Collaborative Research: Miocene-Pliocene Paleoelevation of the Bolivian Altiplano

高海拔高原是形成在收缩山区的神秘特征。区域性地壳缩短和增厚导致高原内显著的地表隆起。然而，其他地壳和次地壳岩石圈过程，如岩石圈减薄、地壳岩浆补充、构造底侵作用或下地壳流动，也可能在产生这些广阔的高海拔地区方面发挥重要作用。要了解高原抬升的机制，就需要了解地表抬升的历史。地表抬升的大小和时间尺度都受引起抬升的潜在机制控制。除了测试一种新的宇宙成因同位素古高度计外，国际空间局还建议使用氧同位素古高度计技术来研究玻利维亚高原的隆升历史。这些古海拔数据将有助于解决目前关于高原早期和最近隆起的争论。此外，这些数据将对随时间推移的抬升幅度和速率施加限制，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，面积和横向面积仅次于青藏高原。这是开展长期地表抬升的系统研究的理想环境，原因如下：1)以前基于树叶地貌的古海拔研究提供了一个记录，可以用来比较我们的古海拔数据。这些数据表明，只有不到一半的高原抬升时间为~10 Ma(Gregory-Wodzicki，1998)，并提供了氧同位素古高度测量所需的古温度估计。2)氧同位素古海拔测量只能应用于蒸发最少的地区(潮湿环境)，而宇宙成因的同位素古海拔测量需要最小的侵蚀(干旱环境)。玻利维亚高原内的气候变化范围很大，使我们能够在更潮湿的高原北部进行氧同位素古高度测量，在干旱的高原南部进行宇宙成因的同位素古高度测量。3)高原内部有长期的沉积历史，产生了广泛的碳酸盐记录，从中可以获得氧同位素古海拔估计。根据互层凝灰岩的40Ar/39Ar测年(马歇尔等人，1992年)和磁性地层学(罗佩奇等人，1999年)，我们的目标剖面科克盆地的沉积岩年龄已被确定为~14 Ma至~5 Ma之间。4)收集了月和年降雨量的氧同位素组成数据，提供了良好的d18O与海拔梯度(Gonfiantini等人，2001年)。这一梯度将被用来估计从那里沉淀碳酸盐的古气象水的海拔。5)高原内广泛的中新世至现代火山作用提供了丰富的可测年古地表，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。在玻利维亚高原南部，证据表明这些古地表在过去15 Ma期间经历了很少的侵蚀(Horton，1998)。氧同位素初步数据显示，从晚渐新世到晚中新世，氧同位素比值发生了显著的漂移，约为-7.5，表明地表抬升超过3公里。对磷灰石和辉长岩的初步测量表明，宇宙成因的3He和38Ar都保存在高原现代低纬表面。这项研究是罗切斯特大学、哈佛-史密森天体物理中心以及伯克利和加州理工大学的研究人员共同努力的结果。该项目更广泛的影响是，它将为两名博士生和两名或更多本科生提供部分支持。本科生的研究将导致完成高级论文项目。此外，这项研究将直接指导正在进行的关于学生对地球过程的概念性理解的研究。板块构造过程的科学观点不断演变，以回应这里提出的研究，这些观点是学生想法必须与之比较的基准。

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.