Collaborative Research: Stratigraphic Signatures of Orogeny: Assessing the Timing of Initial Andean Crustal Shortening

造山作用的地层学特征的性质仍然是一个争论的主题，在许多情况下，对造山运动开始的地层学估计远远落后于基于构造、变质和地质年代学数据的估计。作为高海拔弧后冲断带的典型代表，中央安第斯山脉提供了检验同造山带地层演化相互矛盾的模式的机会。该项目涉及安第斯造山作用的一个基本方面--由于地壳缩短和增厚而最初造山的时间。为此，来自亚利桑那大学、德克萨斯大学和怀俄明大学的一组研究人员与阿根廷科学家合作，记录了阿根廷西北部古近纪盆地的演化。目前正在验证三种盆地演化模式：(1)裂谷后热沉降，(2)前陆盆地沉降，(3)安第斯前陆构造破裂期间的山间盆地发展。每个模型都对沉积环境、沉积物分布模式、物源、盆地结构、沉降和热史作出具体预测。研究小组正在收集约3万平方公里区域内古近纪地层的沉积学数据(岩相、厚度、古流方向、物源)和年代学数据(40Ar/39Ar凝灰岩年龄、孢粉学、锆石和磷灰石U-Pb年龄和磷灰石裂变径迹年龄)。这项研究的预期成果包括阿根廷西北部古近纪地层学的全面数据集，以及两种新方法(碎屑锆石和磷灰石U-Pb年代学)在安第斯山脉中部早新生代构造问题上的协同应用。安第斯山脉被广泛认为是响应大陆之下大洋岩石圈俯冲而形成的山带的典型地区，基于安第斯山脉的模型经常被用来理解全球古代山带的发展。然而，安第斯造山作用的一个基本方面--由于地壳缩短和增厚而形成的初始造山年龄--仍然存在争议。通过应用新的分析技术和对沉积记录的解释，该项目将解决中央安第斯山脉的这一争议。该项目将为地质学家提供新的方法来确定过去地质历史上重大造山事件的时间。

The nature of the stratigraphic signature of orogeny continues to be a subject of debate, and in many cases stratigraphic estimates for the onset of orogeny lag significantly behind estimates based on structural, metamorphic, and geochronological data. As the type-example of a high-elevation retroarc thrust belt, the central Andes provide an opportunity to test conflicting models for synorogenic stratigraphic evolution. This project addresses a fundamental aspect of Andean orogeny - the timing of initial mountain building owing to crustal shortening and thickening. To this end, a team of researchers from the University of Arizona, the University of Texas, and the University of Wyoming in collaboration with Argentinean scientists are documenting the evolution of Paleogene basins in northwestern Argentina. Three models for basin evolution are being tested: (1) post-rift thermal subsidence, (2) foreland basin subsidence, and (3) intermontane basin development during tectonic disruption of the Andean foreland. Each model makes specific predictions about depositional environments, sediment distribution patterns, provenance, basin architecture, subsidence, and thermal history. The research team is collecting sedimentological data (lithofacies, thickness, paleocurrent directions, provenance) and chronological data (40Ar/39Ar tuff ages, palynology, U-Pb zircon and apatite ages, and apatite fission track ages) from Paleogene strata in a roughly 30,000 square kilometer region. Expected products of the research include a comprehensive dataset on the Paleogene stratigraphy of northwestern Argentina, and a concerted application of two new methods (detrital zircon and apatite U-Pb geochronology) to early Cenozoic tectonic problems in the central Andes. Moreover, the results will directly address the issue of how to read orogenic signatures in the stratigraphic record.The Andes are widely regarded as the type area for mountain belts formed in response to subduction of oceanic lithosphere beneath continents and models based on the Andes are regularly used to understand the development of ancient mountain belts around the globe. Nevertheless, a fundamental aspect of Andean orogeny - the age of initial mountain building owing to crustal shortening and thickening -remains controversial. Through application of new analytical techniques and interpretation of the sedimentary record, this project would resolve this controversy for the Central Andes. The project would provide geologists with new methods to determine the timing of major mountain building events in the geological past.