Collaborative Research: Miocene-Pliocene Paleoelevation of the Bolivian Altiplano

高海拔高原是在收缩山带中形成的神秘特征。区域地壳的缩短和增厚导致高原内大量的地表隆起。然而，其他的地壳和地壳下岩石圈过程，如岩石圈减薄、岩浆对地壳的补充、构造底板或地壳下流，也可能在形成这些广阔的、高海拔地区方面发挥重要作用。了解高原抬升的机制需要了解地表抬升的历史。地表抬升的幅度和时间尺度都受抬升机制的控制。除了测试一种新的宇宙成因同位素古高度计外，PI还建议使用氧同位素古高度计技术研究玻利维亚Altiplano的隆起历史。这些古海拔数据将有助于解决目前关于高原早期和近期隆起的争论。此外，这些数据将限制随时间的抬升幅度和速率，使他们能够确定某些抬升机制的重要性。高原平均海拔3700米，面积和横向范围仅次于青藏高原。这是开展长期地表抬升系统研究的理想环境，原因如下：1)先前基于叶片地貌的古高程研究为比较古高程数据提供了一个记录。这些数据表明，Altiplano的隆升不超过一半（Gregory-Wodzicki, 1998），并提供了氧同位素古高程测量所需的古温度值。2)氧同位素古高程测量只能应用于蒸发最小的地区（湿润环境），而宇宙成因同位素古高程测量需要最小的侵蚀（干旱环境）。玻利维亚高原具有大范围的气候变化，使我们能够在湿润的北部高原进行氧同位素古高程测量，在干旱的南部高原进行宇宙成因同位素古高程测量。3)高原有长期的沉积历史，产生了广泛的碳酸盐记录，可以从中获得氧同位素古高程估计。根据40 Ar/ 39 Ar的互层凝灰岩年代（Marshall et al., 1992）和磁地层学（Roperch et al., 1999），我们的目标剖面Corque盆地的沉积岩年龄确定在~14 Ma ~ ~5 Ma之间。4)收集的月和年降雨量氧同位素组成数据提供了良好的d18o随海拔梯度的变化（Gonfiantini et al., 2001）。这个梯度将被用来估计沉积碳酸盐的古大气水的高度。5) Altiplano内广泛分布的中新世至近代火山活动提供了丰富的可测古表面，其中包含应用宇宙成因同位素技术确定古海拔所需的矿物学。在玻利维亚南部的高原，有证据表明，这些古地表在过去15万年中经历了很少的侵蚀（Horton, 1998）。初步氧同位素数据显示，晚渐新世至晚中新世期间，氧同位素比值明显变化，约为-7.5，表明地表隆起超过3 km。对磷灰石和水晶石的初步测量表明，在高原现代低纬度地表上，宇宙成因的3 - He和38 - Ar都被保存了下来。这项研究是罗彻斯特大学、哈佛-史密森天体物理中心以及伯克利和加州理工学院的研究人员共同努力的结果。该项目的更广泛影响是，它将为两名博士生和两名或更多本科生提供部分支持。本科研究将导致完成高级论文项目。此外，这项研究将直接为正在进行的学生对地球过程概念理解的研究提供信息。关于板块构造过程的科学观点随着这里提出的研究而不断发展，这些观点是学生们的观点必须进行比较的基准。

High elevation plateaus are enigmatic features that form in contractional mountain belts.Regional crustal shortening and thickening cause significant amounts of surface uplift within plateaus. However, other crustal and subcrustal lithospheric processes, such as lithospheric thinning, magmatic additions to the crust, tectonic underplating, or lower crustal flow may also play an important role in producing these broad, high elevation regions. Understanding the mechanisms responsible for plateau uplift requires knowledge of the history of surface elevation. Both the magnitude and time scale of surface uplift are controlled by the underlying mechanisms responsible for uplift. The PI's propose to study the uplift history of the Altiplano in Bolivia using oxygen isotope paleoaltimetry techniques in addition to testing a new cosmogenic isotope paleoaltimeter. These paleoelevation data will help resolve current debates over the early versus recent uplift of the Altiplano. In addition, these data will place constraints on the magnitudes and rates of uplift over time, enabling them to determine the importance of certain uplift mechanisms. The Altiplano has an average elevation of 3700 m, second only to the Tibetan plateau in size and lateral extent. This is an ideal setting in which to carry out a systematic study of long term surface uplift for the following reasons: 1) A previous paleoelevation study, based on leaf physiognomy, provides a record by which to compare our paleoelevation data. These data suggest that no more than half of the uplift of the Altiplano took by ~10 Ma (Gregory-Wodzicki, 1998) and also provide paleotemperature estimates required for oxygen isotope paleoaltimetry. 2) Oxygen isotope paleoaltimetry can only be applied in regions that experience minimal evaporation (humid environments), whereas cosmogenic isotope paleoaltimetry requires minimal erosion (arid environment). There is a large range of climatic variability within the Bolivian Altiplano, enabling us carry out oxygen isotope paleoaltimetry in the more humid northern Altiplano and cosmogenic isotope paleoaltimetry in the arid southern Altiplano. 3) There is a long-term history of sedimentation within the Altiplano, producing an extensive carbonate record from which oxygen isotope paleoelevation estimates can be obtained. The ages of sedimentary rocks in the Corque basin, our target section, have been determined to be between ~14 Ma and ~5 Ma by 40 Ar/ 39 Ar dates of interbedded tuffs (Marshall et al., 1992) and magnetostratigraphy (Roperch et al., 1999). 4) Data on the oxygen isotope composition of monthly and yearly rainfall has been collected providing an excellent d 18 O vs. altitude gradient (Gonfiantini et al., 2001). This gradient will be used to estimate the elevation of paleometeoric water from which carbonates were precipitated. 5) Widespread Miocene to recent volcanism within the Altiplano provides abundant datable paleosurfaces containing the mineralogies needed to apply cosmogenic isotope techniques to determining paleoelevation. In the southern Bolivian Altiplano evidence suggests that these paleosurfaces have experienced very little erosion over the past 15 Ma (Horton, 1998). The preliminary oxygen isotope data show a pronounced shift in oxygen isotopic ratios of about -7.5 between late Oligocene and late Miocene time, which suggests more than 3 km of surface uplift. Initial measurements on apatite and sanidine indicate that both cosmogenic 3 He and 38 Ar are preserved in modern low latitude surfaces on the Altiplano. This research is a collaborative effort between the University of Rochester, the Harvard- Smithsonian Center for Astrophysics, and researchers at Berkeley and Caltech. The broader impacts of this project are that it will provide partial support for two Ph.D. students and two or more undergraduate students. Undergraduate research will lead to the completion of senior thesis projects. In addition, this research will directly inform an ongoing study of student conceptual understandingof earth processes. Scientific views of plate tectonic processes continue to evolve in response to research like that proposed here, and these views are the datum to which student ideas must be compared.