Collaborative Research: Miocene-Pliocene Paleoelevation of the Bolivian Altiplano

高海拔高原是形成于收缩山脉带中的神秘地貌，区域性地壳缩短和增厚导致高原内显著的地表隆起。然而，其他地壳和地壳下的岩石圈过程，如岩石圈减薄，岩浆添加到地壳，构造底侵，或下地壳流动也可能发挥重要作用，在产生这些广泛的，高海拔地区。要了解高原隆升的机制，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测量技术研究玻利维亚高原的隆升历史。这些古高程数据将有助于解决目前关于高原早期与近期隆起的争论。此外，这些数据将限制随着时间的推移抬升的幅度和速率，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，在面积和横向范围上仅次于青藏高原。这是一个理想的设置进行系统研究的长期表面隆起的原因如下：1）以前的古海拔研究，根据叶地貌，提供了一个记录，通过比较我们的古海拔数据。这些数据表明，不超过一半的高原隆起了~10 Ma（Gregory-Wodzicki，1998年），也提供了氧同位素古高度测量所需的古温度估计。2)氧同位素古高度测量只能应用于蒸发量最小的地区（潮湿环境），而宇宙成因同位素古高度测量需要最小的侵蚀（干旱环境）。玻利维亚高原气候变化范围大，使我们能够在较为湿润的北方高原进行氧同位素古高度测量，在干旱的南方高原进行宇宙成因同位素古高度测量。3)高原内有长期的沉积历史，产生了广泛的碳酸盐记录，从中可以获得氧同位素古高程估计。通过层间凝灰岩的40 Ar/ 39 Ar年龄测定，我们的目标剖面Corque盆地的沉积岩年龄在~14 Ma和~5 Ma之间（马歇尔等人，1992）和磁性地层学（Roperch等人，1999年）。4)已收集了关于月降雨和年降雨的氧同位素组成的数据，提供了极好的d 18 O与海拔梯度的关系（Gonfiantini等人，2001年）。这个梯度将被用来估计碳酸盐沉淀的古气象水的海拔。5)广泛的中新世到最近的火山活动在高原提供了丰富的可定年的古地表，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。在玻利维亚高原南部，证据表明，这些古地表在过去15 Ma中几乎没有受到侵蚀（Horton，1998年）。初步的氧同位素数据显示，在渐新世晚期和中新世晚期之间，氧同位素比值发生了约-7.5的显著变化，这表明地表隆起超过3 km。对磷灰石和透长石的初步测量表明，宇宙成因的3 He和38 Ar都保存在现代高原低纬度地表。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学院研究人员的合作成果。该项目更广泛的影响是，它将为两个博士学位提供部分支持。学生和两个或两个以上的本科生。本科研究将导致高级论文项目的完成。此外，这项研究将直接通知正在进行的研究，学生的概念理解的地球进程。板块构造过程的科学观点不断发展，以响应这里提出的研究，这些观点是学生想法必须与之进行比较的数据。

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.