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

构造学中一个突出的问题是地壳增厚的机制。在构造板块汇聚的地方，密度相对较低的地壳可能会从密度较大的地幔龙骨中剥离出来，或者长英质地壳的碎片可能会凹陷或断裂成地壳楔。在这种情况下，压痕或甚至相互压痕结构可能导致地壳增厚。地球物理资料的解释越来越支持构造楔化结构，但在野外很少观察到互穿的地壳条子。因此，互穿楔的机制甚至确认仍然是推测性的。在印第安纳大学和杨百翰大学的合作中，地质学家首次尝试利用地表露头直接描述和确定上地壳与下地壳同步碰撞楔入的原因。他们在倾斜的下地壳剖面中发现了一个构造楔入的例子。康涅狄格沿海裸露岩石的露头以前被标为阿瓦隆地体内的褶皱推覆体。除了来自幼年源的阿瓦洛纪岩石外，我们对钕和铅同位素的初步研究表明，结构上最低的正交性来自与甘德地体相关的古代地壳岩石。他们在该地区的工作表明，阿瓦洛纪岩石既位于甘德基底岩上，也位于北方已知的甘德基底上。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Avalon和Gander带岩石与加拿大沿海相关岩石的身份和对比，(2)测试新英格兰其他内陆Avalon和Gander带岩石的范围和分布，(3)测试单和双压痕模型。(4)通过建立盖地体的淬火时间，监测阿瓦隆在盖地体中的接近时间和速度；(5)评估盖地体边界岩石的流变学。研究方法包括野外露头特征、主微量元素分析识别侵入火成岩、钕铅同位素分析确定岩浆起源等。

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.