的原因和后果。与大多数板块以相对恒定的倾角下降到另一个板块下面的俯冲带不同，平板俯冲带的特点是板块下降到一定深度（在这种情况下约为100公里），然后变平，水平移动数百公里，然后重新下降到地幔中。正是这种不寻常的俯冲几何形状，可能导致了远离构造边缘的大型山脉的形成，比如美国西部的落基山脉或阿根廷中部的Pampeanas山脉。然而，对于这种类型的俯冲带是如何形成的，或者是什么过程将这种几何形状与大型内陆山脉的形成联系起来，人们知之甚少。现有的理论通常涉及俯冲海洋高原，其厚厚的地壳使其在某些深度具有中性浮力，直到地壳物质经历相变，增加其密度。当水平时，平坦的板块会释放水，但由于温度太低，不会形成通常在俯冲带观察到的火山活动。释放出来的水反而会积聚在两个板块之间，直到平板俯冲结束，这时水会与流入的热地幔物质相互作用，在地表产生大规模的火山活动。今天，世界上只有两个平板俯冲带。其中一个位于智利中部和阿根廷，并与Pampeanas山脉联系在一起。现在在这个地区进行了几次地震研究，并发现了令人惊讶的结果。在智利和阿根廷，似乎没有任何证据表明水平板块上方有水，但有证据表明二氧化硅被添加了进来。这可能有助于解释早期大陆的形成，早期大陆需要大量的二氧化硅富集来维持其浮力。然而，智利/阿根廷的平板比美国西部落基山脉的假定成因要窄得多。现今唯一存在的另一块平板，位于秘鲁南部，比它宽得多。因此，这是一个更好的类比。我们建议研究沿着这个广阔的平板俯冲区南半部的地幔结构，看看我们是否发现了与智利和阿根廷相似的结构，并更好地了解这种俯冲几何形状是否可能是落基山脉形成的原因。该项目将利用宽带地震学对秘鲁中南部的地壳、地幔岩石圈、下行板块和亚板块地幔进行成像，以提高我们对平板俯冲的理解。平板俯冲已经成为一个流行的概念，用来解释大量的地质观测，包括弧火山作用的停止，远离构造板块边缘的厚皮变形，以及高原的形成。然而，它作为一种解释的有用性部分是由于缺乏关于其起源的要求和其存在的后果的详细资料。也许最著名的古平板的引用是拉拉米德时期的法拉龙板块，据说它负责形成落基山脉和相关的烟灰煤爆发。一些人还将玻利维亚高原的形成归因于平板俯冲，理由是它的宽度和火山历史。然而，要使这些理论成为真正可测试的假设，需要对两个关键问题有更好的约束：1)平板是如何形成的？2)它们对大陆岩石圈有什么影响？今天，这两块真正“平坦”的板块都位于南美洲边缘：一块位于南纬30度的智利中部和阿根廷西部以下，另一块位于南纬3度至15度之间的秘鲁大部分地区以下。然而，无论是宽（沿走向）还是宽（垂直走向），都不被认为是法拉龙平板造成所有相关的拉腊米德时代构造特征的必要条件。因此，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平板，那么了解哪些因素有助于平板的形成以及平板是如何动态支撑的至关重要。为了了解平板对大陆岩石圈的影响，我们需要了解下行平板的水平部分与上覆地壳底部之间的“填充物”的性质。对这种物质性质的严格限制（包括其成分、应力状态和随时间的演变）是理解平板俯冲和内陆地壳变形之间任何耦合的关键。CHARGE的部署研究了智利中部的平板，其结果与之前的一些假设相矛盾，并提出了许多关于平板俯冲性质的额外问题。秘鲁代表了目前存在的最宽的平板，因此可以被认为是研究Laramide时代平板可能是如何形成的最佳地点，以及我们今天可能期望能够找到什么样的地质观测结果。为了提供关于平板结构和动力学的这些基本问题的答案，我们建议在秘鲁平板的三个大致线性的横断面上部署40个宽带地震仪。这些仪器将部署大约两年，由此获得的数据集将提供一个前所未有的大而宽的平板段的工作情况。我们建议对数据进行多种分析，包括体波和面波层析成像、接收函数分析、横波分裂分析以及各种其他工具。这些分析与上述两个基本问题的研究密切相关，并将对地壳、地幔岩石圈、板块和亚板块地幔的各向同性和各向异性结构提供严格的约束。反过来，这些结构约束将提供有关地幔动力学、应力通过地壳和地幔岩石圈的传递以及改变大陆岩石圈的过程的信息。本项目由国际科学与工程办公室地球物理计划和美洲计划资助