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

地壳增厚的机制是构造学中一个突出的问题。在构造板块汇聚的地方，相对低密度的地壳可能从其密度较高的地幔龙骨上分层，或者长英质地壳的切片可能像地壳楔一样重叠或交错。在这种情况下，锯齿状或甚至相互锯齿状的结构可能导致地壳增厚。构造楔入的结构越来越多地得到地球物理资料解释的支持，但在野外很少观察到互穿的地壳薄片。因此，楔互穿的机制，甚至确认仍然是推测性的。在印第安纳州大学和杨百翰大学之间的合作，地质学家正在进行的第一次尝试，直接表征和确定的原因，同碰撞楔从下地壳使用地表露头。他们发现了一个在倾斜的下地壳剖面中构造楔入的例子。康涅狄格州沿海暴露岩石中的露头以前被绘制为阿瓦隆山脉内的褶皱推覆体。除了阿瓦隆岩石来自年轻的来源，我们的钕和铅同位素的试点研究表明，结构最低的正片麻岩是来自古老的地壳岩石，与甘德岩。他们在这一地区的工作表明，阿瓦隆岩石既位于甘德基底岩石之上，也位于北部已知整合于甘德基底之上的岩石之下。因此，这种地图模式定义了构造楔的几何形状。该项目的目标是：（1）确认新英格兰南部沿海的阿瓦隆和甘德带岩石与加拿大海洋部的相关岩石的同一性和相关性，在加拿大海洋部，阿瓦隆和甘德带岩石具有良好的特征，（2）测试阿瓦隆和甘德带岩石在新英格兰其他内围层中的范围和分布，（3）测试单压痕模型与双压痕模型，（4）通过确定盖层甘德岩的淬火时间，监测阿瓦隆在盖层中接近的时间和速度;（5）评估盖层边界中岩石的流变性。采用的方法包括野外露头的特征、分析主要和微量元素组成以识别侵入火成岩、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。

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.