Collaborative Research: Tectonic Significance of ca. 1.6 Ga Deformation in SW Laurentia and New Insights on a Protracted Mazatzal Orogeny

合作研究：约的构造意义。

• 批准号: 0948501
• 负责人: Michael Williams
• 金额: $ 9.01万
• 依托单位: University of Massachusetts Amherst
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-07-01 至 2014-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0948501&HistoricalAwards=false
• 关键词: Collaborative; Research; Tectonic; Significance; ca.

The period between 1.78 and 1.63 billion years ago (Ga) was a time of important southward growth of the North American continent. The rate of growth within this 150-million-year period far exceeds Phanerozoic (542 million years ago to the present) crustal growth rates. As a result of recent work in southern Wyoming and northern Colorado, in a region known as SW Laurentia, the PIs have identified a deformational, magmatic, and thermal event at 1.62-1.58 Ga, which is surprising because the region was thought to be tectonically quiescent. The researchers will combine detailed kinematic histories of shear zones in northern and central Colorado, with precise, texturally-linked dates on deformation, and well-dated metamorphic pressure-temperature paths to evaluate two end-member hypotheses where the enigmatic tectonic 1.6 Ga event may represent either (1) intracratonic deformation of the newly accreted continental crust with stresses transmitted far inboard of the active plate margin or (2) deformation associated with delayed subduction of a long-lived remnant ocean basin. The 1.6 Ga tectonism may reflect the closing of this basin and the final accretion at the southern edge of the North American continent. The recognition of 1.62 to 1.58 Ga deformation, magmatism, and metamorphism as much as 1200 km away from the continent-ocean plate boundary has important implications on several levels. First, the event may have Phanerozoic analogues in the Laramide (80 million years ago) orogeny or the modern Tibetan Plateau. If correct, this hypothesis would suggest rapid stabilization of, and efficient stress transfer within, newly accreted crust of this age. The present midcrustal level of exposure of rocks affected by the 1.6 Ga event affords an opportunity to investigate the crustal stress transfer in the strongest part of the crust. There are also implications for modern earthquake hazards well away from active plate boundaries. Alternatively, if the event reflects closure of a small ocean basin, then the crustal growth process was not a simple north to south progression and indeed was not completed in some of the northernmost regions until after 1.6 Ga. This might also explain some of the anomalous features on recent deep seismic surveys of the region. In either case, results from this study are likely to result in substantial revision of tectonic models for Paleoproterozoic crustal growth, assembly, and stabilization in SW Laurentia.

距今1.78至16.3亿年前（Ga）是北美大陆向南扩张的重要时期。 这1.5亿年期间的地壳生长速度远远超过了显生宙（5.42亿年前至今）地壳的生长速度。最近在怀俄明州南部和科罗拉多州北部的一个被称为西南劳伦西亚的地区进行的工作结果是，PI 确定了 1.62-1.58 Ga 处的变形、岩浆和热事件，这令人惊讶，因为该地区被认为是构造上的静止的。 研究人员将结合科罗拉多州北部和中部剪切带的详细运动学历史、精确的、与结构相关的变形日期以及确定日期的变质压力-温度路径，以评估神秘的构造 1.6 Ga 事件可能发生的两个端元假设。代表（1）新增生的大陆地壳的克拉通内变形，应力远传到活动板块边缘内侧，或（2）与长寿命板块的延迟俯冲相关的变形残余洋盆。 1.6 Ga构造作用可能反映了该盆地的闭合以及北美大陆南缘的最终增生。 认识距大陆-海洋板块边界1200公里处的1.62至1.58 Ga变形、岩浆作用和变质作用在多个层面上具有重要意义。首先，该事件可能与拉拉米德（8000万年前）造山运动或现代青藏高原的显生宙类似。如果正确的话，这一假设将表明这个时代新增生的地壳内部的快速稳定和有效的应力转移。目前受 1.6 Ga 事件影响的岩石暴露在中地壳水平，为研究地壳最强部分的地壳应力转移提供了机会。对于远离活动板块边界的现代地震灾害也有影响。或者，如果该事件反映了一个小海洋盆地的闭合，那么地壳生长过程就不是简单的从北向南的进展，并且实际上在一些最北端的地区直到1.6 Ga之后才完成。这也可以解释一些异常现象该地区近期深部地震勘探的特点。无论哪种情况，这项研究的结果都可能导致对西劳伦大陆古元古代地壳生长、组装和稳定的构造模型进行重大修正。