Collaborative Research: Neoproterozoic Rifting of Utah: Geochronology, Tectonics, Sequence Stratigraphy

对于Rodinia的分裂和与之相关的西劳伦西亚裂谷历史，已经提出了各种板块重建，但模型仍然存在争议。为了解决这个问题，来自爱达荷州州立大学、犹他州州立大学和韦伯州立大学的一个研究小组正在研究新元古代（11亿年至5.42亿年前）的沉积物，以检验裂谷形成和发展的不同模式：1）约7.5亿年前的长期裂谷，与岩浆柱的初始爆发有关;（2）7.2亿~ 6.7亿年前的大陆分离;（3）寒武纪初附近的裂谷形成（约5.4亿年前）。该小组将对新元古代地层进行综合的野外、地层、构造、锆石来源和地质年代学研究。具体办法包括：对关键地段进行详细的层序地层学、沉积学和岩石学研究;对伴生火山岩进行地球化学微量元素分析，以确定构造-岩浆背景;采用激光烧蚀-多集电极-电感耦合等离子体质谱法、灵敏的高分辨率离子微探针和热离子质谱法相结合的方法，对原生火山岩和碎屑锆石进行U-Pb定年;玄武岩流中角闪石的Ar-Ar定年;锆石的Hf同位素分析;区域沉降史的模拟。碎屑锆石的U-Pb定年和Hf同位素分析将揭示源区和古地理，最年轻的火山锆石的年龄为最大沉积年龄提供了重要的括号。新元古代标志着地球历史上一些最深刻的地质变化，包括超大陆Rodinia的分裂，极端的气候变化和快速的生物多样性。对于Rodinia的分裂和与之相关的西劳伦西亚裂谷历史，已经提出了各种板块重建，但模型仍然存在争议。解决这一争议的重要证据在于犹他州和爱达荷州东南部的新元古代地层。这些地层形成了劳伦大陆裂谷西缘和随后的科迪勒拉造山带的一个关键，但不完全了解的部分。这项研究项目将在理解这些地层的历史方面取得重要进展，并导致美国西部早期构造发展的对立模型之间的歧视。

Various plate reconstructions have been proposed for break up of Rodinia and for the associated rifting history of western Laurentia, but models remain controversial. To address this problem, a research team from Idaho State University, Utah State University, and Weber State University is investigating Neoproterozoic (between 1,100 and 542 million years ago) deposits in Utah and southern Idaho in order to test different models for rift initiation and development: 1) protracted rifting at approximately 750 million years ago associated with an initial breakout of a magmatic plume; 2) continental separation between 720 and 670 million years ago; or 3) rift initiation near the beginning of the Cambrian (about 540 million years ago). The team will employ an integrated field, stratigraphic, structural, zircon provenance, and geochronologic study of Neoproterozoic strata. Specific approaches include: detailed sequence stratigraphic, sedimentologic, and petrologic studies of critical sections; geochemical trace-element analysis of associated volcanic rocks to determine tectono-magmatic setting; U-Pb dating of primary volcanic and detrital zircons using a combination of laser ablation-multicollector-inductively coupled plasma mass spectrometry, sensitive high-resolution ion microprobe, and thermal ion mass spectrometry methods; Ar-Ar dating of hornblende from basalt flows; Hf isotopic analysis of zircons; and modeling of regional subsidence histories. U-Pb dating and Hf isotopic analysis of detrital zircon populations will shed light on source areas and paleogeography, with ages of youngest reworked volcanic zircons providing important brackets for maximum depositional ages.The Neoproterozoic marks some of the most profound geologic changes in Earth history, including break up of the supercontinent Rodinia, extreme climate changes, and rapid biologic diversification. Various plate reconstructions have been proposed for break up of Rodinia and for the associated rifting history of western Laurentia, but models remain controversial. Evidence important for the resolution of this controversy lies in the Neoproterozoic strata of Utah and southeastern Idaho. These strata form a critical, but incompletely understood part of the rifted western margin of Laurentia and subsequent Cordilleran orogenic belt. This research project will make important strides in understanding the history of these beds and lead to discrimination between opposing models for the early tectonic development of the western United States.