Collaborative Research: Subduction Dynamics, Mantle Structure, and Cenozoic Tectonic Evolution of South America

南美构造的几个基本方面背后的物理学仍然难以捉摸。例如，各种地质记录表明，安第斯山脉至少从4000万年前开始显著缩短。然而，古测高代理数据显示，直到2000万或1500万年前，安第斯山脉的大部分地表高度仍然相对较低，这一时间框架与缩短的历史不一致。此外，大约在3000万年前，沿着安第斯山脉中部的早期新生代弧火山活动突然向内陆移动了500多公里，形成了持续至今的广泛的硅火山活动。因此，这些构造事件之间缺乏明确的关系。一个重要的原因是，人们对南美洲地下的地幔结构及其动态演化仍然知之甚少。在该提案中，该团队计划开展一项多学科研究项目，试图提高我们对地表构造，深部地幔地震特性及其与大陆俯冲时间演化的地球动力学关系的观测记录的认识。通过收集更多与南美洲构造史和现今地幔内部有关的数据，他们将利用数据同化建立一个复杂的四维地球动力学模型，以便定量地再现过去的俯冲历史。最终，研究小组希望更好地了解南美大陆的新生代演化。该团队计划通过一项涉及地震学（由PI Beck领导）、地质学（由PI DeCelles领导）和地球动力学（由PI Liu领导）的合作研究来实现这一目标。他们建议在地球物理和地质观测的约束下，利用地球动力学模型再现南美洲的新生代俯冲史和相关的大陆变形。该项目包括对两所机构的几名研究生进行跨学科培训，也包括两所机构的本科生，他们将参与研究，所有学生将参加美国西部科迪勒拉的夏季实地考察，作为对南美科迪勒拉的模拟。这种多学科的结合将为学生在板块尺度上理解造山系统提供一个独特的机会。关于南美洲新生代构造史，存在着许多基本问题，包括安第斯山脉的不同步地壳缩短和地表隆升沉降史，以及晚新生代发生的神秘的安第斯山脉中部爆发岩浆活动。早期提出的关于南美洲新生代演化的地球动力学模型都不能同时解释所有这些构造记录，而且这些不同的物理过程在现实中是否能共存和相互作用尚不清楚。这些备选模型存在的一个重要原因是，由于我们对南美洲深部地幔结构的认识不完善，导致俯冲和地幔动力学不确定。地球动力学和构造演化的关键观测约束条件包括：现今地幔地震结构的改善，特别是下地幔地震结构的改善；南美西部构造变形、地表隆起和岩浆历史之间的关系得到更好的识别；与这些观测记录相一致的时变地球动力学模型。

The physics behind several fundamental aspects of the South American tectonics remains elusive. For example, various geological records suggest that the Andes Mountains started to shorten significantly since at least 40 million years ago. However, paleo-altimetry proxy data reveal that most of the surface elevation of the Andes remained relatively low until as late as 20 or 15 million years ago, a time frame that is inconsistent with the shortening history. Furthermore, the early Cenozoic arc volcanism along the central Andes suddenly shifted more than 500 km inland around 30 million years ago and formed widespread silicic volcanic activity that continues to the present. Consequently, a clear relationship among these tectonic events is lacking. An important reason is that both the mantle structures beneath South America and their dynamic evolution remain poorly understood. In this proposal, the team plans to carry out a multidisciplinary research project to attempt to improve our knowledge about the observational records of surface tectonics, deep mantle seismic properties, and their geodynamic relationship with the temporal evolution of subduction beneath the continent. By collecting more data related to the tectonic history of South America and to the present-day mantle interior, they will build a sophisticated 4-dimentional geodynamic model using data assimilation, in order to quantitatively reproduce the past subduction history. Ultimately, the team hopes to better understand the Cenozoic evolution of the South American continent. The team plans to achieve this goal through a collaborative research effort that involves seismology (led by PI Beck), geology (led by PI DeCelles), and geodynamics (led by PI Liu). They propose to reproduce the Cenozoic subduction history beneath South America and associated continental deformation using geodynamic models constrained by geophysical and geological observations. This project includes interdisciplinary training of several graduate students at two institutions, and also involves undergraduates at both institutions who will be involved in the research and will all students will participate in a summer field trip to western U.S. Cordillera as an analog for the South American Cordillera. This multi-disciplinary combination will provide a unique opportunity for the students to understand orogenic systems at plate scale.Multiple fundamental questions exist about the Cenozoic tectonic history of South America, including the asynchronous crustal shortening and surface uplift/subsidence history of the Andes, and the enigmatic Central Andean flare-up magmatism occurring during the late Cenozoic. None of the earlier proposed geodynamic models for the Cenozoic evolution of South America could simultaneously explain all these tectonic records and it is unclear whether these different physical processes could co-exist and interact in reality. An important reason for the existence of these alternative models is the uncertain subduction and mantle dynamics due to our imperfect knowledge of deep mantle structures beneath South America. Key observational constraints on the geodynamic and tectonic evolution include an improved present-day mantle seismic structure, especially in the lower mantle, a better identified relationship among structural deformation, surface uplift and magmatic history within western South America, and time-dependent geodynamic models that are consistent with these observational records.