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

17.8亿至16.3亿年前(GA)之间的时期是北美大陆向南发展的重要时期。在这1.5亿年期间的增长率远远超过显生宙(5.42亿年前到现在)的地壳增长率。由于最近在怀俄明州南部和科罗拉多州北部一个被称为西南劳伦蒂亚的地区进行了研究，PI发现了1.62-1.58Ga的变形、岩浆和热事件，这是令人惊讶的，因为该地区被认为是构造静止的。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，以及精确的、结构上相关的变形日期，以及日期良好的变质压力-温度路径来评估两个端元假说，其中神秘的构造1.6Ga事件可能代表(1)新增生的大陆地壳的克拉内变形，应力传递到活动板块边缘的内侧，或(2)与长期残存的洋盆的延迟俯冲有关的变形。1.6Ga构造作用可能反映了该盆地的闭合和北美大陆南缘的最终聚积。在距陆洋板块边界1200公里范围内的1.62~1.58Ga的变形、岩浆和变质作用的识别，在多个层面上具有重要的意义。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中的显生宙类似。如果这一假设是正确的，那么这一假说将表明，在这个时代，新沉积的地壳迅速稳定，并在内部有效地传递应力。目前受1.6Ga事件影响的地壳中段岩石的暴露程度为研究地壳最强部位的地应力传递提供了机会。这对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洋盆的关闭，那么地壳增长过程不是一个简单的从北向南的过程，实际上在最北端的一些地区直到1.6Ga之后才完成。这也可能解释了最近该地区深部地震调查的一些异常特征。无论是哪种情况，这项研究的结果都可能导致对劳伦西亚西南部古元古代地壳生长、组装和稳定的构造模型进行实质性修订。

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.