RUI: Paleomagnetic, Geochemical and Age Discrimination of ~18-19 Ma Tuffs of the Colorado River Extensional Corridor

区域广泛的火山灰流凝灰岩代表了复杂变形区域内重要的、近似等时的对比单元。在美国西部盆地和山脉省科罗拉多河延伸走廊高度延伸的范围内尤其如此，在那里，前第三系沉积段在中新世和更年轻的延伸之前被侵蚀剥离。火山灰流凝灰岩，如广泛分布的1850万年前的桃泉凝灰岩，通常是孤立山脉中以火山为主的部分之间唯一相关的单元。最近对科罗拉多河伸展走廊内1,800万-1,900万年前的火山灰流动的研究表明：(1)被广泛用于对比的桃泉凝灰岩可能在地层上更加复杂，喷发的时间比以前认识的要长得多；(2)桃泉凝灰岩的古地磁特征可能大大偏离了被广泛引用的桃泉凝灰岩的参考方向，这可能是由于与磁场漂移有关的快速长期变化期间的延长。(3)其他凝灰岩与桃泉凝灰岩具有相同的地层位置，可能具有区域重要性，包括先前已知的库克峡谷和Rock Creek流纹岩凝灰岩，可能是在Cerbat山脉西部发现的一种新的凝灰岩，以及其他可能的凝灰岩。古地磁、地球化学和地质年代学技术正被用于研究科罗拉多河伸展走廊1800万-1900万年前的凝灰岩。研究目标针对以下问题：(1)桃泉凝灰岩喷发的时间确实比之前认为的要长得多吗？(2)科罗拉多河伸展走廊中存在多少不同的、截然不同的凝灰岩，年龄约为18-19 Ma，我们如何区分它们？(3)科罗拉多河伸展走廊的18-19 Ma凝灰岩中，如果有的话，哪些凝灰岩具有适合用作构造标志的古地磁方向？(4)这些不同凝灰岩的源区在哪里？这些研究将能够更好地解决伸展生长断陷盆地内的地层对比和时间问题，为评估伸展过程中的构造旋转和测试伸展变形模型提供更好的基础，并可能突出主要火山口喷发中心和区域伸展构造之间的关系。

Regionally extensive ash flow tuffs represent important, approximately isochronous units for correlation within complexly deformed regions. This is particularly true within the highly extended ranges of the Colorado River extensional corridor of the Basin and Range Province of the western U.S., where the pre-Tertiary sedimentary section was erosionally removed prior to Miocene and younger extension. Ash flow tuffs, such as the widely distributed 18.5 million year old Peach Springs Tuff, often represent the only correlatable units between the largely volcanic sections of isolated mountain ranges. In addition to their value to regional correlation and as time horizons within extensional growth fault basins, the paleomagnetic signatures of ash flow units are also highly useful markers for assessing structural rotations due to extensional events.Recent research on 18-19 million year old ash flows within the Colorado River extensional corridor suggests that (1) the Peach Springs Tuff, widely used for correlation, may be more stratigraphically complex and have erupted over a significantly longer period than previously realized, (2) the paleomagnetic signature of the Peach Springs Tuff can deviate substantially from the widely cited Peach Springs Tuff reference direction, possibly due to protracted eruption during rapid secular variation related to a magnetic excursion, and (3) other tuffs occupy the same stratigraphic position as the Peach Springs Tuff and may be regionally important, including the previously known Cook Canyon and Rock Creek rhyodacite tuffs, a potentially new tuff discovered in the western Cerbat Mountains, and possibly others. Paleomagnetic, geochemical and geochronologic techniques are being used to study the 18-19 million year old tuffs of the Colorado River extensional corridor. Research goals are directed at the following questions: (1) Did the Peach Springs Tuff eruptions indeed occur over a much longer time interval than previously thought? (2) How many different and distinct tuffs of approximately 18-19 Ma age exist in the Colorado River extensional corridor and how do we discriminate them? (3) Which of the 18-19 Ma tuffs of the Colorado River extensional corridor, if any, have a paleomagnetic direction suitable for use as a structural marker? (4) Where are the source areas for these various tuffs? These studies will allow better resolution of stratigraphic correlation and timing within extensional growth fault basins, provide a better basis for assessing structural rotations during extension and for testing models of extensional deformation and, potentially highlight relationships between major caldera eruptive centers and regional extensional structures.