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

区域性广泛的灰流凝灰岩是复杂变形区域内重要的、近似等时的相关单位。在美国西部盆地和山脉省的科罗拉多河伸展走廊的高度延伸范围内尤其如此，那里的前第三纪沉积剖面在中新世和更年轻的伸展之前被侵蚀掉了。火山灰流凝灰岩，如广泛分布于1850万年前的桃源凝灰岩，通常代表了孤立山脉的大部分火山段之间唯一可关联的单位。火山灰流单元的古地磁特征除了具有区域对比价值和作为伸展生长断陷盆地的时间尺度外，对于评估伸展事件引起的构造旋转也是非常有用的标志。最近对科罗拉多河伸展走廊内1800万年至1900万年的火山灰流的研究表明：(1)广泛用于对比的桃泉凝灰岩可能具有更复杂的地层结构，并且喷发的时间比以前认识到的要长得多；(2)桃泉凝灰岩的古地磁特征可能与广泛引用的桃泉凝灰岩参考方向有很大的偏离。(3)其他凝灰岩与桃泉凝灰岩占据相同的地层位置，可能具有区域性重要性，包括先前已知的库克峡谷和Rock Creek流纹石凝灰岩，可能是在西Cerbat山脉发现的一种新凝灰岩，可能还有其他凝灰岩。古地磁、地球化学和地质年代学技术被用来研究1800万到1900万年前科罗拉多河伸展走廊的凝灰岩。研究目标是针对以下问题：(1)桃源凝灰岩喷发的时间间隔是否确实比以前认为的要长得多？(2)科罗拉多河伸展走廊中存在多少种不同的凝灰岩，其年龄约为18-19 Ma，我们如何区分它们？(3)在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作为构造标志的古地磁方向？(4)这些不同凝灰岩的来源在哪里？这些研究将有助于更好地解决伸展生长断陷盆地内的地层对比和时序问题，为评估伸展期间的构造旋转和测试伸展变形模型提供更好的基础，并有可能突出主要火山口喷发中心与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.