Collaborative Research: Syncollisional Delamination: A Test of Crustal-scale Tectonic Wedging Exposed in the Avalon Terrane of New England

合作研究：同碰撞分层：新英格兰阿瓦隆地体中地壳尺度构造楔入的测试

• 批准号: 0510857
• 负责人: Michael Dorais
• 金额: $ 4.36万
• 依托单位: Brigham Young University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-07-01 至 2008-06-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0510857&HistoricalAwards=false
• 关键词: Collaborative; Research; Syncollisional; Delamination; Test

One outstanding problem in tectonics is the mechanism of crustal thickening. Where tectonic plates converge, relatively low density crust may become delaminated from its denser mantle keel, or slices of felsic crust may indent or interfinger as crustal wedges. In this case indenting or even mutually indenting structures may lead to thickened crust. The structure of tectonic wedging has received increasing support from the interpretations of geophysical data, but interpenetrating crustal slivers have rarely been observed in the field. Consequently the mechanisms and even the confirmation of interpenetrating wedges remain speculative.In this collaboration between Indiana University and Brigham Young University geologists are making one of the first attempts to directly characterize and identify the causes of syn-collisional wedging of upper crust from lower crust using surface outcrop. They have discovered an example of tectonic wedging in a tilted lower crustal profile. The outcrops in coastal Connecticut exposure rocks formerly mapped as fold nappes within the Avalon terrane. In addition to Avalonian rocks derived from juvenile sources, our pilot study of neodymium and lead isotopes is showing that the structurally lowest orthogneisses is derived from ancient crustal rocks that correlate with the Gander terrane. Their work in this region shows that Avalonian rocks both overlie Gander basement rocks and underlie rocks known to the north to rest conformably over Gander basement. Thus this map pattern defines the geometry of a tectonic wedge. The goals of the project are (1) to confirm the identity and correlation of Avalon and Gander zone rocks in southern coastal New England with correlative rocks in maritime Canada where they are well characterized, (2) to test the extent and distribution of Avalon and Gander zone rocks in other inliers of New England, (3) to test single vs. double indenting models, (4) to monitor the timing and rate of approach of Avalon in the cover terranes by establishing the time of quenching of the cover Gander terranes, and (5) to assess the rheology of the rocks in the terrane boundary. Methods to be used include characterization of outcrop in the field, analysis of major and trace element compositions to identify intrusive igneous rocks, and neodymium and lead isotopic analysis of theses igneous rocks to ascertain the origins of the initial magmas.

构造中的一个突出问题是地壳增厚的机制。在构造板块汇聚的地方，相对低密度的地壳可能会从密度较高的地幔龙骨上剥离，或者长英质地壳切片可能会凹陷或相互指间形成地壳楔形。在这种情况下，凹陷甚至相互凹陷的结构可能导致结皮增厚。构造楔入构造得到了地球物理数据解释的越来越多的支持，但在野外很少观察到相互穿透的地壳碎屑。在印第安纳大学和杨百翰大学的这项合作中，地质学家们正在进行第一次尝试，利用地表露头直接表征和识别上地壳与下地壳同碰撞楔入的原因。他们在倾斜的下地壳剖面中发现了一个构造楔形的例子。康涅狄格州沿海裸露岩石的露头以前被绘制为阿瓦隆地体内的褶皱推覆构造。除了来自年轻来源的阿瓦伦期岩石外，我们对钕和铅同位素的初步研究表明，结构最低的正交片麻岩来自与甘德地体相关的古老地壳岩石。他们在该地区的研究表明，阿瓦洛尼亚岩石既覆盖在甘德基底岩之上，也覆盖在北部已知的位于甘德基底岩之上的岩石之下。因此，该地图模式定义了构造楔形的几何形状。该项目的目标是(1)确认新英格兰南部沿海Avalon和Gander带岩石与加拿大海上相关岩石的身份和相关性，这些岩石具有良好的特征；(2)测试Avalon和Gander带岩石在新英格兰其他内部的范围和分布；(3)测试单凹陷和双凹陷模式；(4)通过建立盖层Gander地体的淬灭时间来监测Avalon在盖层地体中接近的时间和速率；以及(5)评估地体边界的岩石流变性。使用的方法包括野外露头特征、主要元素和微量元素分析以识别侵入火成岩，以及对这些火成岩进行钕和铅同位素分析以确定初始岩浆的来源。