Uplift History of the Colorado Plateau Since the Late Miocene: Analysis Using Vesicular Basalts as a Paleoaltimeter

PI开发了一种根据熔岩流的泡状来确定高原地区古海拔的方法。他们通过对已知海拔位置的近期熔岩进行测试，证明了该方法的有效性，并最近将该技术应用于科罗拉多高原的探索性研究，以获得隆起时间的一般约束。这一直是地质研究界相当关注和争论的主题。探索性研究结果表明，科罗拉多高原在25 - IO Ma期间经历了缓慢的隆升，而在IO Ma之后经历了快速的隆升。这些结果通过提供科罗拉多高原的隆起历史曲线，调和了长期以来关于古隆起与近代隆起解释之间的争议。PI的最初调查并没有解决慢-快过渡的性质，也没有解决高原不同部分之间差异隆起的细节，但随后的年轻流（10 Ma）采样应该揭示这些细节和高原内部变形的程度（如果有的话）。他们现在建议在我们的勘探项目成果的基础上，重点对近10万年的科罗拉多高原隆升进行详细的分析。建立科罗拉多高原和落基山脉隆升历史的重要性在于它在古地理重建和古气候变化中的关键作用，以及它对岩石圈当时结构和地幔过程的反映。过去已经尝试了各种确定隆升时间的方法，包括地貌学、古植物学、沉积学和同位素研究，但每种方法都有不确定性的来源，这些不确定性源于使用依赖于海拔以外因素的代用物。由于PI最近开发的技术测量的是古大气压力，它不受这些不确定性的影响（尽管像所有测量一样，它有自己潜在的误差来源）。利用提出的分析，PI将能够重建科罗拉多高原和邻近的落基山脉的详细隆起历史。泡状熔岩保存了熔岩就位时间和地点的古压力记录，因为熔岩流底部和顶部气泡内部压力的差异取决于大气压力和熔岩流厚度。在流动的顶部，压力是简单的大气压力，而在底部，有额外的rgH熔岩覆盖层的贡献。因此，囊泡（泡）种群的模态大小在顶部比在底部大。因为流动的厚度可以很容易地在现场测量，而囊泡的大小可以在实验室测量，这直接导致了古大气压力。考虑到海平面压力在新生代没有明显变化，熔岩中“保存”的古海拔高度与现在海拔高度的差异反映了隆升的程度。熔岩可以很好地确定年代，因此，一套不同年龄的样本将限制造陆隆起的时间，而不受气候、侵蚀速率或任何其他环境因素的影响。在他们最近的工作中，PI开发了分析水泡状火山岩的新工具。这些工具最显著的包括确定适当的现场采样位置的方法，以及允许操纵层析成像数据以产生确定大小分布所需的囊泡种群统计的计算软件。所提出的方法是基于囊泡群的模态大小，因此能够准确地测量从流动的顶部和底部收集的样品的大小分布是至关重要的。新的分析技术包括计算机x射线断层扫描来测量囊泡大小。基于我们测量囊泡大小的能力（体积比）在1.6%以内，该技术对大气压的敏感性因子分析提供了+-300英寸的高度。正如他们与当地实地专家合作的初步项目所做的那样，PI建议从目前处于高海拔的各种已确定的河流中收集样本。他们将准备样品进行x射线断层扫描，并在美国德州大学的设备中完成。结果数据将用他们之前开发的软件进行分析。大小分布将用于重建古压力，从而重建古海拔。这应该为该地区隆起历史的细节提供一个明确的文件，并阐明科罗拉多高原内的任何差异隆起。

The PI's have developed a method for deten-nining paleoelevations of highland areas on the basis of the vesicularity of lava flows. They demonstrated the validity of the approach by testing the method against recent lavas emplaced at known elevations, and recently applied the technique in an exploratory study on the Colorado Plateau to obtain a general constraint on the timing of uplift. This has been a subject of considerable concern and debate in the geological research community. The results of the exploratory study indicate that the Colorado Plateau experienced slow uplift from 25 to IO Ma, and rapid uplift after IO Ma. These results reconcile the longstanding controversy between interpretations of ancient vs. recent uplift by providing an uplift history curve for the Colorado Plateau. The PI's initial investigation did not resolve the nature of the slow-fast transition, nor the details of differential uplift between different parts of the plateau, but subsequent sampling of young flows (10 Ma) should reveal these details and the extent (if any) of internal plateau deformation. They now propose to conduct a detailed analysis of the uplift of the Colorado Plateau, building on the results of our exploratory project, and focusing on the last 10 m.y.The importance of establishing the uplift history for the Colorado Plateau and Rocky Mountains lies in its pivotal role in paleogeographic reconstructions and paleoclimatic changes, as well as its reflection of lithospheric then-nal structure and mantle processes. Various approaches for deten-nining timing of uplift have been tried in the past including geomorphologic, paleofloristic, sedimentary, and isotopic investigations, but each has had sources of uncertainty stemming from using proxies that depend on factors other than elevation alone. Because the PI's recently developed technique measures paleoatmospheric pressure, it is not subject to these uncertainties (although like all measurements, has its own potential sources of error). Using the proposed analysis, the PI's will be able to reconstruct the detailed uplift history of the Colorado Plateau and adjacent Rocky Mountains.Vesicular lavas preserve a record of paleopressure at the time and place of lava emplacement because the difference in internal pressure in bubbles at the base and top of a lava flow depends on atmospheric pressure and lava flow thickness. At the top of the flow, the pressure is simply atmospheric pressure, while at the base, there is an additional contribution of rgH lava overburden. Thus the modal size of the vesicle (bubble) population is larger at the top than at the bottom. Because the thickness of the flow can easily be measured in the field, and the vesicle sizes can be measured in the lab, this leads directly to paleoatmospheric pressure. Given that sea level pressure has not changed significantly in Cenozoic time, the difference between the paleoelevation "preserved" in the lavas and their present elevation reflects the amount of uplift. Lava can be well dated, therefore a suite of samples of various ages will constrain the timing of epeirogenic uplift independent of climate, erosion rates or any other envirom-nental factors. In their recent work, the PI's developed new tools for analysis of vesicular volcanic rocks. These tools most notably include methods for identifying appropriate field sampling locations, and the computational software which allows tomographic data to be manipulated to produce vesicle population statistics necessary for deten-nining size distributions. The proposed method is based on the modal size of the vesicle population, so it is critical to be able to accurately measure the size distribution in collected samples from the tops and bottoms of flows. The new analytical techniques involve Computed X-Ray Tomography to measure vesicle sizes. Based on our ability to measure vesicle size to within 1.6% (by volume), a factor analysis of the sensitivity of the technique to atmospheric pressure provides an elevation within +-300 in. As done in their preliminary project in conjunction with local field experts, the PI's propose to collect samples from various identified flows that are now at high elevations. They will prepare samples for Xray Tomographic scanning to be done at the facility at U. Texas. The resulting data will be analyzed with software they have developed previously. The size distributions will be used to reconstruct paleopressure and thus paleoelevation. This should provide an unambiguous documentation of the details of uplift history of the region, as well as elucidate any differential uplift within the Colorado Plateau.