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

PI研究人员已经开发出一种根据熔岩流的泡状性确定高地地区古海拔的方法。 他们证明了该方法的有效性，通过测试的方法对最近的熔岩侵位在已知的海拔高度，并在最近的探索性研究中应用该技术在科罗拉多高原，以获得一个一般的限制隆升的时间。 这一直是地质研究界相当关注和争论的问题。 探索性研究结果表明，科罗拉多高原经历了25 ~ 10 Ma的缓慢隆升和10 Ma后的快速隆升。 这些结果通过提供科罗拉多高原的隆升历史曲线，调和了古代与近代隆升解释之间长期存在的争议。 PI的初步调查没有解决慢-快过渡的性质，也没有高原不同部分之间差异隆升的细节，但随后对年轻流动（10 Ma）的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）。 现在，他们建议进行详细的分析，科罗拉多高原的隆起，我们的勘探项目的结果的基础上，并集中在过去的10百万年。建立科罗拉多高原和落基山脉的隆升历史的重要性在于其在古地理重建和古气候变化的关键作用，以及它的岩石圈热结构和地幔过程的反映。 过去曾尝试过各种确定隆升时间的方法，包括地貌、古植物、沉积和同位素调查，但每种方法都有不确定性的来源，因为使用的代用指标仅取决于海拔以外的因素。 由于PI最近开发的技术测量古大气压力，它不受这些不确定性的影响（尽管像所有测量一样，有自己的潜在误差来源）。 使用所提出的分析，PI的将能够重建详细的隆升历史的科罗拉多高原和邻近的落基山脉。泡状熔岩保存在时间和地点的熔岩侵位的古压力的记录，因为在底部和顶部的熔岩流气泡的内部压力差取决于大气压力和熔岩流的厚度。 在流动的顶部，压力仅仅是大气压力，而在底部，有rgH熔岩覆盖层的额外贡献。 因此，囊泡（气泡）群体的模态尺寸在顶部比在底部大。 由于流动的厚度可以很容易地在野外测量，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。 由于海平面压力在新生代没有显著变化，熔岩中“保存”的古海拔与现在的海拔之间的差异反映了抬升的程度。 熔岩可以很好地定年，因此，一套不同年龄的样品将限制造陆隆升的时间，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开发了用于分析泡状火山岩的新工具。 这些工具最值得注意的包括用于确定适当的现场采样位置的方法，以及允许对断层扫描数据进行操作以产生确定尺寸分布所需的囊泡群体统计的计算软件。 所提出的方法是基于模态大小的囊泡人口，所以它是至关重要的，能够准确地测量从顶部和底部的流动收集的样品中的尺寸分布。 新的分析技术包括计算机X射线断层扫描来测量囊泡的大小。 基于我们测量囊泡大小的能力，在1.6%（按体积计）内，该技术对大气压的敏感性的因子分析提供了± 300英寸内的升高。 正如他们在与当地实地专家合作的初步项目中所做的那样，PI建议从现在高海拔的各种已确定的水流中收集样本。 他们将准备X射线断层扫描的样本，这些样本将在美国的设施中完成。德克萨斯 将使用他们以前开发的软件分析所得数据。 粒度分布将被用来重建古压力，从而古海拔。 这应该提供一个明确的文件的细节隆升的历史，以及阐明任何差异隆升内的科罗拉多高原。

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.