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

17.8 - 16.3亿年前（Ga）是北美大陆向南生长的重要时期。在这一亿五千万年间的生长速度远远超过显生宙（5.42亿年前到现在）的地壳生长速度。根据最近在怀俄明州南部和科罗拉多州北部的工作结果，在一个被称为SW Laurentia的地区，pi已经确定了1.62-1.58 Ga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、纹理相关的变形日期，以及年代久远的变质压力-温度路径，以评估两种端元假说，其中神秘的1.6 Ga构造事件可能代表(1)新增加的大陆地壳的克拉通内变形，应力传递到活动板块边缘的远内侧，或(2)与长寿命残余海洋盆地的延迟俯冲有关的变形。1.6 Ga构造运动可能反映了该盆地的闭合和北美大陆南缘的最终增生。对距离大陆-海洋板块边界1200公里的1.62 - 1.58 Ga变形、岩浆作用和变质作用的识别在几个层面上具有重要意义。首先，这一事件可能与拉腊米（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形成的地壳的快速稳定和有效的应力传递。目前受1.6 Ga事件影响的岩石的中地壳暴露水平为研究地壳最强部分的地应力传递提供了机会。这也暗示了远离活动板块边界的现代地震危险。或者，如果这一事件反映了一个小海洋盆地的关闭，那么地壳生长过程就不是一个简单的由北向南的过程，在一些最北端地区直到1.6 Ga之后才完成。这也可以解释最近该地区深地震调查的一些异常特征。无论哪种情况，本研究的结果都可能导致对西南劳伦西亚古元古代地壳生长、组合和稳定的构造模式的实质性修订。

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.